Node.js v8.0.0-nightly201702011b30df1003 Documentation

Table of Contents

About this Documentation#

The goal of this documentation is to comprehensively explain the Node.js API, both from a reference as well as a conceptual point of view. Each section describes a built-in module or high-level concept.

Where appropriate, property types, method arguments, and the arguments provided to event handlers are detailed in a list underneath the topic heading.

Every .html document has a corresponding .json document presenting the same information in a structured manner. This feature is experimental, and added for the benefit of IDEs and other utilities that wish to do programmatic things with the documentation.

Every .html and .json file is generated based on the corresponding .md file in the doc/api/ folder in Node.js's source tree. The documentation is generated using the tools/doc/generate.js program. The HTML template is located at doc/template.html.

If you find an error in this documentation, please submit an issue or see the contributing guide for directions on how to submit a patch.

Stability Index#

Throughout the documentation, you will see indications of a section's stability. The Node.js API is still somewhat changing, and as it matures, certain parts are more reliable than others. Some are so proven, and so relied upon, that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous and in the process of being redesigned.

The stability indices are as follows:

Stability: 0 - Deprecated
This feature is known to be problematic, and changes are
planned.  Do not rely on it.  Use of the feature may cause warnings.  Backwards
compatibility should not be expected.
Stability: 1 - Experimental
This feature is subject to change, and is gated by a command line flag.
It may change or be removed in future versions.
Stability: 2 - Stable
The API has proven satisfactory. Compatibility with the npm ecosystem
is a high priority, and will not be broken unless absolutely necessary.
Stability: 3 - Locked
Only fixes related to security, performance, or bug fixes will be accepted.
Please do not suggest API changes in this area; they will be refused.

JSON Output#

Stability: 1 - Experimental

Every HTML file in the markdown has a corresponding JSON file with the same data.

This feature was added in Node.js v0.6.12. It is experimental.

Syscalls and man pages#

System calls like open(2) and read(2) define the interface between user programs and the underlying operating system. Node functions which simply wrap a syscall, like fs.open(), will document that. The docs link to the corresponding man pages (short for manual pages) which describe how the syscalls work.

Caveat: some syscalls, like lchown(2), are BSD-specific. That means, for example, that fs.lchown() only works on Mac OS X and other BSD-derived systems, and is not available on Linux.

Most Unix syscalls have Windows equivalents, but behavior may differ on Windows relative to Linux and OS X. For an example of the subtle ways in which it's sometimes impossible to replace Unix syscall semantics on Windows, see Node issue 4760.

Usage#

node [options] [v8 options] [script.js | -e "script"] [arguments]

Please see the Command Line Options document for information about different options and ways to run scripts with Node.js.

Example#

An example of a web server written with Node.js which responds with 'Hello World':

const http = require('http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

To run the server, put the code into a file called example.js and execute it with Node.js:

$ node example.js
Server running at http://127.0.0.1:3000/

All of the examples in the documentation can be run similarly.

Addons#

Node.js Addons are dynamically-linked shared objects, written in C or C++, that can be loaded into Node.js using the require() function, and used just as if they were an ordinary Node.js module. They are used primarily to provide an interface between JavaScript running in Node.js and C/C++ libraries.

At the moment, the method for implementing Addons is rather complicated, involving knowledge of several components and APIs :

  • V8: the C++ library Node.js currently uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the v8.h header file (deps/v8/include/v8.h in the Node.js source tree), which is also available online.

  • libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the filesystem, sockets, timers and system events. libuv also provides a pthreads-like threading abstraction that may be used to power more sophisticated asynchronous Addons that need to move beyond the standard event loop. Addon authors are encouraged to think about how to avoid blocking the event loop with I/O or other time-intensive tasks by off-loading work via libuv to non-blocking system operations, worker threads or a custom use of libuv's threads.

  • Internal Node.js libraries. Node.js itself exports a number of C/C++ APIs that Addons can use — the most important of which is the node::ObjectWrap class.

  • Node.js includes a number of other statically linked libraries including OpenSSL. These other libraries are located in the deps/ directory in the Node.js source tree. Only the V8 and OpenSSL symbols are purposefully re-exported by Node.js and may be used to various extents by Addons. See Linking to Node.js' own dependencies for additional information.

All of the following examples are available for download and may be used as a starting-point for your own Addon.

Hello world#

This "Hello world" example is a simple Addon, written in C++, that is the equivalent of the following JavaScript code:

module.exports.hello = () => 'world';

First, create the file hello.cc:

// hello.cc
#include <node.h>

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void Method(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(isolate, "world"));
}

void init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "hello", Method);
}

NODE_MODULE(addon, init)

}  // namespace demo

Note that all Node.js Addons must export an initialization function following the pattern:

void Initialize(Local<Object> exports);
NODE_MODULE(module_name, Initialize)

There is no semi-colon after NODE_MODULE as it's not a function (see node.h).

The module_name must match the filename of the final binary (excluding the .node suffix).

In the hello.cc example, then, the initialization function is init and the Addon module name is addon.

Building#

Once the source code has been written, it must be compiled into the binary addon.node file. To do so, create a file called binding.gyp in the top-level of the project describing the build configuration of your module using a JSON-like format. This file is used by node-gyp -- a tool written specifically to compile Node.js Addons.

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "hello.cc" ]
    }
  ]
}

Note: A version of the node-gyp utility is bundled and distributed with Node.js as part of npm. This version is not made directly available for developers to use and is intended only to support the ability to use the npm install command to compile and install Addons. Developers who wish to use node-gyp directly can install it using the command npm install -g node-gyp. See the node-gyp installation instructions for more information, including platform-specific requirements.

Once the binding.gyp file has been created, use node-gyp configure to generate the appropriate project build files for the current platform. This will generate either a Makefile (on Unix platforms) or a vcxproj file (on Windows) in the build/ directory.

Next, invoke the node-gyp build command to generate the compiled addon.node file. This will be put into the build/Release/ directory.

When using npm install to install a Node.js Addon, npm uses its own bundled version of node-gyp to perform this same set of actions, generating a compiled version of the Addon for the user's platform on demand.

Once built, the binary Addon can be used from within Node.js by pointing require() to the built addon.node module:

// hello.js
const addon = require('./build/Release/addon');

console.log(addon.hello());
// Prints: 'world'

Please see the examples below for further information or https://github.com/arturadib/node-qt for an example in production.

Because the exact path to the compiled Addon binary can vary depending on how it is compiled (i.e. sometimes it may be in ./build/Debug/), Addons can use the bindings package to load the compiled module.

Note that while the bindings package implementation is more sophisticated in how it locates Addon modules, it is essentially using a try-catch pattern similar to:

try {
  return require('./build/Release/addon.node');
} catch (err) {
  return require('./build/Debug/addon.node');
}

Linking to Node.js' own dependencies#

Node.js uses a number of statically linked libraries such as V8, libuv and OpenSSL. All Addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate #include <...> statements (e.g. #include <v8.h>) and node-gyp will locate the appropriate headers automatically. However, there are a few caveats to be aware of:

  • When node-gyp runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, Addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.

  • node-gyp can be run using the --nodedir flag pointing at a local Node.js source image. Using this option, the Addon will have access to the full set of dependencies.

Loading Addons using require()#

The filename extension of the compiled Addon binary is .node (as opposed to .dll or .so). The require() function is written to look for files with the .node file extension and initialize those as dynamically-linked libraries.

When calling require(), the .node extension can usually be omitted and Node.js will still find and initialize the Addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file addon.js in the same directory as the binary addon.node, then require('addon') will give precedence to the addon.js file and load it instead.

Native Abstractions for Node.js#

Each of the examples illustrated in this document make direct use of the Node.js and V8 APIs for implementing Addons. It is important to understand that the V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, Addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do currently to ensure stability of the V8 APIs.

The Native Abstractions for Node.js (or nan) provide a set of tools that Addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the nan examples for an illustration of how it can be used.

Addon examples#

Following are some example Addons intended to help developers get started. The examples make use of the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

Each of these examples using the following binding.gyp file:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "addon.cc" ]
    }
  ]
}

In cases where there is more than one .cc file, simply add the additional filename to the sources array. For example:

"sources": ["addon.cc", "myexample.cc"]

Once the binding.gyp file is ready, the example Addons can be configured and built using node-gyp:

$ node-gyp configure build

Function arguments#

Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.

The following example illustrates how to read function arguments passed from JavaScript and how to return a result:

// addon.cc
#include <node.h>

namespace demo {

using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    // Throw an Error that is passed back to JavaScript
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong number of arguments")));
    return;
  }

  // Check the argument types
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong arguments")));
    return;
  }

  // Perform the operation
  double value = args[0]->NumberValue() + args[1]->NumberValue();
  Local<Number> num = Number::New(isolate, value);

  // Set the return value (using the passed in
  // FunctionCallbackInfo<Value>&)
  args.GetReturnValue().Set(num);
}

void Init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(addon, Init)

}  // namespace demo

Once compiled, the example Addon can be required and used from within Node.js:

// test.js
const addon = require('./build/Release/addon');

console.log('This should be eight:', addon.add(3, 5));

Callbacks#

It is common practice within Addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:

// addon.cc
#include <node.h>

namespace demo {

using v8::Function;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Null;
using v8::Object;
using v8::String;
using v8::Value;

void RunCallback(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Function> cb = Local<Function>::Cast(args[0]);
  const unsigned argc = 1;
  Local<Value> argv[argc] = { String::NewFromUtf8(isolate, "hello world") };
  cb->Call(Null(isolate), argc, argv);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", RunCallback);
}

NODE_MODULE(addon, Init)

}  // namespace demo

Note that this example uses a two-argument form of Init() that receives the full module object as the second argument. This allows the Addon to completely overwrite exports with a single function instead of adding the function as a property of exports.

To test it, run the following JavaScript:

// test.js
const addon = require('./build/Release/addon');

addon((msg) => {
  console.log(msg);
// Prints: 'hello world'
});

Note that, in this example, the callback function is invoked synchronously.

Object factory#

Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property msg that echoes the string passed to createObject():

// addon.cc
#include <node.h>

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<Object> obj = Object::New(isolate);
  obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString());

  args.GetReturnValue().Set(obj);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(addon, Init)

}  // namespace demo

To test it in JavaScript:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon('hello');
const obj2 = addon('world');
console.log(obj1.msg, obj2.msg);
// Prints: 'hello world'

Function factory#

Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:

// addon.cc
#include <node.h>

namespace demo {

using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void MyFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(isolate, "hello world"));
}

void CreateFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
  Local<Function> fn = tpl->GetFunction();

  // omit this to make it anonymous
  fn->SetName(String::NewFromUtf8(isolate, "theFunction"));

  args.GetReturnValue().Set(fn);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateFunction);
}

NODE_MODULE(addon, Init)

}  // namespace demo

To test:

// test.js
const addon = require('./build/Release/addon');

const fn = addon();
console.log(fn());
// Prints: 'hello world'

Wrapping C++ objects#

It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript new operator:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Local;
using v8::Object;

void InitAll(Local<Object> exports) {
  MyObject::Init(exports);
}

NODE_MODULE(addon, InitAll)

}  // namespace demo

Then, in myobject.h, the wrapper class inherits from node::ObjectWrap:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Local<v8::Object> exports);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Persistent<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

In myobject.cc, implement the various methods that are to be exposed. Below, the method plusOne() is exposed by adding it to the constructor's prototype:

// myobject.cc
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Local<Object> exports) {
  Isolate* isolate = exports->GetIsolate();

  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  constructor.Reset(isolate, tpl->GetFunction());
  exports->Set(String::NewFromUtf8(isolate, "MyObject"),
               tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Context> context = isolate->GetCurrentContext();
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> result =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(result);
  }
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

To build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj = new addon.MyObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

Factory of wrapped objects#

Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript new operator:

const obj = addon.createObject();
// instead of:
// const obj = new addon.Object();

First, the createObject() method is implemented in addon.cc:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void InitAll(Local<Object> exports, Local<Object> module) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(addon, InitAll)

}  // namespace demo

In myobject.h, the static method NewInstance() is added to handle instantiating the object. This method takes the place of using new in JavaScript:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Persistent<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation in myobject.cc is similar to the previous example:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  constructor.Reset(isolate, tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Context> context = isolate->GetCurrentContext();
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

Once again, to build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const createObject = require('./build/Release/addon');

const obj = createObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

const obj2 = createObject(20);
console.log(obj2.plusOne());
// Prints: 21
console.log(obj2.plusOne());
// Prints: 22
console.log(obj2.plusOne());
// Prints: 23

Passing wrapped objects around#

In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function node::ObjectWrap::Unwrap. The following examples shows a function add() that can take two MyObject objects as input arguments:

// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
      args[0]->ToObject());
  MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
      args[1]->ToObject());

  double sum = obj1->value() + obj2->value();
  args.GetReturnValue().Set(Number::New(isolate, sum));
}

void InitAll(Local<Object> exports) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(exports, "createObject", CreateObject);
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(addon, InitAll)

}  // namespace demo

In myobject.h, a new public method is added to allow access to private values after unwrapping the object.

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
  inline double value() const { return value_; }

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Persistent<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation of myobject.cc is similar to before:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  constructor.Reset(isolate, tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Context> context = isolate->GetCurrentContext();
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

}  // namespace demo

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);

console.log(result);
// Prints: 30

AtExit hooks#

An "AtExit" hook is a function that is invoked after the Node.js event loop has ended but before the JavaScript VM is terminated and Node.js shuts down. "AtExit" hooks are registered using the node::AtExit API.

void AtExit(callback, args)#

  • callback: void (*)(void*) - A pointer to the function to call at exit.
  • args: void* - A pointer to pass to the callback at exit.

Registers exit hooks that run after the event loop has ended but before the VM is killed.

AtExit takes two parameters: a pointer to a callback function to run at exit, and a pointer to untyped context data to be passed to that callback.

Callbacks are run in last-in first-out order.

The following addon.cc implements AtExit:

// addon.cc
#undef NDEBUG
#include <assert.h>
#include <stdlib.h>
#include <node.h>

namespace demo {

using node::AtExit;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;

static char cookie[] = "yum yum";
static int at_exit_cb1_called = 0;
static int at_exit_cb2_called = 0;

static void at_exit_cb1(void* arg) {
  Isolate* isolate = static_cast<Isolate*>(arg);
  HandleScope scope(isolate);
  Local<Object> obj = Object::New(isolate);
  assert(!obj.IsEmpty()); // assert VM is still alive
  assert(obj->IsObject());
  at_exit_cb1_called++;
}

static void at_exit_cb2(void* arg) {
  assert(arg == static_cast<void*>(cookie));
  at_exit_cb2_called++;
}

static void sanity_check(void*) {
  assert(at_exit_cb1_called == 1);
  assert(at_exit_cb2_called == 2);
}

void init(Local<Object> exports) {
  AtExit(sanity_check);
  AtExit(at_exit_cb2, cookie);
  AtExit(at_exit_cb2, cookie);
  AtExit(at_exit_cb1, exports->GetIsolate());
}

NODE_MODULE(addon, init);

}  // namespace demo

Test in JavaScript by running:

// test.js
const addon = require('./build/Release/addon');

Assert#

Stability: 3 - Locked

The assert module provides a simple set of assertion tests that can be used to test invariants. The module is intended for internal use by Node.js, but can be used in application code via require('assert'). However, assert is not a testing framework, and is not intended to be used as a general purpose assertion library.

The API for the assert module is Locked. This means that there will be no additions or changes to any of the methods implemented and exposed by the module.

assert(value[, message])#

Added in: v0.5.9

An alias of assert.ok() .

const assert = require('assert');

assert(true);
// OK
assert(1);
// OK
assert(false);
// throws "AssertionError: false == true"
assert(0);
// throws "AssertionError: 0 == true"
assert(false, 'it\'s false');
// throws "AssertionError: it's false"

assert.deepEqual(actual, expected[, message])#

Added in: v0.1.21

Tests for deep equality between the actual and expected parameters. Primitive values are compared with the equal comparison operator ( == ).

Only enumerable "own" properties are considered. The deepEqual() implementation does not test object prototypes, attached symbols, or non-enumerable properties. This can lead to some potentially surprising results. For example, the following example does not throw an AssertionError because the properties on the Error object are non-enumerable:

// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

const assert = require('assert');

const obj1 = {
  a : {
    b : 1
  }
};
const obj2 = {
  a : {
    b : 2
  }
};
const obj3 = {
  a : {
    b : 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK, object is equal to itself

assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different

assert.deepEqual(obj1, obj3);
// OK, objects are equal

assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.deepStrictEqual(actual, expected[, message])#

Added in: v1.2.0

Generally identical to assert.deepEqual() with two exceptions. First, primitive values are compared using the strict equality operator ( === ). Second, object comparisons include a strict equality check of their prototypes.

const assert = require('assert');

assert.deepEqual({a:1}, {a:'1'});
// OK, because 1 == '1'

assert.deepStrictEqual({a:1}, {a:'1'});
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.doesNotThrow(block[, error][, message])#

Added in: v0.1.21

Asserts that the function block does not throw an error. See assert.throws() for more details.

When assert.doesNotThrow() is called, it will immediately call the block function.

If an error is thrown and it is the same type as that specified by the error parameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller.

The following, for instance, will throw the TypeError because there is no matching error type in the assertion:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError
);

However, the following will result in an AssertionError with the message 'Got unwanted exception (TypeError)..':

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError
);

If an AssertionError is thrown and a value is provided for the message parameter, the value of message will be appended to the AssertionError message:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError,
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops

assert.equal(actual, expected[, message])#

Added in: v0.1.21

Tests shallow, coercive equality between the actual and expected parameters using the equal comparison operator ( == ).

const assert = require('assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({a: {b: 1}}, {a: {b: 1}});
//AssertionError: { a: { b: 1 } } == { a: { b: 1 } }

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.fail(actual, expected, message, operator)#

Added in: v0.1.21

Throws an AssertionError. If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. Otherwise, the error message is the value of message.

const assert = require('assert');

assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2

assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops

assert.ifError(value)#

Added in: v0.1.97

Throws value if value is truthy. This is useful when testing the error argument in callbacks.

const assert = require('assert');

assert.ifError(0);
// OK
assert.ifError(1);
// Throws 1
assert.ifError('error');
// Throws 'error'
assert.ifError(new Error());
// Throws Error

assert.notDeepEqual(actual, expected[, message])#

Added in: v0.1.21

Tests for any deep inequality. Opposite of assert.deepEqual().

const assert = require('assert');

const obj1 = {
  a : {
    b : 1
  }
};
const obj2 = {
  a : {
    b : 2
  }
};
const obj3 = {
  a : {
    b : 1
  }
};
const obj4 = Object.create(obj1);

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK, obj1 and obj2 are not deeply equal

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK, obj1 and obj2 are not deeply equal

If the values are deeply equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.notDeepStrictEqual(actual, expected[, message])#

Added in: v1.2.0

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

const assert = require('assert');

assert.notDeepEqual({a:1}, {a:'1'});
// AssertionError: { a: 1 } notDeepEqual { a: '1' }

assert.notDeepStrictEqual({a:1}, {a:'1'});
// OK

If the values are deeply and strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.notEqual(actual, expected[, message])#

Added in: v0.1.21

Tests shallow, coercive inequality with the not equal comparison operator ( != ).

const assert = require('assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'

If the values are equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.notStrictEqual(actual, expected[, message])#

Added in: v0.1.21

Tests strict inequality as determined by the strict not equal operator ( !== ).

const assert = require('assert');

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 1

assert.notStrictEqual(1, '1');
// OK

If the values are strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.ok(value[, message])#

Added in: v0.1.21

Tests if value is truthy. It is equivalent to assert.equal(!!value, true, message).

If value is not truthy, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

const assert = require('assert');

assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok(false);
// throws "AssertionError: false == true"
assert.ok(0);
// throws "AssertionError: 0 == true"
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"

assert.strictEqual(actual, expected[, message])#

Added in: v0.1.21

Tests strict equality as determined by the strict equality operator ( === ).

const assert = require('assert');

assert.strictEqual(1, 2);
// AssertionError: 1 === 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual(1, '1');
// AssertionError: 1 === '1'

If the values are not strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.throws(block[, error][, message])#

Added in: v0.1.21

Expects the function block to throw an error.

If specified, error can be a constructor, RegExp, or validation function.

If specified, message will be the message provided by the AssertionError if the block fails to throw.

Validate instanceof using constructor:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error
);

Validate error message using RegExp:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /value/
);

Custom error validation:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  function(err) {
    if ( (err instanceof Error) && /value/.test(err) ) {
      return true;
    }
  },
  'unexpected error'
);

Note that error can not be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes:

// THIS IS A MISTAKE! DO NOT DO THIS!
assert.throws(myFunction, 'missing foo', 'did not throw with expected message');

// Do this instead.
assert.throws(myFunction, /missing foo/, 'did not throw with expected message');

Buffer#

Stability: 2 - Stable

Prior to the introduction of TypedArray in ECMAScript 2015 (ES6), the JavaScript language had no mechanism for reading or manipulating streams of binary data. The Buffer class was introduced as part of the Node.js API to make it possible to interact with octet streams in the context of things like TCP streams and file system operations.

Now that TypedArray has been added in ES6, the Buffer class implements the Uint8Array API in a manner that is more optimized and suitable for Node.js' use cases.

Instances of the Buffer class are similar to arrays of integers but correspond to fixed-sized, raw memory allocations outside the V8 heap. The size of the Buffer is established when it is created and cannot be resized.

The Buffer class is a global within Node.js, making it unlikely that one would need to ever use require('buffer').Buffer.

Examples:

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10, filled with 0x1.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using either fill() or write().
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing [0x1, 0x2, 0x3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing ASCII bytes [0x74, 0x65, 0x73, 0x74].
const buf5 = Buffer.from('test');

// Creates a Buffer containing UTF-8 bytes [0x74, 0xc3, 0xa9, 0x73, 0x74].
const buf6 = Buffer.from('tést', 'utf8');

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

In versions of Node.js prior to v6, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided:

  • Passing a number as the first argument to Buffer() (e.g. new Buffer(10)), allocates a new Buffer object of the specified size. The memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be initialized manually by using either buf.fill(0) or by writing to the Buffer completely. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized Buffer versus creating a slower-but-safer Buffer.
  • Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
  • Passing an ArrayBuffer returns a Buffer that shares allocated memory with the given ArrayBuffer.

Because the behavior of new Buffer() changes significantly based on the type of value passed as the first argument, applications that do not properly validate the input arguments passed to new Buffer(), or that fail to appropriately initialize newly allocated Buffer content, can inadvertently introduce security and reliability issues into their code.

To make the creation of Buffer instances more reliable and less error prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

Buffer instances returned by Buffer.allocUnsafe() may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize. Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool.

The --zero-fill-buffers command line option#

Added in: v5.10.0

Node.js can be started using the --zero-fill-buffers command line option to force all newly allocated Buffer instances created using either new Buffer(size), Buffer.allocUnsafe(), Buffer.allocUnsafeSlow() or new SlowBuffer(size) to be automatically zero-filled upon creation. Use of this flag changes the default behavior of these methods and can have a significant impact on performance. Use of the --zero-fill-buffers option is recommended only when necessary to enforce that newly allocated Buffer instances cannot contain potentially sensitive data.

Example:

$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>

What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?#

When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow(), the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read.

While there are clear performance advantages to using Buffer.allocUnsafe(), extra care must be taken in order to avoid introducing security vulnerabilities into an application.

Buffers and Character Encodings#

Buffer instances are commonly used to represent sequences of encoded characters such as UTF-8, UCS2, Base64 or even Hex-encoded data. It is possible to convert back and forth between Buffer instances and ordinary JavaScript strings by using an explicit character encoding.

Example:

const buf = Buffer.from('hello world', 'ascii');

// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('hex'));

// Prints: aGVsbG8gd29ybGQ=
console.log(buf.toString('base64'));

The character encodings currently supported by Node.js include:

  • 'ascii' - for 7-bit ASCII data only. This encoding is fast and will strip the high bit if set.

  • 'utf8' - Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8.

  • 'utf16le' - 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported.

  • 'ucs2' - Alias of 'utf16le'.

  • 'base64' - Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC4648, Section 5.

  • 'latin1' - A way of encoding the Buffer into a one-byte encoded string (as defined by the IANA in RFC1345, page 63, to be the Latin-1 supplement block and C0/C1 control codes).

  • 'binary' - Alias for 'latin1'.

  • 'hex' - Encode each byte as two hexadecimal characters.

Note: Today's browsers follow the WHATWG spec which aliases both 'latin1' and ISO-8859-1 to win-1252. This means that while doing something like http.get(), if the returned charset is one of those listed in the WHATWG spec it's possible that the server actually returned win-1252-encoded data, and using 'latin1' encoding may incorrectly decode the characters.

Buffers and TypedArray#

Buffer instances are also Uint8Array instances. However, there are subtle incompatibilities with the TypedArray specification in ECMAScript 2015. For example, while ArrayBuffer#slice() creates a copy of the slice, the implementation of Buffer#slice() creates a view over the existing Buffer without copying, making Buffer#slice() far more efficient.

It is also possible to create new TypedArray instances from a Buffer with the following caveats:

  1. The Buffer object's memory is copied to the TypedArray, not shared.

  2. The Buffer object's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4], not a Uint32Array with a single element [0x1020304] or [0x4030201].

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypeArray object's .buffer property.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`
const buf1 = Buffer.from(arr);

// Shares memory with `arr`
const buf2 = Buffer.from(arr.buffer);

// Prints: <Buffer 88 a0>
console.log(buf1);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf2);

arr[1] = 6000;

// Prints: <Buffer 88 a0>
console.log(buf1);

// Prints: <Buffer 88 13 70 17>
console.log(buf2);

Note that when creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

Example:

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

// Prints: 16
console.log(buf.length);

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

  • TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function:

Buffers and ES6 iteration#

Buffer instances can be iterated over using the ECMAScript 2015 (ES6) for..of syntax.

Example:

const buf = Buffer.from([1, 2, 3]);

// Prints:
//   1
//   2
//   3
for (const b of buf) {
  console.log(b);
}

Additionally, the buf.values(), buf.keys(), and buf.entries() methods can be used to create iterators.

Class: Buffer#

The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

new Buffer(array)#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use Buffer.from(array) instead.
  • array <Array> An array of bytes to copy from

Allocates a new Buffer using an array of octets.

Example:

// Creates a new Buffer containing the ASCII bytes of the string 'buffer'
const buf = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

new Buffer(buffer)#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use Buffer.from(buffer) instead.
  • buffer <Buffer> An existing Buffer to copy data from

Copies the passed buffer data onto a new Buffer instance.

Example:

const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1);

buf1[0] = 0x61;

// Prints: auffer
console.log(buf1.toString());

// Prints: buffer
console.log(buf2.toString());

new Buffer(arrayBuffer[, byteOffset [, length]])#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use
Buffer.from(arrayBuffer[, byteOffset [, length]])
instead.

This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`
const buf = new Buffer(arr.buffer);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf);

// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;

// Prints: <Buffer 88 13 70 17>
console.log(buf);

new Buffer(size)#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use Buffer.alloc() instead (also see
Buffer.allocUnsafe()).
  • size <Integer> The desired length of the new Buffer

Allocates a new Buffer of size bytes. If the size is larger than buffer.kMaxLength or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be created if size is 0.

Unlike ArrayBuffers, the underlying memory for Buffer instances created in this way is not initialized. The contents of a newly created Buffer are unknown and could contain sensitive data. Use Buffer.alloc(size) instead to initialize a Buffer to zeroes.

Example:

const buf = new Buffer(10);

// Prints: (contents may vary): <Buffer 48 21 4b 00 00 00 00 00 30 dd>
console.log(buf);

buf.fill(0);

// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(buf);

new Buffer(string[, encoding])#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated:
Use Buffer.from(string[, encoding]) instead.
  • string <String> String to encode
  • encoding <String> The encoding of string. Default: 'utf8'

Creates a new Buffer containing the given JavaScript string string. If provided, the encoding parameter identifies the character encoding of string.

Examples:

const buf1 = new Buffer('this is a tést');

// Prints: this is a tést
console.log(buf1.toString());

// Prints: this is a tC)st
console.log(buf1.toString('ascii'));


const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex');

// Prints: this is a tést
console.log(buf2.toString());

Class Method: Buffer.alloc(size[, fill[, encoding]])#

Added in: v5.10.0
  • size <Integer> The desired length of the new Buffer
  • fill <String> | <Buffer> | <Integer> A value to pre-fill the new Buffer with. Default: 0
  • encoding <String> If fill is a string, this is its encoding. Default: 'utf8'

Allocates a new Buffer of size bytes. If fill is undefined, the Buffer will be zero-filled.

Example:

const buf = Buffer.alloc(5);

// Prints: <Buffer 00 00 00 00 00>
console.log(buf);

Allocates a new Buffer of size bytes. If the size is larger than buffer.kMaxLength or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be created if size is 0.

If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill).

Example:

const buf = Buffer.alloc(5, 'a');

// Prints: <Buffer 61 61 61 61 61>
console.log(buf);

If both fill and encoding are specified, the allocated Buffer will be initialized by calling buf.fill(fill, encoding).

Example:

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
console.log(buf);

Calling Buffer.alloc() can be significantly slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data.

A TypeError will be thrown if size is not a number.

Class Method: Buffer.allocUnsafe(size)#

Added in: v5.10.0
  • size <Integer> The desired length of the new Buffer

Allocates a new Buffer of size bytes. If the size is larger than buffer.kMaxLength or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be created if size is 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc() instead to initialize Buffer instances to zeroes.

Example:

const buf = Buffer.allocUnsafe(10);

// Prints: (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>
console.log(buf);

buf.fill(0);

// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(buf);

A TypeError will be thrown if size is not a number.

Note that the Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe() and the deprecated new Buffer(size) constructor only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two).

Use of this pre-allocated internal memory pool is a key difference between calling Buffer.alloc(size, fill) vs. Buffer.allocUnsafe(size).fill(fill). Specifically, Buffer.alloc(size, fill) will never use the internal Buffer pool, while Buffer.allocUnsafe(size).fill(fill) will use the internal Buffer pool if size is less than or equal to half Buffer.poolSize. The difference is subtle but can be important when an application requires the additional performance that Buffer.allocUnsafe() provides.

Class Method: Buffer.allocUnsafeSlow(size)#

Added in: v5.10.0
  • size <Integer> The desired length of the new Buffer

Allocates a new Buffer of size bytes. If the size is larger than buffer.kMaxLength or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be created if size is 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize such Buffer instances to zeroes.

When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4KB are, by default, sliced from a single pre-allocated Buffer. This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffer instances. This approach improves both performance and memory usage by eliminating the need to track and cleanup as many Persistent objects.

However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using Buffer.allocUnsafeSlow() then copy out the relevant bits.

Example:

// Need to keep around a few small chunks of memory
const store = [];

socket.on('readable', () => {
  const data = socket.read();

  // Allocate for retained data
  const sb = Buffer.allocUnsafeSlow(10);

  // Copy the data into the new allocation
  data.copy(sb, 0, 0, 10);

  store.push(sb);
});

Use of Buffer.allocUnsafeSlow() should be used only as a last resort after a developer has observed undue memory retention in their applications.

A TypeError will be thrown if size is not a number.

Class Method: Buffer.byteLength(string[, encoding])#

Added in: v0.1.90

Returns the actual byte length of a string. This is not the same as String.prototype.length since that returns the number of characters in a string.

Example:

const str = '\u00bd + \u00bc = \u00be';

// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);

When string is a Buffer/DataView/TypedArray/ArrayBuffer, the actual byte length is returned.

Class Method: Buffer.compare(buf1, buf2)#

Added in: v0.11.13

Compares buf1 to buf2 typically for the purpose of sorting arrays of Buffer instances. This is equivalent to calling buf1.compare(buf2).

Example:

const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];

// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1])
console.log(arr.sort(Buffer.compare));

Class Method: Buffer.concat(list[, totalLength])#

Added in: v0.7.11

Returns a new Buffer which is the result of concatenating all the Buffer instances in the list together.

If the list has no items, or if the totalLength is 0, then a new zero-length Buffer is returned.

If totalLength is not provided, it is calculated from the Buffer instances in list. This however causes an additional loop to be executed in order to calculate the totalLength, so it is faster to provide the length explicitly if it is already known.

Example: Create a single Buffer from a list of three Buffer instances

const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;

// Prints: 42
console.log(totalLength);

const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA);

// Prints: 42
console.log(bufA.length);

Class Method: Buffer.from(array)#

Added in: v5.10.0

Allocates a new Buffer using an array of octets.

Example:

// Creates a new Buffer containing ASCII bytes of the string 'buffer'
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

A TypeError will be thrown if array is not an Array.

Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])#

Added in: v5.10.0

This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`
const buf = Buffer.from(arr.buffer);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf);

// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;

// Prints: <Buffer 88 13 70 17>
console.log(buf);

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

Example:

const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);

// Prints: 2
console.log(buf.length);

A TypeError will be thrown if arrayBuffer is not an ArrayBuffer.

Class Method: Buffer.from(buffer)#

Added in: v5.10.0
  • buffer <Buffer> An existing Buffer to copy data from

Copies the passed buffer data onto a new Buffer instance.

Example:

const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);

buf1[0] = 0x61;

// Prints: auffer
console.log(buf1.toString());

// Prints: buffer
console.log(buf2.toString());

A TypeError will be thrown if buffer is not a Buffer.

Class Method: Buffer.from(string[, encoding])#

Added in: v5.10.0
  • string <String> A string to encode.
  • encoding <String> The encoding of string. Default: 'utf8'

Creates a new Buffer containing the given JavaScript string string. If provided, the encoding parameter identifies the character encoding of string.

Examples:

const buf1 = Buffer.from('this is a tést');

// Prints: this is a tést
console.log(buf1.toString());

// Prints: this is a tC)st
console.log(buf1.toString('ascii'));


const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

// Prints: this is a tést
console.log(buf2.toString());

A TypeError will be thrown if str is not a string.

Class Method: Buffer.isBuffer(obj)#

Added in: v0.1.101

Returns true if obj is a Buffer, false otherwise.

Class Method: Buffer.isEncoding(encoding)#

Added in: v0.9.1

Returns true if encoding contains a supported character encoding, or false otherwise.

Class Property: Buffer.poolSize#

Added in: v0.11.3

This is the number of bytes used to determine the size of pre-allocated, internal Buffer instances used for pooling. This value may be modified.

buf[index]#

The index operator [index] can be used to get and set the octet at position index in buf. The values refer to individual bytes, so the legal value range is between 0x00 and 0xFF (hex) or 0 and 255 (decimal).

Example: Copy an ASCII string into a Buffer, one byte at a time

const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);

for (let i = 0; i < str.length ; i++) {
  buf[i] = str.charCodeAt(i);
}

// Prints: Node.js
console.log(buf.toString('ascii'));

buf.buffer#

The buffer property references the underlying ArrayBuffer object based on which this Buffer object is created.

const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);

console.log(buffer.buffer === arrayBuffer);
// Prints: true

buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])#

Added in: v0.11.13
  • target <Buffer> | <Uint8Array> A Buffer or Uint8Array to compare to
  • targetStart <Integer> The offset within target at which to begin comparison. Default: 0
  • targetEnd <Integer> The offset with target at which to end comparison (not inclusive). Ignored when targetStart is undefined. Default: target.length
  • sourceStart <Integer> The offset within buf at which to begin comparison. Ignored when targetStart is undefined. Default: 0
  • sourceEnd <Integer> The offset within buf at which to end comparison (not inclusive). Ignored when targetStart is undefined. Default: buf.length
  • Returns: <Integer>

Compares buf with target and returns a number indicating whether buf comes before, after, or is the same as target in sort order. Comparison is based on the actual sequence of bytes in each Buffer.

  • 0 is returned if target is the same as buf
  • 1 is returned if target should come before buf when sorted.
  • -1 is returned if target should come after buf when sorted.

Examples:

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');

// Prints: 0
console.log(buf1.compare(buf1));

// Prints: -1
console.log(buf1.compare(buf2));

// Prints: -1
console.log(buf1.compare(buf3));

// Prints: 1
console.log(buf2.compare(buf1));

// Prints: 1
console.log(buf2.compare(buf3));

// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2])
console.log([buf1, buf2, buf3].sort(Buffer.compare));

The optional targetStart, targetEnd, sourceStart, and sourceEnd arguments can be used to limit the comparison to specific ranges within target and buf respectively.

Examples:

const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

// Prints: 0
console.log(buf1.compare(buf2, 5, 9, 0, 4));

// Prints: -1
console.log(buf1.compare(buf2, 0, 6, 4));

// Prints: 1
console.log(buf1.compare(buf2, 5, 6, 5));

A RangeError will be thrown if: targetStart < 0, sourceStart < 0, targetEnd > target.byteLength or sourceEnd > source.byteLength.

buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])#

Added in: v0.1.90
  • target <Buffer> A Buffer to copy into.
  • targetStart <Integer> The offset within target at which to begin copying to. Default: 0
  • sourceStart <Integer> The offset within buf at which to begin copying from. Ignored when targetStart is undefined. Default: 0
  • sourceEnd <Integer> The offset within buf at which to stop copying (not inclusive). Ignored when sourceStart is undefined. Default: buf.length
  • Returns: <Integer> The number of bytes copied.

Copies data from a region of buf to a region in target even if the target memory region overlaps with buf.

Example: Create two Buffer instances, buf1 and buf2, and copy buf1 from byte 16 through byte 19 into buf2, starting at the 8th byte in buf2

const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0 ; i < 26 ; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf1[i] = i + 97;
}

buf1.copy(buf2, 8, 16, 20);

// Prints: !!!!!!!!qrst!!!!!!!!!!!!!
console.log(buf2.toString('ascii', 0, 25));

Example: Create a single Buffer and copy data from one region to an overlapping region within the same Buffer

const buf = Buffer.allocUnsafe(26);

for (let i = 0 ; i < 26 ; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

// Prints: efghijghijklmnopqrstuvwxyz
console.log(buf.toString());

buf.entries()#

Added in: v1.1.0

Creates and returns an iterator of [index, byte] pairs from the contents of buf.

Example: Log the entire contents of a Buffer

const buf = Buffer.from('buffer');

// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]
for (const pair of buf.entries()) {
  console.log(pair);
}

buf.equals(otherBuffer)#

Added in: v0.11.13

Returns true if both buf and otherBuffer have exactly the same bytes, false otherwise.

Examples:

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');

// Prints: true
console.log(buf1.equals(buf2));

// Prints: false
console.log(buf1.equals(buf3));

buf.fill(value[, offset[, end]][, encoding])#

Added in: v0.5.0

Fills buf with the specified value. If the offset and end are not given, the entire buf will be filled. This is meant to be a small simplification to allow the creation and filling of a Buffer to be done on a single line.

Example: Fill a Buffer with the ASCII character 'h'

const b = Buffer.allocUnsafe(50).fill('h');

// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
console.log(b.toString());

value is coerced to a uint32 value if it is not a String or Integer.

If the final write of a fill() operation falls on a multi-byte character, then only the first bytes of that character that fit into buf are written.

Example: Fill a Buffer with a two-byte character

// Prints: <Buffer c8 a2 c8>
console.log(Buffer.allocUnsafe(3).fill('\u0222'));

buf.indexOf(value[, byteOffset][, encoding])#

Added in: v1.5.0
  • value <String> | <Buffer> | <Uint8Array> | <Integer> What to search for
  • byteOffset <Integer> Where to begin searching in buf. Default: 0
  • encoding <String> If value is a string, this is its encoding. Default: 'utf8'
  • Returns: <Integer> The index of the first occurrence of value in buf or -1 if buf does not contain value

If value is:

  • a string, value is interpreted according to the character encoding in encoding.
  • a Buffer or Uint8Array, value will be used in its entirety. To compare a partial Buffer, use buf.slice().
  • a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255.

Examples:

const buf = Buffer.from('this is a buffer');

// Prints: 0
console.log(buf.indexOf('this'));

// Prints: 2
console.log(buf.indexOf('is'));

// Prints: 8
console.log(buf.indexOf(Buffer.from('a buffer')));

// Prints: 8
// (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(97));

// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example')));

// Prints: 8
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));


const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');

// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', 0, 'ucs2'));

// Prints: 6
console.log(utf16Buffer.indexOf('\u03a3', -4, 'ucs2'));

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN or 0, like {}, [], null or undefined, will search the whole buffer. This behavior matches String#indexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte
// Prints: 2, equivalent to searching for 99 or 'c'
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));

// Passing a byteOffset that coerces to NaN or 0
// Prints: 1, searching the whole buffer
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));

buf.includes(value[, byteOffset][, encoding])#

Added in: v5.3.0
  • value <String> | <Buffer> | <Integer> What to search for
  • byteOffset <Integer> Where to begin searching in buf. Default: 0
  • encoding <String> If value is a string, this is its encoding. Default: 'utf8'
  • Returns: <Boolean> true if value was found in buf, false otherwise

Equivalent to buf.indexOf() !== -1.

Examples:

const buf = Buffer.from('this is a buffer');

// Prints: true
console.log(buf.includes('this'));

// Prints: true
console.log(buf.includes('is'));

// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));

// Prints: true
// (97 is the decimal ASCII value for 'a')
console.log(buf.includes(97));

// Prints: false
console.log(buf.includes(Buffer.from('a buffer example')));

// Prints: true
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));

// Prints: false
console.log(buf.includes('this', 4));

buf.keys()#

Added in: v1.1.0

Creates and returns an iterator of buf keys (indices).

Example:

const buf = Buffer.from('buffer');

// Prints:
//   0
//   1
//   2
//   3
//   4
//   5
for (const key of buf.keys()) {
  console.log(key);
}

buf.lastIndexOf(value[, byteOffset][, encoding])#

Added in: v6.0.0

Identical to buf.indexOf(), except buf is searched from back to front instead of front to back.

Examples:

const buf = Buffer.from('this buffer is a buffer');

// Prints: 0
console.log(buf.lastIndexOf('this'));

// Prints: 17
console.log(buf.lastIndexOf('buffer'));

// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));

// Prints: 15
// (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(97));

// Prints: -1
console.log(buf.lastIndexOf(Buffer.from('yolo')));

// Prints: 5
console.log(buf.lastIndexOf('buffer', 5));

// Prints: -1
console.log(buf.lastIndexOf('buffer', 4));


const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');

// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'ucs2'));

// Prints: 4
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'ucs2'));

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN, like {} or undefined, will search the whole buffer. This behavior matches String#lastIndexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte
// Prints: 2, equivalent to searching for 99 or 'c'
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));

// Passing a byteOffset that coerces to NaN
// Prints: 1, searching the whole buffer
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));

// Passing a byteOffset that coerces to 0
// Prints: -1, equivalent to passing 0
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));

buf.length#

Added in: v0.1.90

Returns the amount of memory allocated for buf in bytes. Note that this does not necessarily reflect the amount of "usable" data within buf.

Example: Create a Buffer and write a shorter ASCII string to it

const buf = Buffer.alloc(1234);

// Prints: 1234
console.log(buf.length);

buf.write('some string', 0, 'ascii');

// Prints: 1234
console.log(buf.length);

While the length property is not immutable, changing the value of length can result in undefined and inconsistent behavior. Applications that wish to modify the length of a Buffer should therefore treat length as read-only and use buf.slice() to create a new Buffer.

Examples:

let buf = Buffer.allocUnsafe(10);

buf.write('abcdefghj', 0, 'ascii');

// Prints: 10
console.log(buf.length);

buf = buf.slice(0, 5);

// Prints: 5
console.log(buf.length);

buf.parent#

Stability: 0 - Deprecated: Use buf.buffer instead.

The buf.parent property is a deprecated alias for buf.buffer.

buf.readDoubleBE(offset[, noAssert])#

buf.readDoubleLE(offset[, noAssert])#

Added in: v0.11.15
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 8
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Number>

Reads a 64-bit double from buf at the specified offset with specified endian format (readDoubleBE() returns big endian, readDoubleLE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

// Prints: 8.20788039913184e-304
console.log(buf.readDoubleBE());

// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readDoubleLE(1));

// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!
console.log(buf.readDoubleLE(1, true));

buf.readFloatBE(offset[, noAssert])#

buf.readFloatLE(offset[, noAssert])#

Added in: v0.11.15
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Number>

Reads a 32-bit float from buf at the specified offset with specified endian format (readFloatBE() returns big endian, readFloatLE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([1, 2, 3, 4]);

// Prints: 2.387939260590663e-38
console.log(buf.readFloatBE());

// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readFloatLE(1));

// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!
console.log(buf.readFloatLE(1, true));

buf.readInt8(offset[, noAssert])#

Added in: v0.5.0
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 1
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Integer>

Reads a signed 8-bit integer from buf at the specified offset.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Integers read from a Buffer are interpreted as two's complement signed values.

Examples:

const buf = Buffer.from([-1, 5]);

// Prints: -1
console.log(buf.readInt8(0));

// Prints: 5
console.log(buf.readInt8(1));

// Throws an exception: RangeError: Index out of range
console.log(buf.readInt8(2));

buf.readInt16BE(offset[, noAssert])#

buf.readInt16LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 2
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Integer>

Reads a signed 16-bit integer from buf at the specified offset with the specified endian format (readInt16BE() returns big endian, readInt16LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Integers read from a Buffer are interpreted as two's complement signed values.

Examples:

const buf = Buffer.from([0, 5]);

// Prints: 5
console.log(buf.readInt16BE());

// Prints: 1280
console.log(buf.readInt16LE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readInt16LE(1));

buf.readInt32BE(offset[, noAssert])#

buf.readInt32LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Integer>

Reads a signed 32-bit integer from buf at the specified offset with the specified endian format (readInt32BE() returns big endian, readInt32LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Integers read from a Buffer are interpreted as two's complement signed values.

Examples:

const buf = Buffer.from([0, 0, 0, 5]);

// Prints: 5
console.log(buf.readInt32BE());

// Prints: 83886080
console.log(buf.readInt32LE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readInt32LE(1));

buf.readIntBE(offset, byteLength[, noAssert])#

buf.readIntLE(offset, byteLength[, noAssert])#

Added in: v0.11.15
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - byteLength
  • byteLength <Integer> How many bytes to read. Must satisfy: 0 < byteLength <= 6
  • noAssert <Boolean> Skip offset and byteLength validation? Default: false
  • Returns: <Integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as a two's complement signed value. Supports up to 48 bits of accuracy.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

// Prints: -546f87a9cbee
console.log(buf.readIntLE(0, 6).toString(16));

// Prints: 1234567890ab
console.log(buf.readIntBE(0, 6).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readIntBE(1, 6).toString(16));

buf.readUInt8(offset[, noAssert])#

Added in: v0.5.0
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 1
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Integer>

Reads an unsigned 8-bit integer from buf at the specified offset.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([1, -2]);

// Prints: 1
console.log(buf.readUInt8(0));

// Prints: 254
console.log(buf.readUInt8(1));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt8(2));

buf.readUInt16BE(offset[, noAssert])#

buf.readUInt16LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 2
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Integer>

Reads an unsigned 16-bit integer from buf at the specified offset with specified endian format (readUInt16BE() returns big endian, readUInt16LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56]);

// Prints: 1234
console.log(buf.readUInt16BE(0).toString(16));

// Prints: 3412
console.log(buf.readUInt16LE(0).toString(16));

// Prints: 3456
console.log(buf.readUInt16BE(1).toString(16));

// Prints: 5634
console.log(buf.readUInt16LE(1).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt16LE(2).toString(16));

buf.readUInt32BE(offset[, noAssert])#

buf.readUInt32LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Skip offset validation? Default: false
  • Returns: <Integer>

Reads an unsigned 32-bit integer from buf at the specified offset with specified endian format (readUInt32BE() returns big endian, readUInt32LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

// Prints: 12345678
console.log(buf.readUInt32BE(0).toString(16));

// Prints: 78563412
console.log(buf.readUInt32LE(0).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt32LE(1).toString(16));

buf.readUIntBE(offset, byteLength[, noAssert])#

buf.readUIntLE(offset, byteLength[, noAssert])#

Added in: v0.11.15
  • offset <Integer> Where to start reading. Must satisfy: 0 <= offset <= buf.length - byteLength
  • byteLength <Integer> How many bytes to read. Must satisfy: 0 < byteLength <= 6
  • noAssert <Boolean> Skip offset and byteLength validation? Default: false
  • Returns: <Integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned integer. Supports up to 48 bits of accuracy.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

// Prints: 1234567890ab
console.log(buf.readUIntBE(0, 6).toString(16));

// Prints: ab9078563412
console.log(buf.readUIntLE(0, 6).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUIntBE(1, 6).toString(16));

buf.slice([start[, end]])#

Added in: v0.3.0

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

Note that modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap.

Example: Create a Buffer with the ASCII alphabet, take a slice, and then modify one byte from the original Buffer

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0 ; i < 26 ; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf1[i] = i + 97;
}

const buf2 = buf1.slice(0, 3);

// Prints: abc
console.log(buf2.toString('ascii', 0, buf2.length));

buf1[0] = 33;

// Prints: !bc
console.log(buf2.toString('ascii', 0, buf2.length));

Specifying negative indexes causes the slice to be generated relative to the end of buf rather than the beginning.

Examples:

const buf = Buffer.from('buffer');

// Prints: buffe
// (Equivalent to buf.slice(0, 5))
console.log(buf.slice(-6, -1).toString());

// Prints: buff
// (Equivalent to buf.slice(0, 4))
console.log(buf.slice(-6, -2).toString());

// Prints: uff
// (Equivalent to buf.slice(1, 4))
console.log(buf.slice(-5, -2).toString());

buf.swap16()#

Added in: v5.10.0

Interprets buf as an array of unsigned 16-bit integers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 2.

Examples:

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);

buf1.swap16();

// Prints: <Buffer 02 01 04 03 06 05 08 07>
console.log(buf1);


const buf2 = Buffer.from([0x1, 0x2, 0x3]);

// Throws an exception: RangeError: Buffer size must be a multiple of 16-bits
buf2.swap16();

buf.swap32()#

Added in: v5.10.0

Interprets buf as an array of unsigned 32-bit integers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 4.

Examples:

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);

buf1.swap32();

// Prints: <Buffer 04 03 02 01 08 07 06 05>
console.log(buf1);


const buf2 = Buffer.from([0x1, 0x2, 0x3]);

// Throws an exception: RangeError: Buffer size must be a multiple of 32-bits
buf2.swap32();

buf.swap64()#

Added in: v6.3.0

Interprets buf as an array of 64-bit numbers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 8.

Examples:

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);

buf1.swap64();

// Prints: <Buffer 08 07 06 05 04 03 02 01>
console.log(buf1);


const buf2 = Buffer.from([0x1, 0x2, 0x3]);

// Throws an exception: RangeError: Buffer size must be a multiple of 64-bits
buf2.swap64();

Note that JavaScript cannot encode 64-bit integers. This method is intended for working with 64-bit floats.

buf.toString([encoding[, start[, end]]])#

Added in: v0.1.90
  • encoding <String> The character encoding to decode to. Default: 'utf8'
  • start <Integer> The byte offset to start decoding at. Default: 0
  • end <Integer> The byte offset to stop decoding at (not inclusive). Default: buf.length
  • Returns: <String>

Decodes buf to a string according to the specified character encoding in encoding. start and end may be passed to decode only a subset of buf.

Examples:

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0 ; i < 26 ; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf1[i] = i + 97;
}

// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('ascii'));

// Prints: abcde
console.log(buf1.toString('ascii', 0, 5));


const buf2 = Buffer.from('tést');

// Prints: 74c3a97374
console.log(buf2.toString('hex'));

// Prints: té
console.log(buf2.toString('utf8', 0, 3));

// Prints: té
console.log(buf2.toString(undefined, 0, 3));

buf.toJSON()#

Added in: v0.9.2

Returns a JSON representation of buf. JSON.stringify() implicitly calls this function when stringifying a Buffer instance.

Example:

const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);

// Prints: {"type":"Buffer","data":[1,2,3,4,5]}
console.log(json);

const copy = JSON.parse(json, (key, value) => {
  return value && value.type === 'Buffer'
    ? Buffer.from(value.data)
    : value;
});

// Prints: <Buffer 01 02 03 04 05>
console.log(copy);

buf.values()#

Added in: v1.1.0

Creates and returns an iterator for buf values (bytes). This function is called automatically when a Buffer is used in a for..of statement.

Examples:

const buf = Buffer.from('buffer');

// Prints:
//   98
//   117
//   102
//   102
//   101
//   114
for (const value of buf.values()) {
  console.log(value);
}

// Prints:
//   98
//   117
//   102
//   102
//   101
//   114
for (const value of buf) {
  console.log(value);
}

buf.write(string[, offset[, length]][, encoding])#

Added in: v0.1.90
  • string <String> String to be written to buf
  • offset <Integer> Where to start writing string. Default: 0
  • length <Integer> How many bytes to write. Default: buf.length - offset
  • encoding <String> The character encoding of string. Default: 'utf8'
  • Returns: <Integer> Number of bytes written

Writes string to buf at offset according to the character encoding in encoding. The length parameter is the number of bytes to write. If buf did not contain enough space to fit the entire string, only a partial amount of string will be written. However, partially encoded characters will not be written.

Example:

const buf = Buffer.allocUnsafe(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

// Prints: 12 bytes: ½ + ¼ = ¾
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);

buf.writeDoubleBE(value, offset[, noAssert])#

buf.writeDoubleLE(value, offset[, noAssert])#

Added in: v0.11.15
  • value <Number> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 8
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset with specified endian format (writeDoubleBE() writes big endian, writeDoubleLE() writes little endian). value should be a valid 64-bit double. Behavior is undefined when value is anything other than a 64-bit double.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(0xdeadbeefcafebabe, 0);

// Prints: <Buffer 43 eb d5 b7 dd f9 5f d7>
console.log(buf);

buf.writeDoubleLE(0xdeadbeefcafebabe, 0);

// Prints: <Buffer d7 5f f9 dd b7 d5 eb 43>
console.log(buf);

buf.writeFloatBE(value, offset[, noAssert])#

buf.writeFloatLE(value, offset[, noAssert])#

Added in: v0.11.15
  • value <Number> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset with specified endian format (writeFloatBE() writes big endian, writeFloatLE() writes little endian). value should be a valid 32-bit float. Behavior is undefined when value is anything other than a 32-bit float.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

// Prints: <Buffer 4f 4a fe bb>
console.log(buf);

buf.writeFloatLE(0xcafebabe, 0);

// Prints: <Buffer bb fe 4a 4f>
console.log(buf);

buf.writeInt8(value, offset[, noAssert])#

Added in: v0.5.0
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 1
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset. value should be a valid signed 8-bit integer. Behavior is undefined when value is anything other than a signed 8-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

value is interpreted and written as a two's complement signed integer.

Examples:

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

// Prints: <Buffer 02 fe>
console.log(buf);

buf.writeInt16BE(value, offset[, noAssert])#

buf.writeInt16LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 2
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset with specified endian format (writeInt16BE() writes big endian, writeInt16LE() writes little endian). value should be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

value is interpreted and written as a two's complement signed integer.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeInt16BE(0x0102, 0);
buf.writeInt16LE(0x0304, 2);

// Prints: <Buffer 01 02 04 03>
console.log(buf);

buf.writeInt32BE(value, offset[, noAssert])#

buf.writeInt32LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset with specified endian format (writeInt32BE() writes big endian, writeInt32LE() writes little endian). value should be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

value is interpreted and written as a two's complement signed integer.

Examples:

const buf = Buffer.allocUnsafe(8);

buf.writeInt32BE(0x01020304, 0);
buf.writeInt32LE(0x05060708, 4);

// Prints: <Buffer 01 02 03 04 08 07 06 05>
console.log(buf);

buf.writeIntBE(value, offset, byteLength[, noAssert])#

buf.writeIntLE(value, offset, byteLength[, noAssert])#

Added in: v0.11.15
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - byteLength
  • byteLength <Integer> How many bytes to write. Must satisfy: 0 < byteLength <= 6
  • noAssert <Boolean> Skip value, offset, and byteLength validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

// Prints: <Buffer 12 34 56 78 90 ab>
console.log(buf);

buf.writeUIntLE(0x1234567890ab, 0, 6);

// Prints: <Buffer ab 90 78 56 34 12>
console.log(buf);

buf.writeUInt8(value, offset[, noAssert])#

Added in: v0.5.0
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 1
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset. value should be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);

// Prints: <Buffer 03 04 23 42>
console.log(buf);

buf.writeUInt16BE(value, offset[, noAssert])#

buf.writeUInt16LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 2
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset with specified endian format (writeUInt16BE() writes big endian, writeUInt16LE() writes little endian). value should be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

// Prints: <Buffer de ad be ef>
console.log(buf);

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

// Prints: <Buffer ad de ef be>
console.log(buf);

buf.writeUInt32BE(value, offset[, noAssert])#

buf.writeUInt32LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Skip value and offset validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes value to buf at the specified offset with specified endian format (writeUInt32BE() writes big endian, writeUInt32LE() writes little endian). value should be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

// Prints: <Buffer fe ed fa ce>
console.log(buf);

buf.writeUInt32LE(0xfeedface, 0);

// Prints: <Buffer ce fa ed fe>
console.log(buf);

buf.writeUIntBE(value, offset, byteLength[, noAssert])#

buf.writeUIntLE(value, offset, byteLength[, noAssert])#

Added in: v0.5.5
  • value <Integer> Number to be written to buf
  • offset <Integer> Where to start writing. Must satisfy: 0 <= offset <= buf.length - byteLength
  • byteLength <Integer> How many bytes to write. Must satisfy: 0 < byteLength <= 6
  • noAssert <Boolean> Skip value, offset, and byteLength validation? Default: false
  • Returns: <Integer> offset plus the number of bytes written

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

// Prints: <Buffer 12 34 56 78 90 ab>
console.log(buf);

buf.writeUIntLE(0x1234567890ab, 0, 6);

// Prints: <Buffer ab 90 78 56 34 12>
console.log(buf);

buffer.INSPECT_MAX_BYTES#

Added in: v0.5.4

Returns the maximum number of bytes that will be returned when buf.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buf.inspect() behavior.

Note that this is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.kMaxLength#

Added in: v3.0.0
  • <Integer> The largest size allowed for a single Buffer instance

On 32-bit architectures, this value is (2^30)-1 (~1GB). On 64-bit architectures, this value is (2^31)-1 (~2GB).

Note that this is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.transcode(source, fromEnc, toEnc)#

Added in: v7.1.0

Re-encodes the given Buffer or Uint8Array instance from one character encoding to another. Returns a new Buffer instance.

Throws if the fromEnc or toEnc specify invalid character encodings or if conversion from fromEnc to toEnc is not permitted.

The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:

const buffer = require('buffer');

const newBuf = buffer.transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'

Because the Euro (€) sign is not representable in US-ASCII, it is replaced with ? in the transcoded Buffer.

Note that this is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

Class: SlowBuffer#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

Returns an un-pooled Buffer.

In order to avoid the garbage collection overhead of creating many individually allocated Buffer instances, by default allocations under 4KB are sliced from a single larger allocated object. This approach improves both performance and memory usage since v8 does not need to track and cleanup as many Persistent objects.

In the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using SlowBuffer then copy out the relevant bits.

Example:

// Need to keep around a few small chunks of memory
const store = [];

socket.on('readable', () => {
  const data = socket.read();

  // Allocate for retained data
  const sb = SlowBuffer(10);

  // Copy the data into the new allocation
  data.copy(sb, 0, 0, 10);

  store.push(sb);
});

Use of SlowBuffer should be used only as a last resort after a developer has observed undue memory retention in their applications.

new SlowBuffer(size)#

Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.
  • size <Integer> The desired length of the new SlowBuffer

Allocates a new Buffer of size bytes. If the size is larger than buffer.kMaxLength or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be created if size is 0.

The underlying memory for SlowBuffer instances is not initialized. The contents of a newly created SlowBuffer are unknown and could contain sensitive data. Use buf.fill(0) to initialize a SlowBuffer to zeroes.

Example:

const SlowBuffer = require('buffer').SlowBuffer;

const buf = new SlowBuffer(5);

// Prints: (contents may vary): <Buffer 78 e0 82 02 01>
console.log(buf);

buf.fill(0);

// Prints: <Buffer 00 00 00 00 00>
console.log(buf);

Child Process#

Stability: 2 - Stable

The child_process module provides the ability to spawn child processes in a manner that is similar, but not identical, to popen(3). This capability is primarily provided by the child_process.spawn() function:

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

By default, pipes for stdin, stdout and stderr are established between the parent Node.js process and the spawned child. It is possible to stream data through these pipes in a non-blocking way. Note, however, that some programs use line-buffered I/O internally. While that does not affect Node.js, it can mean that data sent to the child process may not be immediately consumed.

The child_process.spawn() method spawns the child process asynchronously, without blocking the Node.js event loop. The child_process.spawnSync() function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated.

For convenience, the child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync(). Note that each of these alternatives are implemented on top of child_process.spawn() or child_process.spawnSync().

For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.

Asynchronous Process Creation#

The child_process.spawn(), child_process.fork(), child_process.exec(), and child_process.execFile() methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs.

Each of the methods returns a ChildProcess instance. These objects implement the Node.js EventEmitter API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process.

The child_process.exec() and child_process.execFile() methods additionally allow for an optional callback function to be specified that is invoked when the child process terminates.

Spawning .bat and .cmd files on Windows#

The importance of the distinction between child_process.exec() and child_process.execFile() can vary based on platform. On Unix-type operating systems (Unix, Linux, OSX) child_process.execFile() can be more efficient because it does not spawn a shell. On Windows, however, .bat and .cmd files are not executable on their own without a terminal, and therefore cannot be launched using child_process.execFile(). When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec(), or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted.

// On Windows Only ...
const spawn = require('child_process').spawn;
const bat = spawn('cmd.exe', ['/c', 'my.bat']);

bat.stdout.on('data', (data) => {
  console.log(data.toString());
});

bat.stderr.on('data', (data) => {
  console.log(data.toString());
});

bat.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

// OR...
const exec = require('child_process').exec;
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});

// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
  // ...
});

child_process.exec(command[, options][, callback])#

Added in: v0.1.90
  • command <String> The command to run, with space-separated arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • env <Object> Environment key-value pairs
    • encoding <String> (Default: 'utf8')
    • shell <String> Shell to execute the command with (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should understand the -c switch on UNIX or /d /s /c on Windows. On Windows, command line parsing should be compatible with cmd.exe.)
    • timeout <Number> (Default: 0)
    • maxBuffer <Number> largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed (Default: 200*1024)
    • killSignal <String> | <Integer> (Default: 'SIGTERM')
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
  • callback <Function> called with the output when process terminates
  • Returns: <ChildProcess>

Spawns a shell then executes the command within that shell, buffering any generated output.

Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

const exec = require('child_process').exec;
exec('cat *.js bad_file | wc -l', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

If a callback function is provided, it is called with the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error. The error.code property will be the exit code of the child process while error.signal will be set to the signal that terminated the process. Any exit code other than 0 is considered to be an error.

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

The options argument may be passed as the second argument to customize how the process is spawned. The default options are:

{
  encoding: 'utf8',
  timeout: 0,
  maxBuffer: 200*1024,
  killSignal: 'SIGTERM',
  cwd: null,
  env: null
}

If timeout is greater than 0, the parent will send the the signal identified by the killSignal property (the default is 'SIGTERM') if the child runs longer than timeout milliseconds.

Note: Unlike the exec(3) POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command.

child_process.execFile(file[, args][, options][, callback])#

Added in: v0.1.91

The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell. Rather, the specified executable file is spawned directly as a new process making it slightly more efficient than child_process.exec().

The same options as child_process.exec() are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported.

const execFile = require('child_process').execFile;
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
  if (error) {
    throw error;
  }
  console.log(stdout);
});

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

child_process.fork(modulePath[, args][, options])#

Added in: v0.5.0
  • modulePath <String> The module to run in the child
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • env <Object> Environment key-value pairs
    • execPath <String> Executable used to create the child process
    • execArgv <Array> List of string arguments passed to the executable (Default: process.execArgv)
    • silent <Boolean> If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the 'pipe' and 'inherit' options for child_process.spawn()'s stdio for more details (Default: false)
    • stdio <Array> | <String> See child_process.spawn()'s stdio. When this option is provided, it overrides silent. If the array variant is used, it must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'].
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
  • Returns: <ChildProcess>

The child_process.fork() method is a special case of child_process.spawn() used specifically to spawn new Node.js processes. Like child_process.spawn(), a ChildProcess object is returned. The returned ChildProcess will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See child.send() for details.

It is important to keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.

By default, child_process.fork() will spawn new Node.js instances using the process.execPath of the parent process. The execPath property in the options object allows for an alternative execution path to be used.

Node.js processes launched with a custom execPath will communicate with the parent process using the file descriptor (fd) identified using the environment variable NODE_CHANNEL_FD on the child process. The input and output on this fd is expected to be line delimited JSON objects.

Note: Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process.

child_process.spawn(command[, args][, options])#

Added in: v0.1.90
  • command <String> The command to run
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • env <Object> Environment key-value pairs
    • argv0 <String> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <Array> | <String> Child's stdio configuration. (See options.stdio)
    • detached <Boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached)
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
    • shell <Boolean> | <String> If true, runs command inside of a shell. Uses '/bin/sh' on UNIX, and 'cmd.exe' on Windows. A different shell can be specified as a string. The shell should understand the -c switch on UNIX, or /d /s /c on Windows. Defaults to false (no shell).
  • Returns: <ChildProcess>

The child_process.spawn() method spawns a new process using the given command, with command line arguments in args. If omitted, args defaults to an empty array.

Note: If the shell option is enabled, do not pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

A third argument may be used to specify additional options, with these defaults:

{
  cwd: undefined,
  env: process.env
}

Use cwd to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory.

Use env to specify environment variables that will be visible to the new process, the default is process.env.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Example: A very elaborate way to run ps ax | grep ssh

const spawn = require('child_process').spawn;
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);

ps.stdout.on('data', (data) => {
  grep.stdin.write(data);
});

ps.stderr.on('data', (data) => {
  console.log(`ps stderr: ${data}`);
});

ps.on('close', (code) => {
  if (code !== 0) {
    console.log(`ps process exited with code ${code}`);
  }
  grep.stdin.end();
});

grep.stdout.on('data', (data) => {
  console.log(data.toString());
});

grep.stderr.on('data', (data) => {
  console.log(`grep stderr: ${data}`);
});

grep.on('close', (code) => {
  if (code !== 0) {
    console.log(`grep process exited with code ${code}`);
  }
});

Example of checking for failed exec:

const spawn = require('child_process').spawn;
const child = spawn('bad_command');

child.on('error', (err) => {
  console.log('Failed to start child process.');
});

Note: Certain platforms (OS X, Linux) will use the value of argv[0] for the process title while others (Windows, SunOS) will use command.

Note: Node.js currently overwrites argv[0] with process.execPath on startup, so process.argv[0] in a Node.js child process will not match the argv0 parameter passed to spawn from the parent, retrieve it with the process.argv0 property instead.

options.detached#

Added in: v0.7.10

On Windows, setting options.detached to true makes it possible for the child process to continue running after the parent exits. The child will have its own console window. Once enabled for a child process, it cannot be disabled.

On non-Windows platforms, if options.detached is set to true, the child process will be made the leader of a new process group and session. Note that child processes may continue running after the parent exits regardless of whether they are detached or not. See setsid(2) for more information.

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given child, use the child.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and parent.

When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal.

Example of a long-running process, by detaching and also ignoring its parent stdio file descriptors, in order to ignore the parent's termination:

const spawn = require('child_process').spawn;

const child = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore'
});

child.unref();

Alternatively one can redirect the child process' output into files:

const fs = require('fs');
const spawn = require('child_process').spawn;
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');

const child = spawn('prg', [], {
  detached: true,
  stdio: [ 'ignore', out, err ]
});

child.unref();

options.stdio#

Added in: v0.7.10

The options.stdio option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding child.stdin, child.stdout, and child.stderr streams on the ChildProcess object. This is equivalent to setting the options.stdio equal to ['pipe', 'pipe', 'pipe'].

For convenience, options.stdio may be one of the following strings:

  • 'pipe' - equivalent to ['pipe', 'pipe', 'pipe'] (the default)
  • 'ignore' - equivalent to ['ignore', 'ignore', 'ignore']
  • 'inherit' - equivalent to [process.stdin, process.stdout, process.stderr] or [0,1,2]

Otherwise, the value of options.stdio is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and child. The value is one of the following:

  1. 'pipe' - Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the child_process object as child.stdio[fd]. Pipes created for fds 0 - 2 are also available as child.stdin, child.stdout and child.stderr, respectively.
  2. 'ipc' - Create an IPC channel for passing messages/file descriptors between parent and child. A ChildProcess may have at most one IPC stdio file descriptor. Setting this option enables the child.send() method. If the child writes JSON messages to this file descriptor, the child.on('message') event handler will be triggered in the parent. If the child is a Node.js process, the presence of an IPC channel will enable process.send(), process.disconnect(), process.on('disconnect'), and process.on('message') within the child.
  3. 'ignore' - Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0 - 2 for the processes it spawns, setting the fd to 'ignore' will cause Node.js to open /dev/null and attach it to the child's fd.
  4. <Stream> object - Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).
  5. Positive integer - The integer value is interpreted as a file descriptor that is is currently open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared.
  6. null, undefined - Use default value. For stdio fds 0, 1 and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore'.

Example:

const spawn = require('child_process').spawn;

// Child will use parent's stdios
spawn('prg', [], { stdio: 'inherit' });

// Spawn child sharing only stderr
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

// Open an extra fd=4, to interact with programs presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });

It is worth noting that when an IPC channel is established between the parent and child processes, and the child is a Node.js process, the child is launched with the IPC channel unreferenced (using unref()) until the child registers an event handler for the process.on('disconnect') event. This allows the child to exit normally without the process being held open by the open IPC channel.

See also: child_process.exec() and child_process.fork()

Synchronous Process Creation#

The child_process.spawnSync(), child_process.execSync(), and child_process.execFileSync() methods are synchronous and WILL block the Node.js event loop, pausing execution of any additional code until the spawned process exits.

Blocking calls like these are mostly useful for simplifying general purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.

child_process.execFileSync(file[, args][, options])#

Added in: v0.11.12
  • file <String> The name or path of the executable file to run
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • input <String> | <Buffer> | <Uint8Array> The value which will be passed as stdin to the spawned process
      • supplying this value will override stdio[0]
    • stdio <String> | <Array> Child's stdio configuration. (Default: 'pipe')
      • stderr by default will be output to the parent process' stderr unless stdio is specified
    • env <Object> Environment key-value pairs
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
    • timeout <Number> In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)
    • killSignal <String> | <Integer> The signal value to be used when the spawned process will be killed. (Default: 'SIGTERM')
    • maxBuffer <Number> largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed
    • encoding <String> The encoding used for all stdio inputs and outputs. (Default: 'buffer')
  • Returns: <Buffer> | <String> The stdout from the command

The child_process.execFileSync() method is generally identical to child_process.execFile() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the child process intercepts and handles the SIGTERM signal and does not exit, the parent process will still wait until the child process has exited.

If the process times out, or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync()

child_process.execSync(command[, options])#

Added in: v0.11.12
  • command <String> The command to run
  • options <Object>
    • cwd <String> Current working directory of the child process
    • input <String> | <Buffer> | <Uint8Array> The value which will be passed as stdin to the spawned process
      • supplying this value will override stdio[0]
    • stdio <String> | <Array> Child's stdio configuration. (Default: 'pipe')
      • stderr by default will be output to the parent process' stderr unless stdio is specified
    • env <Object> Environment key-value pairs
    • shell <String> Shell to execute the command with (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should understand the -c switch on UNIX or /d /s /c on Windows. On Windows, command line parsing should be compatible with cmd.exe.)
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
    • timeout <Number> In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)
    • killSignal <String> | <Integer> The signal value to be used when the spawned process will be killed. (Default: 'SIGTERM')
    • maxBuffer <Number> largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed
    • encoding <String> The encoding used for all stdio inputs and outputs. (Default: 'buffer')
  • Returns: <Buffer> | <String> The stdout from the command

The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the child process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

If the process times out, or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync()

Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.spawnSync(command[, args][, options])#

Added in: v0.11.12
  • command <String> The command to run
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • input <String> | <Buffer> | <Uint8Array> The value which will be passed as stdin to the spawned process
      • supplying this value will override stdio[0]
    • stdio <String> | <Array> Child's stdio configuration.
    • env <Object> Environment key-value pairs
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
    • timeout <Number> In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)
    • killSignal <String> | <Integer> The signal value to be used when the spawned process will be killed. (Default: 'SIGTERM')
    • maxBuffer <Number> largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed
    • encoding <String> The encoding used for all stdio inputs and outputs. (Default: 'buffer')
    • shell <Boolean> | <String> If true, runs command inside of a shell. Uses '/bin/sh' on UNIX, and 'cmd.exe' on Windows. A different shell can be specified as a string. The shell should understand the -c switch on UNIX, or /d /s /c on Windows. Defaults to false (no shell).
  • Returns: <Object>
    • pid <Number> Pid of the child process
    • output <Array> Array of results from stdio output
    • stdout <Buffer> | <String> The contents of output[1]
    • stderr <Buffer> | <String> The contents of output[2]
    • status <Number> The exit code of the child process
    • signal <String> The signal used to kill the child process
    • error <Error> The error object if the child process failed or timed out

The child_process.spawnSync() method is generally identical to child_process.spawn() with the exception that the function will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

Note: If the shell option is enabled, do not pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

Class: ChildProcess#

Added in: v2.2.0

Instances of the ChildProcess class are EventEmitters that represent spawned child processes.

Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn(), child_process.exec(), child_process.execFile(), or child_process.fork() methods to create instances of ChildProcess.

Event: 'close'#

Added in: v0.7.7
  • code <Number> the exit code if the child exited on its own.
  • signal <String> the signal by which the child process was terminated.

The 'close' event is emitted when the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams.

Event: 'disconnect'#

Added in: v0.7.2

The 'disconnect' event is emitted after calling the child.disconnect() method in parent process or process.disconnect() in child process. After disconnecting it is no longer possible to send or receive messages, and the child.connected property is false.

Event: 'error'#

The 'error' event is emitted whenever:

  1. The process could not be spawned, or
  2. The process could not be killed, or
  3. Sending a message to the child process failed.

Note that the 'exit' event may or may not fire after an error has occurred. If you are listening to both the 'exit' and 'error' events, it is important to guard against accidentally invoking handler functions multiple times.

See also child.kill() and child.send().

Event: 'exit'#

Added in: v0.1.90
  • code <Number> the exit code if the child exited on its own.
  • signal <String> the signal by which the child process was terminated.

The 'exit' event is emitted after the child process ends. If the process exited, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null. One of the two will always be non-null.

Note that when the 'exit' event is triggered, child process stdio streams might still be open.

Also, note that Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See waitpid(2).

Event: 'message'#

Added in: v0.5.9

The 'message' event is triggered when a child process uses process.send() to send messages.

child.channel#

Added in: v7.1.0
  • <Object> A pipe representing the IPC channel to the child process.

The child.channel property is a reference to the child's IPC channel. If no IPC channel currently exists, this property is undefined.

child.connected#

Added in: v0.7.2
  • <Boolean> Set to false after child.disconnect() is called

The child.connected property indicates whether it is still possible to send and receive messages from a child process. When child.connected is false, it is no longer possible to send or receive messages.

child.disconnect()#

Added in: v0.7.2

Closes the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the child.connected and process.connected properties in both the parent and child (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling child.disconnect().

Note that when the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

child.kill([signal])#

Added in: v0.1.90

The child.kill() methods sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. See signal(7) for a list of available signals.

const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);

grep.on('close', (code, signal) => {
  console.log(
    `child process terminated due to receipt of signal ${signal}`);
});

// Send SIGHUP to process
grep.kill('SIGHUP');

The ChildProcess object may emit an 'error' event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results.

Note that while the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2) for reference.

Also note: on Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with use of the shell option of ChildProcess, such as in this example:

'use strict';
const spawn = require('child_process').spawn;

const child = spawn('sh', ['-c',
  `node -e "setInterval(() => {
      console.log(process.pid, 'is alive')
    }, 500);"`
  ], {
    stdio: ['inherit', 'inherit', 'inherit']
  });

setTimeout(() => {
  child.kill(); // does not terminate the node process in the shell
}, 2000);

child.pid#

Added in: v0.1.90

Returns the process identifier (PID) of the child process.

Example:

const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

child.send(message[, sendHandle[, options]][, callback])#

Added in: v0.5.9

When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork()), the child.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the process.on('message') event.

For example, in the parent script:

const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`);

n.on('message', (m) => {
  console.log('PARENT got message:', m);
});

n.send({ hello: 'world' });

And then the child script, 'sub.js' might look like this:

process.on('message', (m) => {
  console.log('CHILD got message:', m);
});

process.send({ foo: 'bar' });

Child Node.js processes will have a process.send() method of their own that allows the child to send messages back to the parent.

There is a special case when sending a {cmd: 'NODE_foo'} message. All messages containing a NODE_ prefix in its cmd property are considered to be reserved for use within Node.js core and will not be emitted in the child's process.on('message') event. Rather, such messages are emitted using the process.on('internalMessage') event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice.

The optional sendHandle argument that may be passed to child.send() is for passing a TCP server or socket object to the child process. The child will receive the object as the second argument passed to the callback function registered on the process.on('message') event. Any data that is received and buffered in the socket will not be sent to the child.

The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:

  • keepOpen - A Boolean value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Defaults to false.

The optional callback is a function that is invoked after the message is sent but before the child may have received it. The function is called with a single argument: null on success, or an Error object on failure.

If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited.

child.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

Example: sending a server object#

The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

const child = require('child_process').fork('child.js');

// Open up the server object and send the handle.
const server = require('net').createServer();
server.on('connection', (socket) => {
  socket.end('handled by parent');
});
server.listen(1337, () => {
  child.send('server', server);
});

The child would then receive the server object as:

process.on('message', (m, server) => {
  if (m === 'server') {
    server.on('connection', (socket) => {
      socket.end('handled by child');
    });
  }
});

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the net module, dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind() instead of server.listen(). This is, however, currently only supported on UNIX platforms.

Example: sending a socket object#

Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

const normal = require('child_process').fork('child.js', ['normal']);
const special = require('child_process').fork('child.js', ['special']);

// Open up the server and send sockets to child
const server = require('net').createServer();
server.on('connection', (socket) => {

  // If this is special priority
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority
  normal.send('socket', socket);
});
server.listen(1337);

The child.js would receive the socket handle as the second argument passed to the event callback function:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    socket.end(`Request handled with ${process.argv[2]} priority`);
  }
});

Once a socket has been passed to a child, the parent is no longer capable of tracking when the socket is destroyed. To indicate this, the .connections property becomes null. It is recommended not to use .maxConnections when this occurs.

Note: this function uses JSON.stringify() internally to serialize the message.

child.stderr#

Added in: v0.1.90

A Readable Stream that represents the child process's stderr.

If the child was spawned with stdio[2] set to anything other than 'pipe', then this will be undefined.

child.stderr is an alias for child.stdio[2]. Both properties will refer to the same value.

child.stdin#

Added in: v0.1.90

A Writable Stream that represents the child process's stdin.

Note that if a child process waits to read all of its input, the child will not continue until this stream has been closed via end().

If the child was spawned with stdio[0] set to anything other than 'pipe', then this will be undefined.

child.stdin is an alias for child.stdio[0]. Both properties will refer to the same value.

child.stdio#

Added in: v0.7.10

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'. Note that child.stdio[0], child.stdio[1], and child.stdio[2] are also available as child.stdin, child.stdout, and child.stderr, respectively.

In the following example, only the child's fd 1 (stdout) is configured as a pipe, so only the parent's child.stdio[1] is a stream, all other values in the array are null.

const assert = require('assert');
const fs = require('fs');
const child_process = require('child_process');

const child = child_process.spawn('ls', {
  stdio: [
    0, // Use parent's stdin for child
    'pipe', // Pipe child's stdout to parent
    fs.openSync('err.out', 'w') // Direct child's stderr to a file
  ]
});

assert.strictEqual(child.stdio[0], null);
assert.strictEqual(child.stdio[0], child.stdin);

assert(child.stdout);
assert.strictEqual(child.stdio[1], child.stdout);

assert.strictEqual(child.stdio[2], null);
assert.strictEqual(child.stdio[2], child.stderr);

child.stdout#

Added in: v0.1.90

A Readable Stream that represents the child process's stdout.

If the child was spawned with stdio[1] set to anything other than 'pipe', then this will be undefined.

child.stdout is an alias for child.stdio[1]. Both properties will refer to the same value.

maxBuffer and Unicode#

It is important to keep in mind that the maxBuffer option specifies the largest number of octets allowed on stdout or stderr. If this value is exceeded, then the child process is terminated. This particularly impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, the following will output 13 UTF-8 encoded octets to stdout although there are only 4 characters:

console.log('中文测试');

Cluster#

Stability: 2 - Stable

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems the user will sometimes want to launch a cluster of Node.js processes to handle the load.

The cluster module allows you to easily create child processes that all share server ports.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Running Node.js will now share port 8000 between the workers:

$ node server.js
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

Please note that on Windows, it is not yet possible to set up a named pipe server in a worker.

How It Works#

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because server.listen() hands off most of the work to the master process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

  1. server.listen({fd: 7}) Because the message is passed to the master, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
  2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the master process. If the worker already has the handle, then it's presumed that you know what you are doing.
  3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0). In essence, the port is random the first time, but predictable thereafter. If you want to listen on a unique port, generate a port number based on the cluster worker ID.

There is no routing logic in Node.js, or in your program, and no shared state between the workers. Therefore, it is important to design your program such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on your program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers for you, however. It is your responsibility to manage the worker pool for your application's needs.

Class: Worker#

Added in: v0.7.0

A Worker object contains all public information and method about a worker. In the master it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.

Event: 'disconnect'#

Added in: v0.7.7

Similar to the cluster.on('disconnect') event, but specific to this worker.

cluster.fork().on('disconnect', () => {
  // Worker has disconnected
});

Event: 'error'#

Added in: v0.7.3

This event is the same as the one provided by child_process.fork().

In a worker you can also use process.on('error').

Event: 'exit'#

Added in: v0.11.2
  • code <Number> the exit code, if it exited normally.
  • signal <String> the name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

Similar to the cluster.on('exit') event, but specific to this worker.

const worker = cluster.fork();
worker.on('exit', (code, signal) => {
  if (signal) {
    console.log(`worker was killed by signal: ${signal}`);
  } else if (code !== 0) {
    console.log(`worker exited with error code: ${code}`);
  } else {
    console.log('worker success!');
  }
});

Event: 'listening'#

Added in: v0.7.0

Similar to the cluster.on('listening') event, but specific to this worker.

cluster.fork().on('listening', (address) => {
  // Worker is listening
});

It is not emitted in the worker.

Event: 'message'#

Added in: v0.7.0

Similar to the cluster.on('message') event, but specific to this worker. In a worker you can also use process.on('message').

See process event: 'message'.

As an example, here is a cluster that keeps count of the number of requests in the master process using the message system:

const cluster = require('cluster');
const http = require('http');

if (cluster.isMaster) {

  // Keep track of http requests
  let numReqs = 0;
  setInterval(() => {
    console.log(`numReqs = ${numReqs}`);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd === 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = require('os').cpus().length;
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  for (const id in cluster.workers) {
    cluster.workers[id].on('message', messageHandler);
  }

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // notify master about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}

Event: 'online'#

Added in: v0.7.0

Similar to the cluster.on('online') event, but specific to this worker.

cluster.fork().on('online', () => {
  // Worker is online
});

It is not emitted in the worker.

worker.disconnect()#

Added in: v0.7.7
  • Returns: <Worker> A reference to worker.

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

In the master, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .exitedAfterDisconnect to be set.

Note that after a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

Note that in a worker, process.disconnect exists, but it is not this function, it is disconnect.

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.

if (cluster.isMaster) {
  const worker = cluster.fork();
  let timeout;

  worker.on('listening', (address) => {
    worker.send('shutdown');
    worker.disconnect();
    timeout = setTimeout(() => {
      worker.kill();
    }, 2000);
  });

  worker.on('disconnect', () => {
    clearTimeout(timeout);
  });

} else if (cluster.isWorker) {
  const net = require('net');
  const server = net.createServer((socket) => {
    // connections never end
  });

  server.listen(8000);

  process.on('message', (msg) => {
    if (msg === 'shutdown') {
      // initiate graceful close of any connections to server
    }
  });
}

worker.exitedAfterDisconnect#

Added in: v6.0.0

Set by calling .kill() or .disconnect(). Until then, it is undefined.

The boolean worker.exitedAfterDisconnect lets you distinguish between voluntary and accidental exit, the master may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.exitedAfterDisconnect === true) {
    console.log('Oh, it was just voluntary – no need to worry');
  }
});

// kill worker
worker.kill();

worker.id#

Added in: v0.8.0

Each new worker is given its own unique id, this id is stored in the id.

While a worker is alive, this is the key that indexes it in cluster.workers

worker.isConnected()#

Added in: v0.11.14

This function returns true if the worker is connected to its master via its IPC channel, false otherwise. A worker is connected to its master after it's been created. It is disconnected after the 'disconnect' event is emitted.

worker.isDead()#

Added in: v0.11.14

This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.

worker.kill([signal='SIGTERM'])#

Added in: v0.9.12
  • signal <String> Name of the kill signal to send to the worker process.

This function will kill the worker. In the master, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by disconnecting the channel, and then exiting with code 0.

Causes .exitedAfterDisconnect to be set.

This method is aliased as worker.destroy() for backwards compatibility.

Note that in a worker, process.kill() exists, but it is not this function, it is kill.

worker.process#

Added in: v0.7.0

All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.

See: Child Process module

Note that workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true. This protects against accidental disconnection.

worker.send(message[, sendHandle][, callback])#

Added in: v0.7.0

Send a message to a worker or master, optionally with a handle.

In the master this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker this sends a message to the master. It is identical to process.send().

This example will echo back all messages from the master:

if (cluster.isMaster) {
  const worker = cluster.fork();
  worker.send('hi there');

} else if (cluster.isWorker) {
  process.on('message', (msg) => {
    process.send(msg);
  });
}

worker.suicide#

Added in: v0.7.0 Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use worker.exitedAfterDisconnect instead.

An alias to worker.exitedAfterDisconnect.

Set by calling .kill() or .disconnect(). Until then, it is undefined.

The boolean worker.suicide lets you distinguish between voluntary and accidental exit, the master may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.suicide === true) {
    console.log('Oh, it was just voluntary – no need to worry');
  }
});

// kill worker
worker.kill();

This API only exists for backwards compatibility and will be removed in the future.

Event: 'disconnect'#

Added in: v0.7.9

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).

There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

cluster.on('disconnect', (worker) => {
  console.log(`The worker #${worker.id} has disconnected`);
});

Event: 'exit'#

Added in: v0.7.9
  • worker <cluster.Worker>
  • code <Number> the exit code, if it exited normally.
  • signal <String> the name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

When any of the workers die the cluster module will emit the 'exit' event.

This can be used to restart the worker by calling .fork() again.

cluster.on('exit', (worker, code, signal) => {
  console.log('worker %d died (%s). restarting...',
    worker.process.pid, signal || code);
  cluster.fork();
});

See child_process event: 'exit'.

Event: 'fork'#

Added in: v0.7.0

When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create your own timeout.

const timeouts = [];
function errorMsg() {
  console.error('Something must be wrong with the connection ...');
}

cluster.on('fork', (worker) => {
  timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
  clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
  clearTimeout(timeouts[worker.id]);
  errorMsg();
});

Event: 'listening'#

Added in: v0.7.0

After calling listen() from a worker, when the 'listening' event is emitted on the server, a 'listening' event will also be emitted on cluster in the master.

The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address, port and addressType. This is very useful if the worker is listening on more than one address.

cluster.on('listening', (worker, address) => {
  console.log(
    `A worker is now connected to ${address.address}:${address.port}`);
});

The addressType is one of:

  • 4 (TCPv4)
  • 6 (TCPv6)
  • -1 (unix domain socket)
  • "udp4" or "udp6" (UDP v4 or v6)

Event: 'message'#

Emitted when the cluster master receives a message from any worker.

See child_process event: 'message'.

Before Node.js v6.0, this event emitted only the message and the handle, but not the worker object, contrary to what the documentation stated.

If you need to support older versions and don't need the worker object, you can work around the discrepancy by checking the number of arguments:

cluster.on('message', (worker, message, handle) => {
  if (arguments.length === 2) {
    handle = message;
    message = worker;
    worker = undefined;
  }
  // ...
});

Event: 'online'#

Added in: v0.7.0

After forking a new worker, the worker should respond with an online message. When the master receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the master forks a worker, and 'online' is emitted when the worker is running.

cluster.on('online', (worker) => {
  console.log('Yay, the worker responded after it was forked');
});

Event: 'setup'#

Added in: v0.7.1

Emitted every time .setupMaster() is called.

The settings object is the cluster.settings object at the time .setupMaster() was called and is advisory only, since multiple calls to .setupMaster() can be made in a single tick.

If accuracy is important, use cluster.settings.

cluster.disconnect([callback])#

Added in: v0.7.7
  • callback <Function> called when all workers are disconnected and handles are closed

Calls .disconnect() on each worker in cluster.workers.

When they are disconnected all internal handles will be closed, allowing the master process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the master process.

cluster.fork([env])#

Added in: v0.6.0

Spawn a new worker process.

This can only be called from the master process.

cluster.isMaster#

Added in: v0.8.1

True if the process is a master. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isMaster is true.

cluster.isWorker#

Added in: v0.6.0

True if the process is not a master (it is the negation of cluster.isMaster).

cluster.schedulingPolicy#

Added in: v0.11.2

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once you spawn the first worker or call cluster.setupMaster(), whatever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are "rr" and "none".

cluster.settings#

Added in: v0.7.1
  • <Object>
    • execArgv <Array> list of string arguments passed to the Node.js executable. (Default=process.execArgv)
    • exec <String> file path to worker file. (Default=process.argv[1])
    • args <Array> string arguments passed to worker. (Default=process.argv.slice(2))
    • silent <Boolean> whether or not to send output to parent's stdio. (Default=false)
    • stdio <Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent.
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)

After calling .setupMaster() (or .fork()) this settings object will contain the settings, including the default values.

This object is not supposed to be changed or set manually, by you.

cluster.setupMaster([settings])#

Added in: v0.7.1
  • settings <Object>
    • exec <String> file path to worker file. (Default=process.argv[1])
    • args <Array> string arguments passed to worker. (Default=process.argv.slice(2))
    • silent <Boolean> whether or not to send output to parent's stdio. (Default=false)
    • stdio <Array> Configures the stdio of forked processes. When this option is provided, it overrides silent.

setupMaster is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Note that:

  • any settings changes only affect future calls to .fork() and have no effect on workers that are already running
  • The only attribute of a worker that cannot be set via .setupMaster() is the env passed to .fork()
  • the defaults above apply to the first call only, the defaults for later calls is the current value at the time of cluster.setupMaster() is called

Example:

const cluster = require('cluster');
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true
});
cluster.fork(); // https worker
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'http']
});
cluster.fork(); // http worker

This can only be called from the master process.

cluster.worker#

Added in: v0.7.0

A reference to the current worker object. Not available in the master process.

const cluster = require('cluster');

if (cluster.isMaster) {
  console.log('I am master');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  console.log(`I am worker #${cluster.worker.id}`);
}

cluster.workers#

Added in: v0.7.0

A hash that stores the active worker objects, keyed by id field. Makes it easy to loop through all the workers. It is only available in the master process.

A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before last 'disconnect' or 'exit' event is emitted.

// Go through all workers
function eachWorker(callback) {
  for (const id in cluster.workers) {
    callback(cluster.workers[id]);
  }
}
eachWorker((worker) => {
  worker.send('big announcement to all workers');
});

Should you wish to reference a worker over a communication channel, using the worker's unique id is the easiest way to find the worker.

socket.on('data', (id) => {
  const worker = cluster.workers[id];
});

Command Line Options#

Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.

To view this documentation as a manual page in your terminal, run man node.

Synopsis#

node [options] [v8 options] [script.js | -e "script"] [--] [arguments]

node debug [script.js | -e "script" | <host>:<port>] …

node --v8-options

Execute without arguments to start the REPL.

For more info about node debug, please see the debugger documentation.

Options#

-v, --version#

Added in: v0.1.3

Print node's version.

-h, --help#

Added in: v0.1.3

Print node command line options. The output of this option is less detailed than this document.

-e, --eval "script"#

Added in: v0.5.2

Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in script.

-p, --print "script"#

Added in: v0.6.4

Identical to -e but prints the result.

-c, --check#

Added in: v5.0.0

Syntax check the script without executing.

-i, --interactive#

Added in: v0.7.7

Opens the REPL even if stdin does not appear to be a terminal.

-r, --require module#

Added in: v1.6.0

Preload the specified module at startup.

Follows require()'s module resolution rules. module may be either a path to a file, or a node module name.

--no-deprecation#

Added in: v0.8.0

Silence deprecation warnings.

--trace-deprecation#

Added in: v0.8.0

Print stack traces for deprecations.

--throw-deprecation#

Added in: v0.11.14

Throw errors for deprecations.

--no-warnings#

Added in: v6.0.0

Silence all process warnings (including deprecations).

--trace-warnings#

Added in: v6.0.0

Print stack traces for process warnings (including deprecations).

--redirect-warnings=file#

Added in: REPLACEME

Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.

--trace-sync-io#

Added in: v2.1.0

Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.

--trace-events-enabled#

Added in: REPLACEME

Enables the collection of trace event tracing information.

--trace-event-categories#

Added in: REPLACEME

A comma separated list of categories that should be traced when trace event tracing is enabled using --trace-events-enabled.

--zero-fill-buffers#

Added in: v6.0.0

Automatically zero-fills all newly allocated Buffer and SlowBuffer instances.

--preserve-symlinks#

Added in: v6.3.0

Instructs the module loader to preserve symbolic links when resolving and caching modules.

By default, when Node.js loads a module from a path that is symbolically linked to a different on-disk location, Node.js will dereference the link and use the actual on-disk "real path" of the module as both an identifier and as a root path to locate other dependency modules. In most cases, this default behavior is acceptable. However, when using symbolically linked peer dependencies, as illustrated in the example below, the default behavior causes an exception to be thrown if moduleA attempts to require moduleB as a peer dependency:

{appDir}
 ├── app
 │   ├── index.js
 │   └── node_modules
 │       ├── moduleA -> {appDir}/moduleA
 │       └── moduleB
 │           ├── index.js
 │           └── package.json
 └── moduleA
     ├── index.js
     └── package.json

The --preserve-symlinks command line flag instructs Node.js to use the symlink path for modules as opposed to the real path, allowing symbolically linked peer dependencies to be found.

Note, however, that using --preserve-symlinks can have other side effects. Specifically, symbolically linked native modules can fail to load if those are linked from more than one location in the dependency tree (Node.js would see those as two separate modules and would attempt to load the module multiple times, causing an exception to be thrown).

--track-heap-objects#

Added in: v2.4.0

Track heap object allocations for heap snapshots.

--prof-process#

Added in: v5.2.0

Process v8 profiler output generated using the v8 option --prof.

--v8-options#

Added in: v0.1.3

Print v8 command line options.

Note: v8 options allow words to be separated by both dashes (-) or underscores (_).

For example, --stack-trace-limit is equivalent to --stack_trace_limit.

--tls-cipher-list=list#

Added in: v4.0.0

Specify an alternative default TLS cipher list. (Requires Node.js to be built with crypto support. (Default))

--enable-fips#

Added in: v6.0.0

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with ./configure --openssl-fips)

--force-fips#

Added in: v6.0.0

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as --enable-fips)

--openssl-config=file#

Added in: v6.9.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips.

--use-openssl-ca, --use-bundled-ca#

Added in: v7.5.0

Use OpenSSL's default CA store or use bundled Mozilla CA store as supplied by current NodeJS version. The default store is selectable at build-time.

Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environmental variables.

The bundled CA store, as supplied by NodeJS, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.

See SSL_CERT_DIR and SSL_CERT_FILE.

--icu-data-dir=file#

Added in: v0.11.15

Specify ICU data load path. (overrides NODE_ICU_DATA)

--#

Added in: v7.5.0

Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument will be used as a script filename.

Environment Variables#

NODE_DEBUG=module[,…]#

Added in: v0.1.32

','-separated list of core modules that should print debug information.

NODE_PATH=path[:…]#

Added in: v0.1.32

':'-separated list of directories prefixed to the module search path.

Note: on Windows, this is a ';'-separated list instead.

NODE_DISABLE_COLORS=1#

Added in: v0.3.0

When set to 1 colors will not be used in the REPL.

NODE_ICU_DATA=file#

Added in: v0.11.15

Data path for ICU (Intl object) data. Will extend linked-in data when compiled with small-icu support.

NODE_NO_WARNINGS=1#

Added in: v7.5.0

When set to 1, process warnings are silenced.

NODE_PRESERVE_SYMLINKS=1#

Added in: v7.1.0

When set to 1, instructs the module loader to preserve symbolic links when resolving and caching modules.

NODE_REPL_HISTORY=file#

Added in: v3.0.0

Path to the file used to store the persistent REPL history. The default path is ~/.node_repl_history, which is overridden by this variable. Setting the value to an empty string ("" or " ") disables persistent REPL history.

NODE_TTY_UNSAFE_ASYNC=1#

Added in: v6.4.0

When set to 1, writes to stdout and stderr will be non-blocking and asynchronous when outputting to a TTY on platforms which support async stdio. Setting this will void any guarantee that stdio will not be interleaved or dropped at program exit. Use of this mode is not recommended.

NODE_EXTRA_CA_CERTS=file#

Added in: v7.3.0

When set, the well known "root" CAs (like VeriSign) will be extended with the extra certificates in file. The file should consist of one or more trusted certificates in PEM format. A message will be emitted (once) with process.emitWarning() if the file is missing or misformatted, but any errors are otherwise ignored.

Note that neither the well known nor extra certificates are used when the ca options property is explicitly specified for a TLS or HTTPS client or server.

SSL_CERT_DIR=dir#

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's directory containing trusted certificates.

Note: Be aware that unless the child environment is explicitly set, this evironment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

SSL_CERT_FILE=file#

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's file containing trusted certificates.

Note: Be aware that unless the child environment is explicitly set, this evironment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

NODE_REDIRECT_WARNINGS=file#

Added in: REPLACEME

When set, process warnings will be emitted to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. This is equivalent to using the --redirect-warnings=file command-line flag.

Console#

Stability: 2 - Stable

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

  • A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
  • A global console instance configured to write to stdout and stderr. Because this object is global, it can be used without calling require('console').

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

While the API for the Console class is designed fundamentally around the browser console object, the Console in Node.js is not intended to duplicate the browser's functionality exactly.

Asynchronous vs Synchronous Consoles#

The console functions are usually asynchronous unless the destination is a file. Disks are fast and operating systems normally employ write-back caching; it should be a very rare occurrence indeed that a write blocks, but it is possible.

Additionally, console functions are blocking when outputting to TTYs (terminals) on OS X as a workaround for the OS's very small, 1kb buffer size. This is to prevent interleaving between stdout and stderr.

Class: Console#

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console:

const Console = require('console').Console;
const Console = console.Console;

new Console(stdout[, stderr])#

Creates a new Console by passing one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr is not passed, warning and error output will be sent to stdout.

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, errorOutput);
// use it like console
const count = 5;
logger.log('count: %d', count);
// in stdout.log: count 5

The global console is a special Console whose output is sent to process.stdout and process.stderr. It is equivalent to calling:

new Console(process.stdout, process.stderr);

console.assert(value[, message][, ...args])#

Added in: v0.1.101

A simple assertion test that verifies whether value is truthy. If it is not, an AssertionError is thrown. If provided, the error message is formatted using util.format() and used as the error message.

console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work

Note: the console.assert() method is implemented differently in Node.js than the console.assert() method available in browsers.

Specifically, in browsers, calling console.assert() with a falsy assertion will cause the message to be printed to the console without interrupting execution of subsequent code. In Node.js, however, a falsy assertion will cause an AssertionError to be thrown.

Functionality approximating that implemented by browsers can be implemented by extending Node.js' console and overriding the console.assert() method.

In the following example, a simple module is created that extends and overrides the default behavior of console in Node.js.

'use strict';

// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.create(console, {
  assert: {
    value: function assert(assertion, message, ...args) {
      try {
        console.assert(assertion, message, ...args);
      } catch (err) {
        console.error(err.stack);
      }
    },
    configurable: true,
    enumerable: true,
    writable: true,
  },
});

module.exports = myConsole;

This can then be used as a direct replacement for the built in console:

const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');

console.dir(obj[, options])#

Added in: v0.1.101

Uses util.inspect() on obj and prints the resulting string to stdout. This function bypasses any custom inspect() function defined on obj. An optional options object may be passed to alter certain aspects of the formatted string:

  • showHidden - if true then the object's non-enumerable and symbol properties will be shown too. Defaults to false.

  • depth - tells util.inspect() how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to 2. To make it recurse indefinitely, pass null.

  • colors - if true, then the output will be styled with ANSI color codes. Defaults to false. Colors are customizable; see customizing util.inspect() colors.

console.error([data][, ...args])#

Added in: v0.1.100

Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.info([data][, ...args])#

Added in: v0.1.100

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Added in: v0.1.100

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.time(label)#

Added in: v0.1.104

Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label. Use the same label when you call console.timeEnd() to stop the timer and output the elapsed time in milliseconds to stdout. Timer durations are accurate to the sub-millisecond.

console.timeEnd(label)#

Added in: v0.1.104

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('100-elements');
for (let i = 0; i < 100; i++) {
  ;
}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

Note: As of Node.js v6.0.0, console.timeEnd() deletes the timer to avoid leaking it. On older versions, the timer persisted. This allowed console.timeEnd() to be called multiple times for the same label. This functionality was unintended and is no longer supported.

console.trace(message[, ...args])#

Added in: v0.1.104

Prints to stderr the string 'Trace :', followed by the util.format() formatted message and stack trace to the current position in the code.

console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
//  Trace: Show me
//    at repl:2:9
//    at REPLServer.defaultEval (repl.js:248:27)
//    at bound (domain.js:287:14)
//    at REPLServer.runBound [as eval] (domain.js:300:12)
//    at REPLServer.<anonymous> (repl.js:412:12)
//    at emitOne (events.js:82:20)
//    at REPLServer.emit (events.js:169:7)
//    at REPLServer.Interface._onLine (readline.js:210:10)
//    at REPLServer.Interface._line (readline.js:549:8)
//    at REPLServer.Interface._ttyWrite (readline.js:826:14)

console.warn([data][, ...args])#

Added in: v0.1.100

The console.warn() function is an alias for console.error().

Crypto#

Stability: 2 - Stable

The crypto module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign and verify functions.

Use require('crypto') to access this module.

const crypto = require('crypto');

const secret = 'abcdefg';
const hash = crypto.createHmac('sha256', secret)
                   .update('I love cupcakes')
                   .digest('hex');
console.log(hash);
// Prints:
//   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e

Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the crypto module. In such cases, calling require('crypto') will result in an error being thrown.

let crypto;
try {
  crypto = require('crypto');
} catch (err) {
  console.log('crypto support is disabled!');
}

Class: Certificate#

Added in: v0.11.8

SPKAC is a Certificate Signing Request mechanism originally implemented by Netscape and now specified formally as part of HTML5's keygen element.

The crypto module provides the Certificate class for working with SPKAC data. The most common usage is handling output generated by the HTML5 <keygen> element. Node.js uses OpenSSL's SPKAC implementation internally.

new crypto.Certificate()#

Instances of the Certificate class can be created using the new keyword or by calling crypto.Certificate() as a function:

const crypto = require('crypto');

const cert1 = new crypto.Certificate();
const cert2 = crypto.Certificate();

certificate.exportChallenge(spkac)#

Added in: v0.11.8

The spkac data structure includes a public key and a challenge. The certificate.exportChallenge() returns the challenge component in the form of a Node.js Buffer. The spkac argument can be either a string or a Buffer.

const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

certificate.exportPublicKey(spkac)#

Added in: v0.11.8

The spkac data structure includes a public key and a challenge. The certificate.exportPublicKey() returns the public key component in the form of a Node.js Buffer. The spkac argument can be either a string or a Buffer.

const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

certificate.verifySpkac(spkac)#

Added in: v0.11.8

Returns true if the given spkac data structure is valid, false otherwise. The spkac argument must be a Node.js Buffer.

const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Class: Cipher#

Added in: v0.1.94

Instances of the Cipher class are used to encrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
  • Using the cipher.update() and cipher.final() methods to produce the encrypted data.

The crypto.createCipher() or crypto.createCipheriv() methods are used to create Cipher instances. Cipher objects are not to be created directly using the new keyword.

Example: Using Cipher objects as streams:

const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');

let encrypted = '';
cipher.on('readable', () => {
  const data = cipher.read();
  if (data)
    encrypted += data.toString('hex');
});
cipher.on('end', () => {
  console.log(encrypted);
  // Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
});

cipher.write('some clear text data');
cipher.end();

Example: Using Cipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const cipher = crypto.createCipher('aes192', 'a password');

const input = fs.createReadStream('test.js');
const output = fs.createWriteStream('test.enc');

input.pipe(cipher).pipe(output);

Example: Using the cipher.update() and cipher.final() methods:

const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');

let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
// Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504

cipher.final([output_encoding])#

Added in: v0.1.94

Returns any remaining enciphered contents. If output_encoding parameter is one of 'latin1', 'base64' or 'hex', a string is returned. If an output_encoding is not provided, a Buffer is returned.

Once the cipher.final() method has been called, the Cipher object can no longer be used to encrypt data. Attempts to call cipher.final() more than once will result in an error being thrown.

cipher.setAAD(buffer)#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the cipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

Returns this for method chaining.

cipher.getAuthTag()#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the cipher.getAuthTag() method returns a Buffer containing the authentication tag that has been computed from the given data.

The cipher.getAuthTag() method should only be called after encryption has been completed using the cipher.final() method.

cipher.setAutoPadding(auto_padding=true)#

Added in: v0.7.1

When using block encryption algorithms, the Cipher class will automatically add padding to the input data to the appropriate block size. To disable the default padding call cipher.setAutoPadding(false).

When auto_padding is false, the length of the entire input data must be a multiple of the cipher's block size or cipher.final() will throw an Error. Disabling automatic padding is useful for non-standard padding, for instance using 0x0 instead of PKCS padding.

The cipher.setAutoPadding() method must be called before cipher.final().

Returns this for method chaining.

cipher.update(data[, input_encoding][, output_encoding])#

Added in: v0.1.94

Updates the cipher with data. If the input_encoding argument is given, its value must be one of 'utf8', 'ascii', or 'latin1' and the data argument is a string using the specified encoding. If the input_encoding argument is not given, data must be a Buffer. If data is a Buffer then input_encoding is ignored.

The output_encoding specifies the output format of the enciphered data, and can be 'latin1', 'base64' or 'hex'. If the output_encoding is specified, a string using the specified encoding is returned. If no output_encoding is provided, a Buffer is returned.

The cipher.update() method can be called multiple times with new data until cipher.final() is called. Calling cipher.update() after cipher.final() will result in an error being thrown.

Class: Decipher#

Added in: v0.1.94

Instances of the Decipher class are used to decrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
  • Using the decipher.update() and decipher.final() methods to produce the unencrypted data.

The crypto.createDecipher() or crypto.createDecipheriv() methods are used to create Decipher instances. Decipher objects are not to be created directly using the new keyword.

Example: Using Decipher objects as streams:

const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');

let decrypted = '';
decipher.on('readable', () => {
  const data = decipher.read();
  if (data)
    decrypted += data.toString('utf8');
});
decipher.on('end', () => {
  console.log(decrypted);
  // Prints: some clear text data
});

const encrypted = 'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
decipher.write(encrypted, 'hex');
decipher.end();

Example: Using Decipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const decipher = crypto.createDecipher('aes192', 'a password');

const input = fs.createReadStream('test.enc');
const output = fs.createWriteStream('test.js');

input.pipe(decipher).pipe(output);

Example: Using the decipher.update() and decipher.final() methods:

const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');

const encrypted = 'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data

decipher.final([output_encoding])#

Added in: v0.1.94

Returns any remaining deciphered contents. If output_encoding parameter is one of 'latin1', 'ascii' or 'utf8', a string is returned. If an output_encoding is not provided, a Buffer is returned.

Once the decipher.final() method has been called, the Decipher object can no longer be used to decrypt data. Attempts to call decipher.final() more than once will result in an error being thrown.

decipher.setAAD(buffer)#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the decipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

Returns this for method chaining.

decipher.setAuthTag(buffer)#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the decipher.setAuthTag() method is used to pass in the received authentication tag. If no tag is provided, or if the cipher text has been tampered with, decipher.final() with throw, indicating that the cipher text should be discarded due to failed authentication.

Returns this for method chaining.

decipher.setAutoPadding(auto_padding=true)#

Added in: v0.7.1

When data has been encrypted without standard block padding, calling decipher.setAutoPadding(false) will disable automatic padding to prevent decipher.final() from checking for and removing padding.

Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.

The decipher.setAutoPadding() method must be called before decipher.update().

Returns this for method chaining.

decipher.update(data[, input_encoding][, output_encoding])#

Added in: v0.1.94

Updates the decipher with data. If the input_encoding argument is given, its value must be one of 'latin1', 'base64', or 'hex' and the data argument is a string using the specified encoding. If the input_encoding argument is not given, data must be a Buffer. If data is a Buffer then input_encoding is ignored.

The output_encoding specifies the output format of the enciphered data, and can be 'latin1', 'ascii' or 'utf8'. If the output_encoding is specified, a string using the specified encoding is returned. If no output_encoding is provided, a Buffer is returned.

The decipher.update() method can be called multiple times with new data until decipher.final() is called. Calling decipher.update() after decipher.final() will result in an error being thrown.

Class: DiffieHellman#

Added in: v0.5.0

The DiffieHellman class is a utility for creating Diffie-Hellman key exchanges.

Instances of the DiffieHellman class can be created using the crypto.createDiffieHellman() function.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createDiffieHellman(2048);
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));

diffieHellman.computeSecret(other_public_key[, input_encoding][, output_encoding])#

Added in: v0.5.0

Computes the shared secret using other_public_key as the other party's public key and returns the computed shared secret. The supplied key is interpreted using the specified input_encoding, and secret is encoded using specified output_encoding. Encodings can be 'latin1', 'hex', or 'base64'. If the input_encoding is not provided, other_public_key is expected to be a Buffer.

If output_encoding is given a string is returned; otherwise, a Buffer is returned.

diffieHellman.generateKeys([encoding])#

Added in: v0.5.0

Generates private and public Diffie-Hellman key values, and returns the public key in the specified encoding. This key should be transferred to the other party. Encoding can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getGenerator([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman generator in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrime([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman prime in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrivateKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman private key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPublicKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman public key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.setPrivateKey(private_key[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman private key. If the encoding argument is provided and is either 'latin1', 'hex', or 'base64', private_key is expected to be a string. If no encoding is provided, private_key is expected to be a Buffer.

diffieHellman.setPublicKey(public_key[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman public key. If the encoding argument is provided and is either 'latin1', 'hex' or 'base64', public_key is expected to be a string. If no encoding is provided, public_key is expected to be a Buffer.

diffieHellman.verifyError#

Added in: v0.11.12

A bit field containing any warnings and/or errors resulting from a check performed during initialization of the DiffieHellman object.

The following values are valid for this property (as defined in constants module):

  • DH_CHECK_P_NOT_SAFE_PRIME
  • DH_CHECK_P_NOT_PRIME
  • DH_UNABLE_TO_CHECK_GENERATOR
  • DH_NOT_SUITABLE_GENERATOR

Class: ECDH#

Added in: v0.11.14

The ECDH class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH) key exchanges.

Instances of the ECDH class can be created using the crypto.createECDH() function.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createECDH('secp521r1');
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.createECDH('secp521r1');
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
  // OK

ecdh.computeSecret(other_public_key[, input_encoding][, output_encoding])#

Added in: v0.11.14

Computes the shared secret using other_public_key as the other party's public key and returns the computed shared secret. The supplied key is interpreted using specified input_encoding, and the returned secret is encoded using the specified output_encoding. Encodings can be 'latin1', 'hex', or 'base64'. If the input_encoding is not provided, other_public_key is expected to be a Buffer.

If output_encoding is given a string will be returned; otherwise a Buffer is returned.

ecdh.generateKeys([encoding[, format]])#

Added in: v0.11.14

Generates private and public EC Diffie-Hellman key values, and returns the public key in the specified format and encoding. This key should be transferred to the other party.

The format argument specifies point encoding and can be 'compressed' or 'uncompressed'. If format is not specified, the point will be returned in 'uncompressed' format.

The encoding argument can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPrivateKey([encoding])#

Added in: v0.11.14

Returns the EC Diffie-Hellman private key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPublicKey([encoding[, format]])#

Added in: v0.11.14

Returns the EC Diffie-Hellman public key in the specified encoding and format.

The format argument specifies point encoding and can be 'compressed' or 'uncompressed'. If format is not specified the point will be returned in 'uncompressed' format.

The encoding argument can be 'latin1', 'hex', or 'base64'. If encoding is specified, a string is returned; otherwise a Buffer is returned.

ecdh.setPrivateKey(private_key[, encoding])#

Added in: v0.11.14

Sets the EC Diffie-Hellman private key. The encoding can be 'latin1', 'hex' or 'base64'. If encoding is provided, private_key is expected to be a string; otherwise private_key is expected to be a Buffer. If private_key is not valid for the curve specified when the ECDH object was created, an error is thrown. Upon setting the private key, the associated public point (key) is also generated and set in the ECDH object.

ecdh.setPublicKey(public_key[, encoding])#

Added in: v0.11.14 Deprecated since: v5.2.0 
Stability: 0 - Deprecated

Sets the EC Diffie-Hellman public key. Key encoding can be 'latin1', 'hex' or 'base64'. If encoding is provided public_key is expected to be a string; otherwise a Buffer is expected.

Note that there is not normally a reason to call this method because ECDH only requires a private key and the other party's public key to compute the shared secret. Typically either ecdh.generateKeys() or ecdh.setPrivateKey() will be called. The ecdh.setPrivateKey() method attempts to generate the public point/key associated with the private key being set.

Example (obtaining a shared secret):

const crypto = require('crypto');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1');

// Note: This is a shortcut way to specify one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
  crypto.createHash('sha256').update('alice', 'utf8').digest()
);

// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);

Class: Hash#

Added in: v0.1.92

The Hash class is a utility for creating hash digests of data. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
  • Using the hash.update() and hash.digest() methods to produce the computed hash.

The crypto.createHash() method is used to create Hash instances. Hash objects are not to be created directly using the new keyword.

Example: Using Hash objects as streams:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.on('readable', () => {
  const data = hash.read();
  if (data)
    console.log(data.toString('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
});

hash.write('some data to hash');
hash.end();

Example: Using Hash and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');

const input = fs.createReadStream('test.js');
input.pipe(hash).pipe(process.stdout);

Example: Using the hash.update() and hash.digest() methods:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50

hash.digest([encoding])#

Added in: v0.1.92

Calculates the digest of all of the data passed to be hashed (using the hash.update() method). The encoding can be 'hex', 'latin1' or 'base64'. If encoding is provided a string will be returned; otherwise a Buffer is returned.

The Hash object can not be used again after hash.digest() method has been called. Multiple calls will cause an error to be thrown.

hash.update(data[, input_encoding])#

Added in: v0.1.92

Updates the hash content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

Class: Hmac#

Added in: v0.1.94

The Hmac Class is a utility for creating cryptographic HMAC digests. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
  • Using the hmac.update() and hmac.digest() methods to produce the computed HMAC digest.

The crypto.createHmac() method is used to create Hmac instances. Hmac objects are not to be created directly using the new keyword.

Example: Using Hmac objects as streams:

const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.on('readable', () => {
  const data = hmac.read();
  if (data)
    console.log(data.toString('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
});

hmac.write('some data to hash');
hmac.end();

Example: Using Hmac and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream('test.js');
input.pipe(hmac).pipe(process.stdout);

Example: Using the hmac.update() and hmac.digest() methods:

const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
//   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e

hmac.digest([encoding])#

Added in: v0.1.94

Calculates the HMAC digest of all of the data passed using hmac.update(). The encoding can be 'hex', 'latin1' or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned;

The Hmac object can not be used again after hmac.digest() has been called. Multiple calls to hmac.digest() will result in an error being thrown.

hmac.update(data[, input_encoding])#

Added in: v0.1.94

Updates the Hmac content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

Class: Sign#

Added in: v0.1.92

The Sign Class is a utility for generating signatures. It can be used in one of two ways:

  • As a writable stream, where data to be signed is written and the sign.sign() method is used to generate and return the signature, or
  • Using the sign.update() and sign.sign() methods to produce the signature.

The crypto.createSign() method is used to create Sign instances. Sign objects are not to be created directly using the new keyword.

Example: Using Sign objects as streams:

const crypto = require('crypto');
const sign = crypto.createSign('RSA-SHA256');

sign.write('some data to sign');
sign.end();

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature

Example: Using the sign.update() and sign.sign() methods:

const crypto = require('crypto');
const sign = crypto.createSign('RSA-SHA256');

sign.update('some data to sign');

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature

A Sign instance can also be created by just passing in the digest algorithm name, in which case OpenSSL will infer the full signature algorithm from the type of the PEM-formatted private key, including algorithms that do not have directly exposed name constants, e.g. 'ecdsa-with-SHA256'.

Example: signing using ECDSA with SHA256

const crypto = require('crypto');
const sign = crypto.createSign('sha256');

sign.update('some data to sign');

const privateKey =
`-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIF+jnWY1D5kbVYDNvxxo/Y+ku2uJPDwS0r/VuPZQrjjVoAoGCCqGSM49
AwEHoUQDQgAEurOxfSxmqIRYzJVagdZfMMSjRNNhB8i3mXyIMq704m2m52FdfKZ2
pQhByd5eyj3lgZ7m7jbchtdgyOF8Io/1ng==
-----END EC PRIVATE KEY-----`;

console.log(sign.sign(privateKey).toString('hex'));

sign.sign(private_key[, output_format])#

Added in: v0.1.92

Calculates the signature on all the data passed through using either sign.update() or sign.write().

The private_key argument can be an object or a string. If private_key is a string, it is treated as a raw key with no passphrase. If private_key is an object, it is interpreted as a hash containing two properties:

  • key : <String> - PEM encoded private key
  • passphrase : <String> - passphrase for the private key

The output_format can specify one of 'latin1', 'hex' or 'base64'. If output_format is provided a string is returned; otherwise a Buffer is returned.

The Sign object can not be again used after sign.sign() method has been called. Multiple calls to sign.sign() will result in an error being thrown.

sign.update(data[, input_encoding])#

Added in: v0.1.92

Updates the Sign content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

Class: Verify#

Added in: v0.1.92

The Verify class is a utility for verifying signatures. It can be used in one of two ways:

The crypto.createVerify() method is used to create Verify instances. Verify objects are not to be created directly using the new keyword.

Example: Using Verify objects as streams:

const crypto = require('crypto');
const verify = crypto.createVerify('RSA-SHA256');

verify.write('some data to sign');
verify.end();

const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false

Example: Using the verify.update() and verify.verify() methods:

const crypto = require('crypto');
const verify = crypto.createVerify('RSA-SHA256');

verify.update('some data to sign');

const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false

verifier.update(data[, input_encoding])#

Added in: v0.1.92

Updates the Verify content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

verifier.verify(object, signature[, signature_format])#

Added in: v0.1.92

Verifies the provided data using the given object and signature. The object argument is a string containing a PEM encoded object, which can be one an RSA public key, a DSA public key, or an X.509 certificate. The signature argument is the previously calculated signature for the data, in the signature_format which can be 'latin1', 'hex' or 'base64'. If a signature_format is specified, the signature is expected to be a string; otherwise signature is expected to be a Buffer.

Returns true or false depending on the validity of the signature for the data and public key.

The verifier object can not be used again after verify.verify() has been called. Multiple calls to verify.verify() will result in an error being thrown.

crypto module methods and properties#

crypto.constants#

Added in: v6.3.0

Returns an object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto Constants.

crypto.DEFAULT_ENCODING#

Added in: v0.9.3

The default encoding to use for functions that can take either strings or buffers. The default value is 'buffer', which makes methods default to Buffer objects.

The crypto.DEFAULT_ENCODING mechanism is provided for backwards compatibility with legacy programs that expect 'latin1' to be the default encoding.

New applications should expect the default to be 'buffer'. This property may become deprecated in a future Node.js release.

crypto.fips#

Added in: v6.0.0

Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.

crypto.createCipher(algorithm, password)#

Added in: v0.1.94

Creates and returns a Cipher object that uses the given algorithm and password.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list-cipher-algorithms will display the available cipher algorithms.

The password is used to derive the cipher key and initialization vector (IV). The value must be either a 'latin1' encoded string or a Buffer.

The implementation of crypto.createCipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use pbkdf2 instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.pbkdf2() and to use crypto.createCipheriv() to create the Cipher object.

crypto.createCipheriv(algorithm, key, iv)#

Creates and returns a Cipher object, with the given algorithm, key and initialization vector (iv).

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list-cipher-algorithms will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings or buffers.

crypto.createCredentials(details)#

Added in: v0.1.92 Deprecated since: v0.11.13 
Stability: 0 - Deprecated: Use tls.createSecureContext() instead.

The crypto.createCredentials() method is a deprecated function for creating and returning a tls.SecureContext. It should not be used. Replace it with tls.createSecureContext() which has the exact same arguments and return value.

Returns a tls.SecureContext, as-if tls.createSecureContext() had been called.

crypto.createDecipher(algorithm, password)#

Added in: v0.1.94

Creates and returns a Decipher object that uses the given algorithm and password (key).

The implementation of crypto.createDecipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use pbkdf2 instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.pbkdf2() and to use crypto.createDecipheriv() to create the Decipher object.

crypto.createDecipheriv(algorithm, key, iv)#

Added in: v0.1.94

Creates and returns a Decipher object that uses the given algorithm, key and initialization vector (iv).

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list-cipher-algorithms will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings or buffers.

crypto.createDiffieHellman(prime[, prime_encoding][, generator][, generator_encoding])#

Added in: v0.11.12

Creates a DiffieHellman key exchange object using the supplied prime and an optional specific generator.

The generator argument can be a number, string, or Buffer. If generator is not specified, the value 2 is used.

The prime_encoding and generator_encoding arguments can be 'latin1', 'hex', or 'base64'.

If prime_encoding is specified, prime is expected to be a string; otherwise a Buffer is expected.

If generator_encoding is specified, generator is expected to be a string; otherwise either a number or Buffer is expected.

crypto.createDiffieHellman(prime_length[, generator])#

Added in: v0.5.0

Creates a DiffieHellman key exchange object and generates a prime of prime_length bits using an optional specific numeric generator. If generator is not specified, the value 2 is used.

crypto.createECDH(curve_name)#

Added in: v0.11.14

Creates an Elliptic Curve Diffie-Hellman (ECDH) key exchange object using a predefined curve specified by the curve_name string. Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.

crypto.createHash(algorithm)#

Added in: v0.1.92

Creates and returns a Hash object that can be used to generate hash digests using the given algorithm.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list-message-digest-algorithms will display the available digest algorithms.

Example: generating the sha256 sum of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hash = crypto.createHash('sha256');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});

crypto.createHmac(algorithm, key)#

Added in: v0.1.94

Creates and returns an Hmac object that uses the given algorithm and key.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list-message-digest-algorithms will display the available digest algorithms.

The key is the HMAC key used to generate the cryptographic HMAC hash.

Example: generating the sha256 HMAC of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hmac.update(data);
  else {
    console.log(`${hmac.digest('hex')} ${filename}`);
  }
});

crypto.createSign(algorithm)#

Added in: v0.1.92

Creates and returns a Sign object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms.

crypto.createVerify(algorithm)#

Added in: v0.1.92

Creates and returns a Verify object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms.

crypto.getCiphers()#

Added in: v0.9.3

Returns an array with the names of the supported cipher algorithms.

Example:

const ciphers = crypto.getCiphers();
console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...]

crypto.getCurves()#

Added in: v2.3.0

Returns an array with the names of the supported elliptic curves.

Example:

const curves = crypto.getCurves();
console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]

crypto.getDiffieHellman(group_name)#

Added in: v0.7.5

Creates a predefined DiffieHellman key exchange object. The supported groups are: 'modp1', 'modp2', 'modp5' (defined in RFC 2412, but see Caveats) and 'modp14', 'modp15', 'modp16', 'modp17', 'modp18' (defined in RFC 3526). The returned object mimics the interface of objects created by crypto.createDiffieHellman(), but will not allow changing the keys (with diffieHellman.setPublicKey() for example). The advantage of using this method is that the parties do not have to generate nor exchange a group modulus beforehand, saving both processor and communication time.

Example (obtaining a shared secret):

const crypto = require('crypto');
const alice = crypto.getDiffieHellman('modp14');
const bob = crypto.getDiffieHellman('modp14');

alice.generateKeys();
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);

crypto.getHashes()#

Added in: v0.9.3

Returns an array of the names of the supported hash algorithms, such as RSA-SHA256.

Example:

const hashes = crypto.getHashes();
console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]

crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)#

Added in: v0.5.5

Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

The supplied callback function is called with two arguments: err and derivedKey. If an error occurs, err will be set; otherwise err will be null. The successfully generated derivedKey will be passed as a Buffer.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should also be as unique as possible. It is recommended that the salts are random and their lengths are greater than 16 bytes. See NIST SP 800-132 for details.

Example:

const crypto = require('crypto');
crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, key) => {
  if (err) throw err;
  console.log(key.toString('hex'));  // '3745e48...aa39b34'
});

An array of supported digest functions can be retrieved using crypto.getHashes().

crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)#

Added in: v0.9.3

Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

If an error occurs an Error will be thrown, otherwise the derived key will be returned as a Buffer.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should also be as unique as possible. It is recommended that the salts are random and their lengths are greater than 16 bytes. See NIST SP 800-132 for details.

Example:

const crypto = require('crypto');
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512');
console.log(key.toString('hex'));  // '3745e48...aa39b34'

An array of supported digest functions can be retrieved using crypto.getHashes().

crypto.privateDecrypt(private_key, buffer)#

Added in: v0.11.14

Decrypts buffer with private_key.

private_key can be an object or a string. If private_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING. If private_key is an object, it is interpreted as a hash object with the keys:

  • key : <String> - PEM encoded private key
  • passphrase : <String> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING
    • crypto.constants.RSA_PKCS1_OAEP_PADDING

All paddings are defined in crypto.constants.

crypto.timingSafeEqual(a, b)#

Added in: v6.6.0

Returns true if a is equal to b, without leaking timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls.

a and b must both be Buffers, and they must have the same length.

Note: Use of crypto.timingSafeEqual does not guarantee that the surrounding code is timing-safe. Care should be taken to ensure that the surrounding code does not introduce timing vulnerabilities.

crypto.privateEncrypt(private_key, buffer)#

Added in: v1.1.0

Encrypts buffer with private_key.

private_key can be an object or a string. If private_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING. If private_key is an object, it is interpreted as a hash object with the keys:

  • key : <String> - PEM encoded private key
  • passphrase : <String> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING

All paddings are defined in crypto.constants.

crypto.publicDecrypt(public_key, buffer)#

Added in: v1.1.0

Decrypts buffer with public_key.

public_key can be an object or a string. If public_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING. If public_key is an object, it is interpreted as a hash object with the keys:

  • key : <String> - PEM encoded public key
  • passphrase : <String> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING
    • crypto.constants.RSA_PKCS1_OAEP_PADDING

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

All paddings are defined in crypto.constants.

crypto.publicEncrypt(public_key, buffer)#

Added in: v0.11.14

Encrypts buffer with public_key.

public_key can be an object or a string. If public_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING. If public_key is an object, it is interpreted as a hash object with the keys:

  • key : <String> - PEM encoded public key
  • passphrase : <String> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING
    • crypto.constants.RSA_PKCS1_OAEP_PADDING

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

All paddings are defined in crypto.constants.

crypto.randomBytes(size[, callback])#

Added in: v0.5.8

Generates cryptographically strong pseudo-random data. The size argument is a number indicating the number of bytes to generate.

If a callback function is provided, the bytes are generated asynchronously and the callback function is invoked with two arguments: err and buf. If an error occurs, err will be an Error object; otherwise it is null. The buf argument is a Buffer containing the generated bytes.

// Asynchronous
const crypto = require('crypto');
crypto.randomBytes(256, (err, buf) => {
  if (err) throw err;
  console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});

If the callback function is not provided, the random bytes are generated synchronously and returned as a Buffer. An error will be thrown if there is a problem generating the bytes.

// Synchronous
const buf = crypto.randomBytes(256);
console.log(
  `${buf.length} bytes of random data: ${buf.toString('hex')}`);

The crypto.randomBytes() method will block until there is sufficient entropy. This should normally never take longer than a few milliseconds. The only time when generating the random bytes may conceivably block for a longer period of time is right after boot, when the whole system is still low on entropy.

crypto.setEngine(engine[, flags])#

Added in: v0.11.11

Load and set the engine for some or all OpenSSL functions (selected by flags).

engine could be either an id or a path to the engine's shared library.

The optional flags argument uses ENGINE_METHOD_ALL by default. The flags is a bit field taking one of or a mix of the following flags (defined in crypto.constants):

  • crypto.constants.ENGINE_METHOD_RSA
  • crypto.constants.ENGINE_METHOD_DSA
  • crypto.constants.ENGINE_METHOD_DH
  • crypto.constants.ENGINE_METHOD_RAND
  • crypto.constants.ENGINE_METHOD_ECDH
  • crypto.constants.ENGINE_METHOD_ECDSA
  • crypto.constants.ENGINE_METHOD_CIPHERS
  • crypto.constants.ENGINE_METHOD_DIGESTS
  • crypto.constants.ENGINE_METHOD_STORE
  • crypto.constants.ENGINE_METHOD_PKEY_METHS
  • crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
  • crypto.constants.ENGINE_METHOD_ALL
  • crypto.constants.ENGINE_METHOD_NONE

Notes#

Legacy Streams API (pre Node.js v0.10)#

The Crypto module was added to Node.js before there was the concept of a unified Stream API, and before there were Buffer objects for handling binary data. As such, the many of the crypto defined classes have methods not typically found on other Node.js classes that implement the streams API (e.g. update(), final(), or digest()). Also, many methods accepted and returned 'latin1' encoded strings by default rather than Buffers. This default was changed after Node.js v0.8 to use Buffer objects by default instead.

Recent ECDH Changes#

Usage of ECDH with non-dynamically generated key pairs has been simplified. Now, ecdh.setPrivateKey() can be called with a preselected private key and the associated public point (key) will be computed and stored in the object. This allows code to only store and provide the private part of the EC key pair. ecdh.setPrivateKey() now also validates that the private key is valid for the selected curve.

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful. Either a previously stored private key should be set, which automatically generates the associated public key, or ecdh.generateKeys() should be called. The main drawback of using ecdh.setPublicKey() is that it can be used to put the ECDH key pair into an inconsistent state.

Support for weak or compromised algorithms#

The crypto module still supports some algorithms which are already compromised and are not currently recommended for use. The API also allows the use of ciphers and hashes with a small key size that are considered to be too weak for safe use.

Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.

Based on the recommendations of NIST SP 800-131A:

  • MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
  • The key used with RSA, DSA and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
  • The DH groups of modp1, modp2 and modp5 have a key size smaller than 2048 bits and are not recommended.

See the reference for other recommendations and details.

Crypto Constants#

The following constants exported by crypto.constants apply to various uses of the crypto, tls, and https modules and are generally specific to OpenSSL.

OpenSSL Options#

Constant Description
SSL_OP_ALL Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html for detail.
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CIPHER_SERVER_PREFERENCE Attempts to use the server's preferences instead of the client's when selecting a cipher. Behaviour depends on protocol version. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CISCO_ANYCONNECT Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER.
SSL_OP_COOKIE_EXCHANGE Instructs OpenSSL to turn on cookie exchange.
SSL_OP_CRYPTOPRO_TLSEXT_BUG Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft.
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d.
SSL_OP_EPHEMERAL_RSA Instructs OpenSSL to always use the tmp_rsa key when performing RSA operations.
SSL_OP_LEGACY_SERVER_CONNECT Allows initial connection to servers that do not support RI.
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
SSL_OP_MICROSOFT_SESS_ID_BUG
SSL_OP_MSIE_SSLV2_RSA_PADDING Instructs OpenSSL to disable the workaround for a man-in-the-middle protocol-version vulnerability in the SSL 2.0 server implementation.
SSL_OP_NETSCAPE_CA_DN_BUG
SSL_OP_NETSCAPE_CHALLENGE_BUG
SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
SSL_OP_NO_COMPRESSION Instructs OpenSSL to disable support for SSL/TLS compression.
SSL_OP_NO_QUERY_MTU
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION Instructs OpenSSL to always start a new session when performing renegotiation.
SSL_OP_NO_SSLv2 Instructs OpenSSL to turn off SSL v2
SSL_OP_NO_SSLv3 Instructs OpenSSL to turn off SSL v3
SSL_OP_NO_TICKET Instructs OpenSSL to disable use of RFC4507bis tickets.
SSL_OP_NO_TLSv1 Instructs OpenSSL to turn off TLS v1
SSL_OP_NO_TLSv1_1 Instructs OpenSSL to turn off TLS v1.1
SSL_OP_NO_TLSv1_2 Instructs OpenSSL to turn off TLS v1.2
SSL_OP_PKCS1_CHECK_1
SSL_OP_PKCS1_CHECK_2
SSL_OP_SINGLE_DH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral DH parameters.
SSL_OP_SINGLE_ECDH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral ECDH parameters.
SSL_OP_SSLEAY_080_CLIENT_DH_BUG
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
SSL_OP_TLS_BLOCK_PADDING_BUG
SSL_OP_TLS_D5_BUG
SSL_OP_TLS_ROLLBACK_BUG Instructs OpenSSL to disable version rollback attack detection.

OpenSSL Engine Constants#

Constant Description
ENGINE_METHOD_RSA Limit engine usage to RSA
ENGINE_METHOD_DSA Limit engine usage to DSA
ENGINE_METHOD_DH Limit engine usage to DH
ENGINE_METHOD_RAND Limit engine usage to RAND
ENGINE_METHOD_ECDH Limit engine usage to ECDH
ENGINE_METHOD_ECDSA Limit engine usage to ECDSA
ENGINE_METHOD_CIPHERS Limit engine usage to CIPHERS
ENGINE_METHOD_DIGESTS Limit engine usage to DIGESTS
ENGINE_METHOD_STORE Limit engine usage to STORE
ENGINE_METHOD_PKEY_METHS Limit engine usage to PKEY_METHDS
ENGINE_METHOD_PKEY_ASN1_METHS Limit engine usage to PKEY_ASN1_METHS
ENGINE_METHOD_ALL
ENGINE_METHOD_NONE

Other OpenSSL Constants#

Constant Description
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
NPN_ENABLED
ALPN_ENABLED
RSA_PKCS1_PADDING
RSA_SSLV23_PADDING
RSA_NO_PADDING
RSA_PKCS1_OAEP_PADDING
RSA_X931_PADDING
RSA_PKCS1_PSS_PADDING
POINT_CONVERSION_COMPRESSED
POINT_CONVERSION_UNCOMPRESSED
POINT_CONVERSION_HYBRID

Node.js Crypto Constants#

Constant Description
defaultCoreCipherList Specifies the built-in default cipher list used by Node.js.
defaultCipherList Specifies the active default cipher list used by the current Node.js process.

Debugger#

Stability: 2 - Stable

Node.js includes an out-of-process debugging utility accessible via a TCP-based protocol and built-in debugging client. To use it, start Node.js with the debug argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

$ node debug myscript.js
< debugger listening on port 5858
connecting... ok
break in /home/indutny/Code/git/indutny/myscript.js:1
  1 x = 5;
  2 setTimeout(() => {
  3   debugger;
debug>

Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible.

Inserting the statement debugger; into the source code of a script will enable a breakpoint at that position in the code:

// myscript.js
x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');

Once the debugger is run, a breakpoint will occur at line 4:

$ node debug myscript.js
< debugger listening on port 5858
connecting... ok
break in /home/indutny/Code/git/indutny/myscript.js:1
  1 x = 5;
  2 setTimeout(() => {
  3   debugger;
debug> cont
< hello
break in /home/indutny/Code/git/indutny/myscript.js:3
  1 x = 5;
  2 setTimeout(() => {
  3   debugger;
  4   console.log('world');
  5 }, 1000);
debug> next
break in /home/indutny/Code/git/indutny/myscript.js:4
  2 setTimeout(() => {
  3   debugger;
  4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
debug> repl
Press Ctrl + C to leave debug repl
> x
5
> 2+2
4
debug> next
< world
break in /home/indutny/Code/git/indutny/myscript.js:5
  3   debugger;
  4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
  7
debug> quit

The repl command allows code to be evaluated remotely. The next command steps to the next line. Type help to see what other commands are available.

Pressing enter without typing a command will repeat the previous debugger command.

Watchers#

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type watch('my_expression'). The command watchers will print the active watchers. To remove a watcher, type unwatch('my_expression').

Command reference#

Stepping#

  • cont, c - Continue execution
  • next, n - Step next
  • step, s - Step in
  • out, o - Step out
  • pause - Pause running code (like pause button in Developer Tools)

Breakpoints#

  • setBreakpoint(), sb() - Set breakpoint on current line
  • setBreakpoint(line), sb(line) - Set breakpoint on specific line
  • setBreakpoint('fn()'), sb(...) - Set breakpoint on a first statement in functions body
  • setBreakpoint('script.js', 1), sb(...) - Set breakpoint on first line of script.js
  • clearBreakpoint('script.js', 1), cb(...) - Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that is not loaded yet:

$ node debug test/fixtures/break-in-module/main.js
< debugger listening on port 5858
connecting to port 5858... ok
break in test/fixtures/break-in-module/main.js:1
  1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> setBreakpoint('mod.js', 23)
Warning: script 'mod.js' was not loaded yet.
  1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> c
break in test/fixtures/break-in-module/mod.js:23
 21
 22 exports.hello = () => {
 23   return 'hello from module';
 24 };
 25
debug>

Information#

  • backtrace, bt - Print backtrace of current execution frame
  • list(5) - List scripts source code with 5 line context (5 lines before and after)
  • watch(expr) - Add expression to watch list
  • unwatch(expr) - Remove expression from watch list
  • watchers - List all watchers and their values (automatically listed on each breakpoint)
  • repl - Open debugger's repl for evaluation in debugging script's context
  • exec expr - Execute an expression in debugging script's context

Execution control#

  • run - Run script (automatically runs on debugger's start)
  • restart - Restart script
  • kill - Kill script

Various#

  • scripts - List all loaded scripts
  • version - Display V8's version

Advanced Usage#

TCP-based protocol#

Stability: 0 - Deprecated: Use V8 Inspector Integration instead. The debug protocol used by the --debug flag was removed from V8.

An alternative way of enabling and accessing the debugger is to start Node.js with the --debug command-line flag or by signaling an existing Node.js process with SIGUSR1.

Once a process has been set in debug mode this way, it can be inspected using the Node.js debugger by either connecting to the pid of the running process or via URI reference to the listening debugger:

  • node debug -p <pid> - Connects to the process via the pid
  • node debug <URI> - Connects to the process via the URI such as localhost:5858

V8 Inspector Integration for Node.js#

NOTE: This is an experimental feature.

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome Debugging Protocol.

V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222.

To break on the first line of the application code, provide the --debug-brk flag in addition to --inspect.

$ node --inspect index.js
Debugger listening on port 9229.
Warning: This is an experimental feature and could change at any time.
To start debugging, open the following URL in Chrome:
    chrome-devtools://devtools/remote/serve_file/@60cd6e859b9f557d2312f5bf532f6aec5f284980/inspector.html?experiments=true&v8only=true&ws=localhost:9229/node

UDP / Datagram Sockets#

Stability: 2 - Stable

The dgram module provides an implementation of UDP Datagram sockets.

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  var address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// server listening 0.0.0.0:41234

Class: dgram.Socket#

Added in: v0.1.99

The dgram.Socket object is an EventEmitter that encapsulates the datagram functionality.

New instances of dgram.Socket are created using dgram.createSocket(). The new keyword is not to be used to create dgram.Socket instances.

Event: 'close'#

Added in: v0.1.99

The 'close' event is emitted after a socket is closed with close(). Once triggered, no new 'message' events will be emitted on this socket.

Event: 'error'#

Added in: v0.1.99

The 'error' event is emitted whenever any error occurs. The event handler function is passed a single Error object.

Event: 'listening'#

Added in: v0.1.99

The 'listening' event is emitted whenever a socket begins listening for datagram messages. This occurs as soon as UDP sockets are created.

Event: 'message'#

Added in: v0.1.99

The 'message' event is emitted when a new datagram is available on a socket. The event handler function is passed two arguments: msg and rinfo.

socket.addMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Tells the kernel to join a multicast group at the given multicastAddress and multicastInterface using the IP_ADD_MEMBERSHIP socket option. If the multicastInterface argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call addMembership multiple times, once per interface.

socket.address()#

Added in: v0.1.99

Returns an object containing the address information for a socket. For UDP sockets, this object will contain address, family and port properties.

socket.bind([port][, address][, callback])#

Added in: v0.1.99
  • port <Number> - Integer, Optional
  • address <String>, Optional
  • callback <Function> with no parameters, Optional. Called when binding is complete.

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Note that specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

Example of a UDP server listening on port 41234:

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  var address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// server listening 0.0.0.0:41234

socket.bind(options[, callback])#

Added in: v0.11.14

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address that are passed as properties of an options object passed as the first argument. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Note that specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

The options object may contain an additional exclusive property that is use when using dgram.Socket objects with the cluster module. When exclusive is set to false (the default), cluster workers will use the same underlying socket handle allowing connection handling duties to be shared. When exclusive is true, however, the handle is not shared and attempted port sharing results in an error.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

An example socket listening on an exclusive port is shown below.

socket.bind({
  address: 'localhost',
  port: 8000,
  exclusive: true
});

socket.close([callback])#

Added in: v0.1.99

Close the underlying socket and stop listening for data on it. If a callback is provided, it is added as a listener for the 'close' event.

socket.dropMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Instructs the kernel to leave a multicast group at multicastAddress using the IP_DROP_MEMBERSHIP socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If multicastInterface is not specified, the operating system will attempt to drop membership on all valid interfaces.

socket.send(msg, [offset, length,] port, address[, callback])#

Added in: v0.1.99
  • msg <Buffer> | <String> | <Array> Message to be sent
  • offset <Number> Integer. Optional. Offset in the buffer where the message starts.
  • length <Number> Integer. Optional. Number of bytes in the message.
  • port <Number> Integer. Destination port.
  • address <String> Destination hostname or IP address.
  • callback <Function> Called when the message has been sent. Optional.

Broadcasts a datagram on the socket. The destination port and address must be specified.

The msg argument contains the message to be sent. Depending on its type, different behavior can apply. If msg is a Buffer, the offset and length specify the offset within the Buffer where the message begins and the number of bytes in the message, respectively. If msg is a String, then it is automatically converted to a Buffer with 'utf8' encoding. With messages that contain multi-byte characters, offset and length will be calculated with respect to byte length and not the character position. If msg is an array, offset and length must not be specified.

The address argument is a string. If the value of address is a host name, DNS will be used to resolve the address of the host. If the address is not specified or is an empty string, '127.0.0.1' or '::1' will be used instead.

If the socket has not been previously bound with a call to bind, the socket is assigned a random port number and is bound to the "all interfaces" address ('0.0.0.0' for udp4 sockets, '::0' for udp6 sockets.)

An optional callback function may be specified to as a way of reporting DNS errors or for determining when it is safe to reuse the buf object. Note that DNS lookups delay the time to send for at least one tick of the Node.js event loop.

The only way to know for sure that the datagram has been sent is by using a callback. If an error occurs and a callback is given, the error will be passed as the first argument to the callback. If a callback is not given, the error is emitted as an 'error' event on the socket object.

Offset and length are optional, but if you specify one you would need to specify the other. Also, they are supported only when the first argument is a Buffer.

Example of sending a UDP packet to a random port on localhost;

const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
  client.close();
});

Example of sending a UDP packet composed of multiple buffers to a random port on localhost;

const dgram = require('dgram');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, 'localhost', (err) => {
  client.close();
});

Sending multiple buffers might be faster or slower depending on your application and operating system: benchmark it. Usually it is faster.

A Note about UDP datagram size

The maximum size of an IPv4/v6 datagram depends on the MTU (Maximum Transmission Unit) and on the Payload Length field size.

  • The Payload Length field is 16 bits wide, which means that a normal payload exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks.

  • The MTU is the largest size a given link layer technology can support for datagram messages. For any link, IPv4 mandates a minimum MTU of 68 octets, while the recommended MTU for IPv4 is 576 (typically recommended as the MTU for dial-up type applications), whether they arrive whole or in fragments.

    For IPv6, the minimum MTU is 1280 octets, however, the mandatory minimum fragment reassembly buffer size is 1500 octets. The value of 68 octets is very small, since most current link layer technologies, like Ethernet, have a minimum MTU of 1500.

It is impossible to know in advance the MTU of each link through which a packet might travel. Sending a datagram greater than the receiver MTU will not work because the packet will get silently dropped without informing the source that the data did not reach its intended recipient.

socket.setBroadcast(flag)#

Added in: v0.6.9

Sets or clears the SO_BROADCAST socket option. When set to true, UDP packets may be sent to a local interface's broadcast address.

socket.setMulticastLoopback(flag)#

Added in: v0.3.8

Sets or clears the IP_MULTICAST_LOOP socket option. When set to true, multicast packets will also be received on the local interface.

socket.setMulticastTTL(ttl)#

Added in: v0.3.8

Sets the IP_MULTICAST_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through, specifically for multicast traffic. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.

The argument passed to to socket.setMulticastTTL() is a number of hops between 0 and 255. The default on most systems is 1 but can vary.

socket.setTTL(ttl)#

Added in: v0.1.101

Sets the IP_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded. Changing TTL values is typically done for network probes or when multicasting.

The argument to socket.setTTL() is a number of hops between 1 and 255. The default on most systems is 64 but can vary.

socket.ref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active. The socket.ref() method adds the socket back to the reference counting and restores the default behavior.

Calling socket.ref() multiples times will have no additional effect.

The socket.ref() method returns a reference to the socket so calls can be chained.

socket.unref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active, allowing the process to exit even if the socket is still listening.

Calling socket.unref() multiple times will have no addition effect.

The socket.unref() method returns a reference to the socket so calls can be chained.

Change to asynchronous socket.bind() behavior#

As of Node.js v0.10, dgram.Socket#bind() changed to an asynchronous execution model. Legacy code that assumes synchronous behavior, as in the following example:

const s = dgram.createSocket('udp4');
s.bind(1234);
s.addMembership('224.0.0.114');

Must be changed to pass a callback function to the dgram.Socket#bind() function:

const s = dgram.createSocket('udp4');
s.bind(1234, () => {
  s.addMembership('224.0.0.114');
});

dgram module functions#

dgram.createSocket(options[, callback])#

Added in: v0.11.13

Creates a dgram.Socket object. The options argument is an object that should contain a type field of either udp4 or udp6 and an optional boolean reuseAddr field.

When reuseAddr is true socket.bind() will reuse the address, even if another process has already bound a socket on it. reuseAddr defaults to false. An optional callback function can be passed specified which is added as a listener for 'message' events.

Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

dgram.createSocket(type[, callback])#

Added in: v0.1.99

Creates a dgram.Socket object of the specified type. The type argument can be either udp4 or udp6. An optional callback function can be passed which is added as a listener for 'message' events.

Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

DNS#

Stability: 2 - Stable

The dns module contains functions belonging to two different categories:

1) Functions that use the underlying operating system facilities to perform name resolution, and that do not necessarily perform any network communication. This category contains only one function: dns.lookup(). Developers looking to perform name resolution in the same way that other applications on the same operating system behave should use dns.lookup().

For example, looking up iana.org.

const dns = require('dns');

dns.lookup('iana.org', (err, addresses, family) => {
  console.log('addresses:', addresses);
});

2) Functions that connect to an actual DNS server to perform name resolution, and that always use the network to perform DNS queries. This category contains all functions in the dns module except dns.lookup(). These functions do not use the same set of configuration files used by dns.lookup() (e.g. /etc/hosts). These functions should be used by developers who do not want to use the underlying operating system's facilities for name resolution, and instead want to always perform DNS queries.

Below is an example that resolves 'archive.org' then reverse resolves the IP addresses that are returned.

const dns = require('dns');

dns.resolve4('archive.org', (err, addresses) => {
  if (err) throw err;

  console.log(`addresses: ${JSON.stringify(addresses)}`);

  addresses.forEach((a) => {
    dns.reverse(a, (err, hostnames) => {
      if (err) {
        throw err;
      }
      console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
    });
  });
});

There are subtle consequences in choosing one over the other, please consult the Implementation considerations section for more information.

dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings that are being used for name resolution.

dns.lookup(hostname[, options], callback)#

Added in: v0.1.90

Resolves a hostname (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. options can be an object or integer. If options is not provided, then IPv4 and IPv6 addresses are both valid. If options is an integer, then it must be 4 or 6.

Alternatively, options can be an object containing these properties:

  • family <Number> - The record family. If present, must be the integer 4 or 6. If not provided, both IP v4 and v6 addresses are accepted.
  • hints: <Number> - If present, it should be one or more of the supported getaddrinfo flags. If hints is not provided, then no flags are passed to getaddrinfo. Multiple flags can be passed through hints by bitwise ORing their values. See supported getaddrinfo flags for more information on supported flags.
  • all: <Boolean> - When true, the callback returns all resolved addresses in an array, otherwise returns a single address. Defaults to false.

All properties are optional. An example usage of options is shown below.

{
  family: 4,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
  all: false
}

The callback function has arguments (err, address, family). address is a string representation of an IPv4 or IPv6 address. family is either the integer 4 or 6 and denotes the family of address (not necessarily the value initially passed to lookup).

With the all option set to true, the arguments change to (err, addresses), with addresses being an array of objects with the properties address and family.

On error, err is an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOENT' not only when the hostname does not exist but also when the lookup fails in other ways such as no available file descriptors.

dns.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dns.lookup().

Supported getaddrinfo flags#

The following flags can be passed as hints to dns.lookup().

  • dns.ADDRCONFIG: Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered.
  • dns.V4MAPPED: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. Note that it is not supported on some operating systems (e.g FreeBSD 10.1).

dns.lookupService(address, port, callback)#

Added in: v0.11.14

Resolves the given address and port into a hostname and service using the operating system's underlying getnameinfo implementation.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

The callback has arguments (err, hostname, service). The hostname and service arguments are strings (e.g. 'localhost' and 'http' respectively).

On error, err is an Error object, where err.code is the error code.

const dns = require('dns');
dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
  console.log(hostname, service);
  // Prints: localhost ssh
});

dns.resolve(hostname[, rrtype], callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org') into an array of the record types specified by rrtype.

Valid values for rrtype are:

  • 'A' - IPV4 addresses, default
  • 'AAAA' - IPV6 addresses
  • 'MX' - mail exchange records
  • 'TXT' - text records
  • 'SRV' - SRV records
  • 'PTR' - PTR records
  • 'NS' - name server records
  • 'CNAME' - canonical name records
  • 'SOA' - start of authority record
  • 'NAPTR' - name authority pointer record

The callback function has arguments (err, addresses). When successful, addresses will be an array, except when resolving an SOA record which returns an object structured in the same manner as one returned by the dns.resolveSoa() method. The type of each item in addresses is determined by the record type, and described in the documentation for the corresponding lookup methods.

On error, err is an Error object, where err.code is one of the error codes listed here.

dns.resolve4(hostname[, options], callback)#

Added in: v0.1.16

Uses the DNS protocol to resolve a IPv4 addresses (A records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

  • hostname <String> Hostname to resolve.
  • options <Object>
    • ttl <Boolean> Retrieve the Time-To-Live value (TTL) of each record. The callback receives an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings. The TTL is expressed in seconds.
  • callback <Function> An (err, result) callback function.

dns.resolve6(hostname[, options], callback)#

Added in: v0.1.16

Uses the DNS protocol to resolve a IPv6 addresses (AAAA records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv6 addresses.

  • hostname <String> Hostname to resolve.
  • options <Object>
    • ttl <Boolean> Retrieve the Time-To-Live value (TTL) of each record. The callback receives an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings. The TTL is expressed in seconds.
  • callback <Function> An (err, result) callback function.

dns.resolveCname(hostname, callback)#

Added in: v0.3.2

Uses the DNS protocol to resolve CNAME records for the hostname. The addresses argument passed to the callback function will contain an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dns.resolveMx(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. The addresses argument passed to the callback function will contain an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dns.resolveNaptr(hostname, callback)#

Added in: v0.9.12

Uses the DNS protocol to resolve regular expression based records (NAPTR records) for the hostname. The callback function has arguments (err, addresses). The addresses argument passed to the callback function will contain an array of objects with the following properties:

  • flags
  • service
  • regexp
  • replacement
  • order
  • preference

For example:

{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dns.resolveNs(hostname, callback)#

Added in: v0.1.90

Uses the DNS protocol to resolve name server records (NS records) for the hostname. The addresses argument passed to the callback function will contain an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dns.resolveSoa(hostname, callback)#

Added in: v0.11.10

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. The addresses argument passed to the callback function will be an object with the following properties:

  • nsname
  • hostmaster
  • serial
  • refresh
  • retry
  • expire
  • minttl
{
  nsname: 'ns.example.com',
  hostmaster: 'root.example.com',
  serial: 2013101809,
  refresh: 10000,
  retry: 2400,
  expire: 604800,
  minttl: 3600
}

dns.resolveSrv(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve service records (SRV records) for the hostname. The addresses argument passed to the callback function will be an array of objects with the following properties:

  • priority
  • weight
  • port
  • name
{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dns.resolvePtr(hostname, callback)#

Added in: v6.0.0

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. The addresses argument passed to the callback function will be an array of strings containing the reply records.

dns.resolveTxt(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. The addresses argument passed to the callback function is is a two-dimensional array of the text records available for hostname (e.g., [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

dns.reverse(ip, callback)#

Added in: v0.1.16

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames.

The callback function has arguments (err, hostnames), where hostnames is an array of resolved hostnames for the given ip.

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.setServers(servers)#

Added in: v0.11.3

Sets the IP addresses of the servers to be used when resolving. The servers argument is an array of IPv4 or IPv6 addresses.

If a port specified on the address it will be removed.

An error will be thrown if an invalid address is provided.

The dns.setServers() method must not be called while a DNS query is in progress.

Error codes#

Each DNS query can return one of the following error codes:

  • dns.NODATA: DNS server returned answer with no data.
  • dns.FORMERR: DNS server claims query was misformatted.
  • dns.SERVFAIL: DNS server returned general failure.
  • dns.NOTFOUND: Domain name not found.
  • dns.NOTIMP: DNS server does not implement requested operation.
  • dns.REFUSED: DNS server refused query.
  • dns.BADQUERY: Misformatted DNS query.
  • dns.BADNAME: Misformatted hostname.
  • dns.BADFAMILY: Unsupported address family.
  • dns.BADRESP: Misformatted DNS reply.
  • dns.CONNREFUSED: Could not contact DNS servers.
  • dns.TIMEOUT: Timeout while contacting DNS servers.
  • dns.EOF: End of file.
  • dns.FILE: Error reading file.
  • dns.NOMEM: Out of memory.
  • dns.DESTRUCTION: Channel is being destroyed.
  • dns.BADSTR: Misformatted string.
  • dns.BADFLAGS: Illegal flags specified.
  • dns.NONAME: Given hostname is not numeric.
  • dns.BADHINTS: Illegal hints flags specified.
  • dns.NOTINITIALIZED: c-ares library initialization not yet performed.
  • dns.LOADIPHLPAPI: Error loading iphlpapi.dll.
  • dns.ADDRGETNETWORKPARAMS: Could not find GetNetworkParams function.
  • dns.CANCELLED: DNS query cancelled.

Implementation considerations#

Although dns.lookup() and the various dns.resolve*()/dns.reverse() functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs.

dns.lookup()#

Under the hood, dns.lookup() uses the same operating system facilities as most other programs. For instance, dns.lookup() will almost always resolve a given name the same way as the ping command. On most POSIX-like operating systems, the behavior of the dns.lookup() function can be modified by changing settings in nsswitch.conf(5) and/or resolv.conf(5), but note that changing these files will change the behavior of all other programs running on the same operating system.

Though the call to dns.lookup() will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs on libuv's threadpool. Because libuv's threadpool has a fixed size, it means that if for whatever reason the call to getaddrinfo(3) takes a long time, other operations that could run on libuv's threadpool (such as filesystem operations) will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the 'UV_THREADPOOL_SIZE' environment variable to a value greater than 4 (its current default value). For more information on libuv's threadpool, see the official libuv documentation.

dns.resolve(), dns.resolve*() and dns.reverse()#

These functions are implemented quite differently than dns.lookup(). They do not use getaddrinfo(3) and they always perform a DNS query on the network. This network communication is always done asynchronously, and does not use libuv's threadpool.

As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that dns.lookup() can have.

They do not use the same set of configuration files than what dns.lookup() uses. For instance, they do not use the configuration from /etc/hosts.

Domain#

Stability: 0 - Deprecated

This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most end users should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.

Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an 'error' event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the process.on('uncaughtException') handler, or causing the program to exit immediately with an error code.

Warning: Don't Ignore Errors!#

Domain error handlers are not a substitute for closing down your process when an error occurs.

By the very nature of how throw works in JavaScript, there is almost never any way to safely "pick up where you left off", without leaking references, or creating some other sort of undefined brittle state.

The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, you might have many connections open, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.

The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.

In this way, domain usage goes hand-in-hand with the cluster module, since the master process can fork a new worker when a worker encounters an error. For Node.js programs that scale to multiple machines, the terminating proxy or service registry can take note of the failure, and react accordingly.

For example, this is not a good idea:

// XXX WARNING!  BAD IDEA!

var d = require('domain').create();
d.on('error', (er) => {
  // The error won't crash the process, but what it does is worse!
  // Though we've prevented abrupt process restarting, we are leaking
  // resources like crazy if this ever happens.
  // This is no better than process.on('uncaughtException')!
  console.log('error, but oh well', er.message);
});
d.run(() => {
  require('http').createServer((req, res) => {
    handleRequest(req, res);
  }).listen(PORT);
});

By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.

// Much better!

const cluster = require('cluster');
const PORT = +process.env.PORT || 1337;

if (cluster.isMaster) {
  // In real life, you'd probably use more than just 2 workers,
  // and perhaps not put the master and worker in the same file.
  //
  // You can also of course get a bit fancier about logging, and
  // implement whatever custom logic you need to prevent DoS
  // attacks and other bad behavior.
  //
  // See the options in the cluster documentation.
  //
  // The important thing is that the master does very little,
  // increasing our resilience to unexpected errors.

  cluster.fork();
  cluster.fork();

  cluster.on('disconnect', (worker) => {
    console.error('disconnect!');
    cluster.fork();
  });

} else {
  // the worker
  //
  // This is where we put our bugs!

  const domain = require('domain');

  // See the cluster documentation for more details about using
  // worker processes to serve requests.  How it works, caveats, etc.

  const server = require('http').createServer((req, res) => {
    var d = domain.create();
    d.on('error', (er) => {
      console.error('error', er.stack);

      // Note: we're in dangerous territory!
      // By definition, something unexpected occurred,
      // which we probably didn't want.
      // Anything can happen now!  Be very careful!

      try {
        // make sure we close down within 30 seconds
        var killtimer = setTimeout(() => {
          process.exit(1);
        }, 30000);
        // But don't keep the process open just for that!
        killtimer.unref();

        // stop taking new requests.
        server.close();

        // Let the master know we're dead.  This will trigger a
        // 'disconnect' in the cluster master, and then it will fork
        // a new worker.
        cluster.worker.disconnect();

        // try to send an error to the request that triggered the problem
        res.statusCode = 500;
        res.setHeader('content-type', 'text/plain');
        res.end('Oops, there was a problem!\n');
      } catch (er2) {
        // oh well, not much we can do at this point.
        console.error('Error sending 500!', er2.stack);
      }
    });

    // Because req and res were created before this domain existed,
    // we need to explicitly add them.
    // See the explanation of implicit vs explicit binding below.
    d.add(req);
    d.add(res);

    // Now run the handler function in the domain.
    d.run(() => {
      handleRequest(req, res);
    });
  });
  server.listen(PORT);
}

// This part is not important.  Just an example routing thing.
// You'd put your fancy application logic here.
function handleRequest(req, res) {
  switch(req.url) {
    case '/error':
      // We do some async stuff, and then...
      setTimeout(() => {
        // Whoops!
        flerb.bark();
      });
      break;
    default:
      res.end('ok');
  }
}

Additions to Error objects#

Any time an Error object is routed through a domain, a few extra fields are added to it.

  • error.domain The domain that first handled the error.
  • error.domainEmitter The event emitter that emitted an 'error' event with the error object.
  • error.domainBound The callback function which was bound to the domain, and passed an error as its first argument.
  • error.domainThrown A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.

Implicit Binding#

If domains are in use, then all new EventEmitter objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation.

Additionally, callbacks passed to lowlevel event loop requests (such as to fs.open, or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error.

In order to prevent excessive memory usage, Domain objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected.

If you want to nest Domain objects as children of a parent Domain, then you must explicitly add them.

Implicit binding routes thrown errors and 'error' events to the Domain's 'error' event, but does not register the EventEmitter on the Domain, so domain.dispose() will not shut down the EventEmitter. Implicit binding only takes care of thrown errors and 'error' events.

Explicit Binding#

Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.

For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.

That is possible via explicit binding.

For example:

// create a top-level domain for the server
const domain = require('domain');
const http = require('http');
const serverDomain = domain.create();

serverDomain.run(() => {
  // server is created in the scope of serverDomain
  http.createServer((req, res) => {
    // req and res are also created in the scope of serverDomain
    // however, we'd prefer to have a separate domain for each request.
    // create it first thing, and add req and res to it.
    var reqd = domain.create();
    reqd.add(req);
    reqd.add(res);
    reqd.on('error', (er) => {
      console.error('Error', er, req.url);
      try {
        res.writeHead(500);
        res.end('Error occurred, sorry.');
      } catch (er) {
        console.error('Error sending 500', er, req.url);
      }
    });
  }).listen(1337);
});

domain.create()#

  • Returns: <Domain>

Returns a new Domain object.

Class: Domain#

The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object.

Domain is a child class of EventEmitter. To handle the errors that it catches, listen to its 'error' event.

domain.run(fn[, ...args])#

Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function.

This is the most basic way to use a domain.

Example:

const domain = require('domain');
const fs = require('fs');
const d = domain.create();
d.on('error', (er) => {
  console.error('Caught error!', er);
});
d.run(() => {
  process.nextTick(() => {
    setTimeout(() => { // simulating some various async stuff
      fs.open('non-existent file', 'r', (er, fd) => {
        if (er) throw er;
        // proceed...
      });
    }, 100);
  });
});

In this example, the d.on('error') handler will be triggered, rather than crashing the program.

domain.members#

An array of timers and event emitters that have been explicitly added to the domain.

domain.add(emitter)#

Explicitly adds an emitter to the domain. If any event handlers called by the emitter throw an error, or if the emitter emits an 'error' event, it will be routed to the domain's 'error' event, just like with implicit binding.

This also works with timers that are returned from setInterval() and setTimeout(). If their callback function throws, it will be caught by the domain 'error' handler.

If the Timer or EventEmitter was already bound to a domain, it is removed from that one, and bound to this one instead.

domain.remove(emitter)#

The opposite of domain.add(emitter). Removes domain handling from the specified emitter.

domain.bind(callback)#

The returned function will be a wrapper around the supplied callback function. When the returned function is called, any errors that are thrown will be routed to the domain's 'error' event.

Example#

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.bind((er, data) => {
    // if this throws, it will also be passed to the domain
    return cb(er, data ? JSON.parse(data) : null);
  }));
}

d.on('error', (er) => {
  // an error occurred somewhere.
  // if we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.intercept(callback)#

This method is almost identical to domain.bind(callback). However, in addition to catching thrown errors, it will also intercept Error objects sent as the first argument to the function.

In this way, the common if (err) return callback(err); pattern can be replaced with a single error handler in a single place.

Example#

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.intercept((data) => {
    // note, the first argument is never passed to the
    // callback since it is assumed to be the 'Error' argument
    // and thus intercepted by the domain.

    // if this throws, it will also be passed to the domain
    // so the error-handling logic can be moved to the 'error'
    // event on the domain instead of being repeated throughout
    // the program.
    return cb(null, JSON.parse(data));
  }));
}

d.on('error', (er) => {
  // an error occurred somewhere.
  // if we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.enter()#

The enter method is plumbing used by the run, bind, and intercept methods to set the active domain. It sets domain.active and process.domain to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see domain.exit() for details on the domain stack). The call to enter delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain.

Calling enter changes only the active domain, and does not alter the domain itself. enter and exit can be called an arbitrary number of times on a single domain.

If the domain on which enter is called has been disposed, enter will return without setting the domain.

domain.exit()#

The exit method exits the current domain, popping it off the domain stack. Any time execution is going to switch to the context of a different chain of asynchronous calls, it's important to ensure that the current domain is exited. The call to exit delimits either the end of or an interruption to the chain of asynchronous calls and I/O operations bound to a domain.

If there are multiple, nested domains bound to the current execution context, exit will exit any domains nested within this domain.

Calling exit changes only the active domain, and does not alter the domain itself. enter and exit can be called an arbitrary number of times on a single domain.

If the domain on which exit is called has been disposed, exit will return without exiting the domain.

domain.dispose()#

Stability: 0 - Deprecated.  Please recover from failed IO actions
explicitly via error event handlers set on the domain.

Once dispose has been called, the domain will no longer be used by callbacks bound into the domain via run, bind, or intercept, and a 'dispose' event is emitted.

Errors#

Applications running in Node.js will generally experience four categories of errors:

  • Standard JavaScript errors such as:
    • <EvalError> : thrown when a call to eval() fails.
    • <SyntaxError> : thrown in response to improper JavaScript language syntax.
    • <RangeError> : thrown when a value is not within an expected range
    • <ReferenceError> : thrown when using undefined variables
    • <TypeError> : thrown when passing arguments of the wrong type
    • <URIError> : thrown when a global URI handling function is misused.
  • System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist, attempting to send data over a closed socket, etc;
  • And User-specified errors triggered by application code.
  • Assertion Errors are a special class of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module.

All JavaScript and System errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.

Error Propagation and Interception#

Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called.

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try / catch construct provided by the JavaScript language.

// Throws with a ReferenceError because z is undefined
try {
  const m = 1;
  const n = m + z;
} catch (err) {
  // Handle the error here.
}

Any use of the JavaScript throw mechanism will raise an exception that must be handled using try / catch or the Node.js process will exit immediately.

With few exceptions, Synchronous APIs (any blocking method that does not accept a callback function, such as fs.readFileSync), will use throw to report errors.

Errors that occur within Asynchronous APIs may be reported in multiple ways:

  • Most asynchronous methods that accept a callback function will accept an Error object passed as the first argument to that function. If that first argument is not null and is an instance of Error, then an error occurred that should be handled.

    const fs = require('fs');
fs.readFile('a file that does not exist', (err, data) => {
  if (err) {
    console.error('There was an error reading the file!', err);
    return;
  }
  // Otherwise handle the data
});

  • When an asynchronous method is called on an object that is an EventEmitter, errors can be routed to that object's 'error' event.

    const net = require('net');
const connection = net.connect('localhost');

// Adding an 'error' event handler to a stream:
connection.on('error', (err) => {
  // If the connection is reset by the server, or if it can't
  // connect at all, or on any sort of error encountered by
  // the connection, the error will be sent here.
  console.error(err);
});

connection.pipe(process.stdout);

  • A handful of typically asynchronous methods in the Node.js API may still use the throw mechanism to raise exceptions that must be handled using try / catch. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.

The use of the 'error' event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail).

For all EventEmitter objects, if an 'error' event handler is not provided, the error will be thrown, causing the Node.js process to report an unhandled exception and crash unless either: The domain module is used appropriately or a handler has been registered for the process.on('uncaughtException') event.

const EventEmitter = require('events');
const ee = new EventEmitter();

setImmediate(() => {
  // This will crash the process because no 'error' event
  // handler has been added.
  ee.emit('error', new Error('This will crash'));
});

Errors generated in this way cannot be intercepted using try / catch as they are thrown after the calling code has already exited.

Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.

Node.js style callbacks#

Most asynchronous methods exposed by the Node.js core API follow an idiomatic pattern referred to as a "Node.js style callback". With this pattern, a callback function is passed to the method as an argument. When the operation either completes or an error is raised, the callback function is called with the Error object (if any) passed as the first argument. If no error was raised, the first argument will be passed as null.

const fs = require('fs');

function nodeStyleCallback(err, data) {
 if (err) {
   console.error('There was an error', err);
   return;
 }
 console.log(data);
}

fs.readFile('/some/file/that/does-not-exist', nodeStyleCallback);
fs.readFile('/some/file/that/does-exist', nodeStyleCallback)

The JavaScript try / catch mechanism cannot be used to intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside a Node.js style callback:

// THIS WILL NOT WORK:
const fs = require('fs');

try {
  fs.readFile('/some/file/that/does-not-exist', (err, data) => {
    // mistaken assumption: throwing here...
    if (err) {
      throw err;
    }
  });
} catch(err) {
  // This will not catch the throw!
  console.log(err);
}

This will not work because the callback function passed to fs.readFile() is called asynchronously. By the time the callback has been called, the surrounding code (including the try { } catch(err) { } block will have already exited. Throwing an error inside the callback can crash the Node.js process in most cases. If domains are enabled, or a handler has been registered with process.on('uncaughtException'), such errors can be intercepted.

Class: Error#

A generic JavaScript Error object that does not denote any specific circumstance of why the error occurred. Error objects capture a "stack trace" detailing the point in the code at which the Error was instantiated, and may provide a text description of the error.

All errors generated by Node.js, including all System and JavaScript errors, will either be instances of, or inherit from, the Error class.

new Error(message)#

Creates a new Error object and sets the error.message property to the provided text message. If an object is passed as message, the text message is generated by calling message.toString(). The error.stack property will represent the point in the code at which new Error() was called. Stack traces are dependent on V8's stack trace API. Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property Error.stackTraceLimit, whichever is smaller.

Error.captureStackTrace(targetObject[, constructorOpt])#

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called.

const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack  // similar to `new Error().stack`

The first line of the trace, instead of being prefixed with ErrorType: message, will be the result of calling targetObject.toString().

The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

The constructorOpt argument is useful for hiding implementation details of error generation from an end user. For instance:

function MyError() {
  Error.captureStackTrace(this, MyError);
}

// Without passing MyError to captureStackTrace, the MyError
// frame would show up in the .stack property. By passing
// the constructor, we omit that frame and all frames above it.
new MyError().stack

Error.stackTraceLimit#

The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

error.message#

Returns the string description of error as set by calling new Error(message). The message passed to the constructor will also appear in the first line of the stack trace of the Error, however changing this property after the Error object is created may not change the first line of the stack trace.

const err = new Error('The message');
console.log(err.message);
// Prints: The message

error.stack#

Returns a string describing the point in the code at which the Error was instantiated.

For example:

Error: Things keep happening!
   at /home/gbusey/file.js:525:2
   at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
   at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
   at increaseSynergy (/home/gbusey/actors.js:701:6)

The first line is formatted as <error class name>: <error message>, and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses.

It is important to note that frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify, which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in the stack traces:

const cheetahify = require('./native-binding.node');

function makeFaster() {
  // cheetahify *synchronously* calls speedy.
  cheetahify(function speedy() {
    throw new Error('oh no!');
  });
}

makeFaster(); // will throw:
  // /home/gbusey/file.js:6
  //     throw new Error('oh no!');
  //           ^
  // Error: oh no!
  //     at speedy (/home/gbusey/file.js:6:11)
  //     at makeFaster (/home/gbusey/file.js:5:3)
  //     at Object.<anonymous> (/home/gbusey/file.js:10:1)
  //     at Module._compile (module.js:456:26)
  //     at Object.Module._extensions..js (module.js:474:10)
  //     at Module.load (module.js:356:32)
  //     at Function.Module._load (module.js:312:12)
  //     at Function.Module.runMain (module.js:497:10)
  //     at startup (node.js:119:16)
  //     at node.js:906:3

The location information will be one of:

  • native, if the frame represents a call internal to V8 (as in [].forEach).
  • plain-filename.js:line:column, if the frame represents a call internal to Node.js.
  • /absolute/path/to/file.js:line:column, if the frame represents a call in a user program, or its dependencies.

The string representing the stack trace is lazily generated when the error.stack property is accessed.

The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick.

System-level errors are generated as augmented Error instances, which are detailed here.

Class: RangeError#

A subclass of Error that indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.

For example:

require('net').connect(-1);
  // throws RangeError, port should be > 0 && < 65536

Node.js will generate and throw RangeError instances immediately as a form of argument validation.

Class: ReferenceError#

A subclass of Error that indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.

While client code may generate and propagate these errors, in practice, only V8 will do so.

doesNotExist;
  // throws ReferenceError, doesNotExist is not a variable in this program.

ReferenceError instances will have an error.arguments property whose value is an array containing a single element: a string representing the variable that was not defined.

const assert = require('assert');
try {
  doesNotExist;
} catch(err) {
  assert(err.arguments[0], 'doesNotExist');
}

Unless an application is dynamically generating and running code, ReferenceError instances should always be considered a bug in the code or its dependencies.

Class: SyntaxError#

A subclass of Error that indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval, Function, require, or vm. These errors are almost always indicative of a broken program.

try {
  require('vm').runInThisContext('binary ! isNotOk');
} catch(err) {
  // err will be a SyntaxError
}

SyntaxError instances are unrecoverable in the context that created them – they may only be caught by other contexts.

Class: TypeError#

A subclass of Error that indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError.

require('url').parse(() => { });
  // throws TypeError, since it expected a string

Node.js will generate and throw TypeError instances immediately as a form of argument validation.

Exceptions vs. Errors#

A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values are instances of Error or classes which inherit from Error, all exceptions thrown by Node.js or the JavaScript runtime will be instances of Error.

Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include assert() checks or abort() calls in the C++ layer.

System Errors#

System errors are generated when exceptions occur within the program's runtime environment. Typically, these are operational errors that occur when an application violates an operating system constraint such as attempting to read a file that does not exist or when the user does not have sufficient permissions.

System errors are typically generated at the syscall level: an exhaustive list of error codes and their meanings is available by running man 2 intro or man 3 errno on most Unices; or online.

In Node.js, system errors are represented as augmented Error objects with added properties.

Class: System Error#

error.code#

Returns a string representing the error code, which is always E followed by a sequence of capital letters, and may be referenced in man 2 intro.

error.errno#

Returns a number corresponding to the negated error code, which may be referenced in man 2 intro. For example, an ENOENT error has an errno of -2 because the error code for ENOENT is 2.

error.syscall#

Returns a string describing the syscall that failed.

Common System Errors#

This list is not exhaustive, but enumerates many of the common system errors encountered when writing a Node.js program. An exhaustive list may be found here.

  • EACCES (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.

  • EADDRINUSE (Address already in use): An attempt to bind a server (net, http, or https) to a local address failed due to another server on the local system already occupying that address.

  • ECONNREFUSED (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.

  • ECONNRESET (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the http and net modules.

  • EEXIST (File exists): An existing file was the target of an operation that required that the target not exist.

  • EISDIR (Is a directory): An operation expected a file, but the given pathname was a directory.

  • EMFILE (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, OS X) where there is a low file descriptor limit for processes. To remedy a low limit, run ulimit -n 2048 in the same shell that will run the Node.js process.

  • ENOENT (No such file or directory): Commonly raised by fs operations to indicate that a component of the specified pathname does not exist -- no entity (file or directory) could be found by the given path.

  • ENOTDIR (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir.

  • ENOTEMPTY (Directory not empty): A directory with entries was the target of an operation that requires an empty directory -- usually fs.unlink.

  • EPERM (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.

  • EPIPE (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the net and http layers, indicative that the remote side of the stream being written to has been closed.

  • ETIMEDOUT (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by http or net -- often a sign that a socket.end() was not properly called.

Events#

Stability: 2 - Stable

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") periodically emit named events that cause Function objects ("listeners") to be called.

For instance: a net.Server object emits an event each time a peer connects to it; a fs.ReadStream emits an event when the file is opened; a stream emits an event whenever data is available to be read.

All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and will be discarded.

The following example shows a simple EventEmitter instance with a single listener. The eventEmitter.on() method is used to register listeners, while the eventEmitter.emit() method is used to trigger the event.

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');

Passing arguments and this to listeners#

The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. It is important to keep in mind that when an ordinary listener function is called by the EventEmitter, the standard this keyword is intentionally set to reference the EventEmitter to which the listener is attached.

const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this);
  // Prints:
  //   a b MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined }
});
myEmitter.emit('event', 'a', 'b');

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');

Asynchronous vs. Synchronous#

The EventListener calls all listeners synchronously in the order in which they were registered. This is important to ensure the proper sequencing of events and to avoid race conditions or logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');

Handling events only once#

When a listener is registered using the eventEmitter.on() method, that listener will be invoked every time the named event is emitted.

const myEmitter = new MyEmitter();
var m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2

Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

const myEmitter = new MyEmitter();
var m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored

Error events#

When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.

If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.

const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js

To guard against crashing the Node.js process, a listener can be registered on the process object's uncaughtException event or the domain module can be used. (Note, however, that the domain module has been deprecated)

const myEmitter = new MyEmitter();

process.on('uncaughtException', (err) => {
  console.log('whoops! there was an error');
});

myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

As a best practice, listeners should always be added for the 'error' events.

const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.log('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

Class: EventEmitter#

Added in: v0.1.26

The EventEmitter class is defined and exposed by the events module:

const EventEmitter = require('events');

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

Event: 'newListener'#

Added in: v0.1.26

The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.

Listeners registered for the 'newListener' event will be passed the event name and a reference to the listener being added.

The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback will be inserted before the listener that is in the process of being added.

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A

Event: 'removeListener'#

Added in: v0.9.3

The 'removeListener' event is emitted after the listener is removed.

EventEmitter.listenerCount(emitter, eventName)#

Added in: v0.9.12 Deprecated since: v4.0.0 
Stability: 0 - Deprecated: Use emitter.listenerCount() instead.

A class method that returns the number of listeners for the given eventName registered on the given emitter.

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2

EventEmitter.defaultMaxListeners#

Added in: v0.11.2

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for all EventEmitter instances, the EventEmitter.defaultMaxListeners property can be used.

Take caution when setting the EventEmitter.defaultMaxListeners because the change effects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners.

Note that this is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type and count properties, referring to the event emitter instance, the event’s name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

emitter.addListener(eventName, listener)#

Added in: v0.1.26

Alias for emitter.on(eventName, listener).

emitter.emit(eventName[, ...args])#

Added in: v0.1.26

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

emitter.eventNames()#

Added in: v6.0.0

Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings or Symbols.

const EventEmitter = require('events');
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

emitter.getMaxListeners()#

Added in: v1.0.0

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.

emitter.listenerCount(eventName)#

Added in: v3.2.0

Returns the number of listeners listening to the event named eventName.

emitter.listeners(eventName)#

Added in: v0.1.26

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

emitter.on(eventName, listener)#

Added in: v0.1.101

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.once(eventName, listener)#

Added in: v0.3.0

Adds a one time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.prependListener(eventName, listener)#

Added in: v6.0.0

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.prependOnceListener(eventName, listener)#

Added in: v6.0.0

Adds a one time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeAllListeners([eventName])#

Added in: v0.1.26

Removes all listeners, or those of the specified eventName.

Note that it is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeListener(eventName, listener)#

Added in: v0.1.26

Removes the specified listener from the listener array for the event named eventName.

var callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener must be called multiple times to remove each instance.

Note that once an event has been emitted, all listeners attached to it at the time of emitting will be called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events will behave as expected.

const myEmitter = new MyEmitter();

var callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

var callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.setMaxListeners(n)#

Added in: v0.3.5

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. Obviously, not all events should be limited to just 10 listeners. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

File System#

Stability: 2 - Stable

File I/O is provided by simple wrappers around standard POSIX functions. To use this module do require('fs'). All the methods have asynchronous and synchronous forms.

The asynchronous form always takes a completion callback as its last argument. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation was completed successfully, then the first argument will be null or undefined.

When using the synchronous form any exceptions are immediately thrown. You can use try/catch to handle exceptions or allow them to bubble up.

Here is an example of the asynchronous version:

const fs = require('fs');

fs.unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

Here is the synchronous version:

const fs = require('fs');

fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');

With the asynchronous methods there is no guaranteed ordering. So the following is prone to error:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
  if (err) throw err;
  console.log(`stats: ${JSON.stringify(stats)}`);
});

It could be that fs.stat is executed before fs.rename. The correct way to do this is to chain the callbacks.

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  fs.stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});

In busy processes, the programmer is strongly encouraged to use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete--halting all connections.

The relative path to a filename can be used. Remember, however, that this path will be relative to process.cwd().

Most fs functions let you omit the callback argument. If you do, a default callback is used that rethrows errors. To get a trace to the original call site, set the NODE_DEBUG environment variable:

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();

$ env NODE_DEBUG=fs node script.js
fs.js:88
        throw backtrace;
        ^
Error: EISDIR: illegal operation on a directory, read
    <stack trace.>

Buffer API#

Added in: v6.0.0

fs functions support passing and receiving paths as both strings and Buffers. The latter is intended to make it possible to work with filesystems that allow for non-UTF-8 filenames. For most typical uses, working with paths as Buffers will be unnecessary, as the string API converts to and from UTF-8 automatically.

Note that on certain file systems (such as NTFS and HFS+) filenames will always be encoded as UTF-8. On such file systems, passing non-UTF-8 encoded Buffers to fs functions will not work as expected.

Class: fs.FSWatcher#

Added in: v0.5.8

Objects returned from fs.watch() are of this type.

The listener callback provided to fs.watch() receives the returned FSWatcher's change events.

The object itself emits these events:

Event: 'change'#

Added in: v0.5.8
  • eventType <String> The type of fs change
  • filename <String> | <Buffer> The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file. See more details in fs.watch().

The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a Buffer if fs.watch() is called with its encoding option set to 'buffer', otherwise filename will be a string.

// Example when handled through fs.watch listener
fs.watch('./tmp', {encoding: 'buffer'}, (eventType, filename) => {
  if (filename)
    console.log(filename);
    // Prints: <Buffer ...>
});

Event: 'error'#

Added in: v0.5.8

Emitted when an error occurs.

watcher.close()#

Added in: v0.5.8

Stop watching for changes on the given fs.FSWatcher.

Class: fs.ReadStream#

Added in: v0.1.93

ReadStream is a Readable Stream.

Event: 'open'#

Added in: v0.1.93
  • fd <Integer> Integer file descriptor used by the ReadStream.

Emitted when the ReadStream's file is opened.

Event: 'close'#

Added in: v0.1.93

Emitted when the ReadStream's underlying file descriptor has been closed using the fs.close() method.

readStream.bytesRead#

Added in: 6.4.0

The number of bytes read so far.

readStream.path#

Added in: v0.1.93

The path to the file the stream is reading from as specified in the first argument to fs.createReadStream(). If path is passed as a string, then readStream.path will be a string. If path is passed as a Buffer, then readStream.path will be a Buffer.

Class: fs.Stats#

Added in: v0.1.21

Objects returned from fs.stat(), fs.lstat() and fs.fstat() and their synchronous counterparts are of this type.

  • stats.isFile()
  • stats.isDirectory()
  • stats.isBlockDevice()
  • stats.isCharacterDevice()
  • stats.isSymbolicLink() (only valid with fs.lstat())
  • stats.isFIFO()
  • stats.isSocket()

For a regular file util.inspect(stats) would return a string very similar to this:

{
  dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT
}

Please note that atime, mtime, birthtime, and ctime are instances of Date object and to compare the values of these objects you should use appropriate methods. For most general uses getTime() will return the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC and this integer should be sufficient for any comparison, however there are additional methods which can be used for displaying fuzzy information. More details can be found in the MDN JavaScript Reference page.

Stat Time Values#

The times in the stat object have the following semantics:

  • atime "Access Time" - Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.
  • mtime "Modified Time" - Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.
  • ctime "Change Time" - Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.
  • birthtime "Birth Time" - Time of file creation. Set once when the file is created. On filesystems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, unix epoch timestamp 0). Note that this value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call.

Prior to Node v0.12, the ctime held the birthtime on Windows systems. Note that as of v0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.WriteStream#

Added in: v0.1.93

WriteStream is a Writable Stream.

Event: 'open'#

Added in: v0.1.93
  • fd <Integer> Integer file descriptor used by the WriteStream.

Emitted when the WriteStream's file is opened.

Event: 'close'#

Added in: v0.1.93

Emitted when the WriteStream's underlying file descriptor has been closed using the fs.close() method.

writeStream.bytesWritten#

Added in: v0.4.7

The number of bytes written so far. Does not include data that is still queued for writing.

writeStream.path#

Added in: v0.1.93

The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream(). If path is passed as a string, then writeStream.path will be a string. If path is passed as a Buffer, then writeStream.path will be a Buffer.

fs.access(path[, mode], callback)#

Added in: v0.11.15

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. The following constants define the possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values.

  • fs.constants.F_OK - path is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
  • fs.constants.R_OK - path can be read by the calling process.
  • fs.constants.W_OK - path can be written by the calling process.
  • fs.constants.X_OK - path can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

The final argument, callback, is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be populated. The following example checks if the file /etc/passwd can be read and written by the current process.

fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => {
  console.log(err ? 'no access!' : 'can read/write');
});

Using fs.access() to check for the accessibility of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

For example:

write (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (!err) {
    console.error('myfile already exists');
    return;
  }

  fs.open('myfile', 'wx', (err, fd) => {
    if (err) throw err;
    writeMyData(fd);
  });
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === "EEXIST") {
      console.error('myfile already exists');
      return;
    } else {
      throw err;
    }
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (err) {
    if (err.code === "ENOENT") {
      console.error('myfile does not exist');
      return;
    } else {
      throw err;
    }
  }

  fs.open('myfile', 'r', (err, fd) => {
    if (err) throw err;
    readMyData(fd);
  });
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === "ENOENT") {
      console.error('myfile does not exist');
      return;
    } else {
      throw err;
    }
  }

  readMyData(fd);
});

The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the accessibility of a file only if the file won’t be used directly, for example when its accessibility is a signal from another process.

fs.accessSync(path[, mode])#

Added in: v0.11.15

Synchronous version of fs.access(). This throws if any accessibility checks fail, and does nothing otherwise.

fs.appendFile(file, data[, options], callback)#

Added in: v0.6.7

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a buffer.

Example:

fs.appendFile('message.txt', 'data to append', (err) => {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
});

If options is a string, then it specifies the encoding. Example:

fs.appendFile('message.txt', 'data to append', 'utf8', callback);

Any specified file descriptor has to have been opened for appending.

Note: If a file descriptor is specified as the file, it will not be closed automatically.

fs.appendFileSync(file, data[, options])#

Added in: v0.6.7

The synchronous version of fs.appendFile(). Returns undefined.

fs.chmod(path, mode, callback)#

Added in: v0.1.30

Asynchronous chmod(2). No arguments other than a possible exception are given to the completion callback.

fs.chmodSync(path, mode)#

Added in: v0.6.7

Synchronous chmod(2). Returns undefined.

fs.chown(path, uid, gid, callback)#

Added in: v0.1.97

Asynchronous chown(2). No arguments other than a possible exception are given to the completion callback.

fs.chownSync(path, uid, gid)#

Added in: v0.1.97

Synchronous chown(2). Returns undefined.

fs.close(fd, callback)#

Added in: v0.0.2

Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.

fs.closeSync(fd)#

Added in: v0.1.21

Synchronous close(2). Returns undefined.

fs.constants#

Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.

fs.createReadStream(path[, options])#

Added in: v0.1.31

Returns a new ReadStream object. (See Readable Stream).

Be aware that, unlike the default value set for highWaterMark on a readable stream (16 kb), the stream returned by this method has a default value of 64 kb for the same parameter.

options is an object or string with the following defaults:

{
  flags: 'r',
  encoding: null,
  fd: null,
  mode: 0o666,
  autoClose: true
}

options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0. If fd is specified and start is omitted or undefined, fs.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by Buffer.

If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. Note that fd should be blocking; non-blocking fds should be passed to net.Socket.

If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is your responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on error or end the file descriptor will be closed automatically.

mode sets the file mode (permission and sticky bits), but only if the file was created.

An example to read the last 10 bytes of a file which is 100 bytes long:

fs.createReadStream('sample.txt', {start: 90, end: 99});

If options is a string, then it specifies the encoding.

fs.createWriteStream(path[, options])#

Added in: v0.1.31

Returns a new WriteStream object. (See Writable Stream).

options is an object or string with the following defaults:

{
  flags: 'w',
  defaultEncoding: 'utf8',
  fd: null,
  mode: 0o666,
  autoClose: true
}

options may also include a start option to allow writing data at some position past the beginning of the file. Modifying a file rather than replacing it may require a flags mode of r+ rather than the default mode w. The defaultEncoding can be any one of those accepted by Buffer.

If autoClose is set to true (default behavior) on error or end the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is your responsibility to close it and make sure there's no file descriptor leak.

Like ReadStream, if fd is specified, WriteStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. Note that fd should be blocking; non-blocking fds should be passed to net.Socket.

If options is a string, then it specifies the encoding.

fs.exists(path, callback)#

Added in: v0.0.2 Deprecated since: v1.0.0 
Stability: 0 - Deprecated: Use fs.stat() or fs.access() instead.

Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false. Example:

fs.exists('/etc/passwd', (exists) => {
  console.log(exists ? 'it\'s there' : 'no passwd!');
});

Note that the parameter to this callback is not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().

Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

For example:

write (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    console.error('myfile already exists');
  } else {
    fs.open('myfile', 'wx', (err, fd) => {
      if (err) throw err;
      writeMyData(fd);
    });
  }
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === "EEXIST") {
      console.error('myfile already exists');
      return;
    } else {
      throw err;
    }
  }
  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    fs.open('myfile', 'r', (err, fd) => {
      readMyData(fd);
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === "ENOENT") {
      console.error('myfile does not exist');
      return;
    } else {
      throw err;
    }
  } else {
    readMyData(fd);
  }
});

The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process.

fs.existsSync(path)#

Added in: v0.1.21

Synchronous version of fs.exists(). Returns true if the file exists, false otherwise.

Note that fs.exists() is deprecated, but fs.existsSync() is not. (The callback parameter to fs.exists() accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync() does not use a callback.)

fs.fchmod(fd, mode, callback)#

Added in: v0.4.7

Asynchronous fchmod(2). No arguments other than a possible exception are given to the completion callback.

fs.fchmodSync(fd, mode)#

Added in: v0.4.7

Synchronous fchmod(2). Returns undefined.

fs.fchown(fd, uid, gid, callback)#

Added in: v0.4.7

Asynchronous fchown(2). No arguments other than a possible exception are given to the completion callback.

fs.fchownSync(fd, uid, gid)#

Added in: v0.4.7

Synchronous fchown(2). Returns undefined.

fs.fdatasync(fd, callback)#

Added in: v0.1.96

Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.

fs.fdatasyncSync(fd)#

Added in: v0.1.96

Synchronous fdatasync(2). Returns undefined.

fs.fstat(fd, callback)#

Added in: v0.1.95

Asynchronous fstat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object. fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor fd.

fs.fstatSync(fd)#

Added in: v0.1.95

Synchronous fstat(2). Returns an instance of fs.Stats.

fs.fsync(fd, callback)#

Added in: v0.1.96

Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.

fs.fsyncSync(fd)#

Added in: v0.1.96

Synchronous fsync(2). Returns undefined.

fs.ftruncate(fd, len, callback)#

Added in: v0.8.6

Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.

If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to first four bytes
fs.ftruncate(fd, 4, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt', 'utf8'));
});
// Prints: Node

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'). For example,

console.log(fs.readFileSync('temp.txt', 'utf-8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to 10 bytes, whereas the actual size is 7 bytes
fs.ftruncate(fd, 10, (err) => {
  assert.ifError(!err);
  console.log(fs.readFileSync('temp.txt'));
});
// Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
// ('Node.js\0\0\0' in UTF8)

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

fs.ftruncateSync(fd, len)#

Added in: v0.8.6

Synchronous ftruncate(2). Returns undefined.

fs.futimes(fd, atime, mtime, callback)#

Added in: v0.4.2

Change the file timestamps of a file referenced by the supplied file descriptor.

fs.futimesSync(fd, atime, mtime)#

Added in: v0.4.2

Synchronous version of fs.futimes(). Returns undefined.

fs.lchmod(path, mode, callback)#

Deprecated since: v0.4.7

Asynchronous lchmod(2). No arguments other than a possible exception are given to the completion callback.

Only available on Mac OS X.

fs.lchmodSync(path, mode)#

Deprecated since: v0.4.7

Synchronous lchmod(2). Returns undefined.

fs.lchown(path, uid, gid, callback)#

Deprecated since: v0.4.7

Asynchronous lchown(2). No arguments other than a possible exception are given to the completion callback.

fs.lchownSync(path, uid, gid)#

Deprecated since: v0.4.7

Synchronous lchown(2). Returns undefined.

fs.link(existingPath, newPath, callback)#

Added in: v0.1.31

Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.

fs.linkSync(existingPath, newPath)#

Added in: v0.1.31

Synchronous link(2). Returns undefined.

fs.lstat(path, callback)#

Added in: v0.1.30

Asynchronous lstat(2). The callback gets two arguments (err, stats) where stats is a fs.Stats object. lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to.

fs.lstatSync(path)#

Added in: v0.1.30

Synchronous lstat(2). Returns an instance of fs.Stats.

fs.mkdir(path[, mode], callback)#

Added in: v0.1.8

Asynchronous mkdir(2). No arguments other than a possible exception are given to the completion callback. mode defaults to 0o777.

fs.mkdirSync(path[, mode])#

Added in: v0.1.21

Synchronous mkdir(2). Returns undefined.

fs.mkdtemp(prefix[, options], callback)#

Added in: v5.10.0

Creates a unique temporary directory.

Generates six random characters to be appended behind a required prefix to create a unique temporary directory.

The created folder path is passed as a string to the callback's second parameter.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

Example:

fs.mkdtemp('/tmp/foo-', (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Prints: /tmp/foo-itXde2
});

Note: The fs.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp, if the intention is to create a temporary directory within /tmp, the prefix must end with a trailing platform-specific path separator (require('path').sep).

// The parent directory for the new temporary directory
const tmpDir = '/tmp';

// This method is *INCORRECT*:
fs.mkdtemp(tmpDir, (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Will print something similar to `/tmpabc123`.
  // Note that a new temporary directory is created
  // at the file system root rather than *within*
  // the /tmp directory.
});

// This method is *CORRECT*:
const path = require('path');
fs.mkdtemp(tmpDir + path.sep, (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Will print something similar to `/tmp/abc123`.
  // A new temporary directory is created within
  // the /tmp directory.
});

fs.mkdtempSync(prefix[, options])#

Added in: v5.10.0

The synchronous version of fs.mkdtemp(). Returns the created folder path.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

fs.open(path, flags[, mode], callback)#

Added in: v0.0.2

Asynchronous file open. See open(2). flags can be:

  • 'r' - Open file for reading. An exception occurs if the file does not exist.

  • 'r+' - Open file for reading and writing. An exception occurs if the file does not exist.

  • 'rs+' - Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.

    This is primarily useful for opening files on NFS mounts as it allows you to skip the potentially stale local cache. It has a very real impact on I/O performance so don't use this flag unless you need it.

    Note that this doesn't turn fs.open() into a synchronous blocking call. If that's what you want then you should be using fs.openSync()

  • 'w' - Open file for writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx' - Like 'w' but fails if path exists.

  • 'w+' - Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx+' - Like 'w+' but fails if path exists.

  • 'a' - Open file for appending. The file is created if it does not exist.

  • 'ax' - Like 'a' but fails if path exists.

  • 'a+' - Open file for reading and appending. The file is created if it does not exist.

  • 'ax+' - Like 'a+' but fails if path exists.

mode sets the file mode (permission and sticky bits), but only if the file was created. It defaults to 0666, readable and writable.

The callback gets two arguments (err, fd).

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems.

flags can also be a number as documented by open(2); commonly used constants are available from fs.constants. On Windows, flags are translated to their equivalent ones where applicable, e.g. O_WRONLY to FILE_GENERIC_WRITE, or O_EXCL|O_CREAT to CREATE_NEW, as accepted by CreateFileW.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

Note: The behavior of fs.open() is platform specific for some flags. As such, opening a directory on OS X and Linux with the 'a+' flag - see example below - will return an error. In contrast, on Windows and FreeBSD, a file descriptor will be returned.

// OS X and Linux
fs.open('<directory>', 'a+', (err, fd) => {
  // => [Error: EISDIR: illegal operation on a directory, open <directory>]
});

// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
  // => null, <fd>
});

fs.openSync(path, flags[, mode])#

Added in: v0.1.21

Synchronous version of fs.open(). Returns an integer representing the file descriptor.

fs.read(fd, buffer, offset, length, position, callback)#

Added in: v0.0.2

Read data from the file specified by fd.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an integer specifying where to begin reading from in the file. If position is null, data will be read from the current file position.

The callback is given the three arguments, (err, bytesRead, buffer).

fs.readdir(path[, options], callback)#

Added in: v0.1.8

Asynchronous readdir(3). Reads the contents of a directory. The callback gets two arguments (err, files) where files is an array of the names of the files in the directory excluding '.' and '..'.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

fs.readdirSync(path[, options])#

Added in: v0.1.21

Synchronous readdir(3). Returns an array of filenames excluding '.' and '..'.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

fs.readFile(file[, options], callback)#

Added in: v0.1.29

Asynchronously reads the entire contents of a file. Example:

fs.readFile('/etc/passwd', (err, data) => {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the contents of the file.

If no encoding is specified, then the raw buffer is returned.

If options is a string, then it specifies the encoding. Example:

fs.readFile('/etc/passwd', 'utf8', callback);

Any specified file descriptor has to support reading.

Note: If a file descriptor is specified as the file, it will not be closed automatically.

fs.readFileSync(file[, options])#

Added in: v0.1.8

Synchronous version of fs.readFile. Returns the contents of the file.

If the encoding option is specified then this function returns a string. Otherwise it returns a buffer.

fs.readlink(path[, options], callback)#

Added in: v0.1.31

Asynchronous readlink(2). The callback gets two arguments (err, linkString).

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fs.readlinkSync(path[, options])#

Added in: v0.1.31

Synchronous readlink(2). Returns the symbolic link's string value.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fs.readSync(fd, buffer, offset, length, position)#

Added in: v0.1.21

Synchronous version of fs.read(). Returns the number of bytesRead.

fs.realpath(path[, options], callback)#

Added in: v0.1.31

Asynchronous realpath(3). The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

fs.realpathSync(path[, options])#

Added in: v0.1.31

Synchronous realpath(3). Returns the resolved path.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the returned value. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

fs.rename(oldPath, newPath, callback)#

Added in: v0.0.2

Asynchronous rename(2). No arguments other than a possible exception are given to the completion callback.

fs.renameSync(oldPath, newPath)#

Added in: v0.1.21

Synchronous rename(2). Returns undefined.

fs.rmdir(path, callback)#

Added in: v0.0.2

Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.

fs.rmdirSync(path)#

Added in: v0.1.21

Synchronous rmdir(2). Returns undefined.

fs.stat(path, callback)#

Added in: v0.0.2

Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object.

In case of an error, the err.code will be one of Common System Errors.

Using fs.stat() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available.

To check if a file exists without manipulating it afterwards, fs.access() is recommended.

fs.statSync(path)#

Added in: v0.1.21

Synchronous stat(2). Returns an instance of fs.Stats.

fs.symlink(target, path[, type], callback)#

Added in: v0.1.31

Asynchronous symlink(2). No arguments other than a possible exception are given to the completion callback. The type argument can be set to 'dir', 'file', or 'junction' (default is 'file') and is only available on Windows (ignored on other platforms). Note that Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Here is an example below:

fs.symlink('./foo', './new-port');

It creates a symbolic link named "new-port" that points to "foo".

fs.symlinkSync(target, path[, type])#

Added in: v0.1.31

Synchronous symlink(2). Returns undefined.

fs.truncate(path, len, callback)#

Added in: v0.8.6

Asynchronous truncate(2). No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called.

fs.truncateSync(path, len)#

Added in: v0.8.6

Synchronous truncate(2). Returns undefined. A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync() is called.

fs.unlink(path, callback)#

Added in: v0.0.2

Asynchronous unlink(2). No arguments other than a possible exception are given to the completion callback.

fs.unlinkSync(path)#

Added in: v0.1.21

Synchronous unlink(2). Returns undefined.

fs.unwatchFile(filename[, listener])#

Added in: v0.1.31

Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed and you have effectively stopped watching filename.

Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error.

Note: fs.watch() is more efficient than fs.watchFile() and fs.unwatchFile(). fs.watch() should be used instead of fs.watchFile() and fs.unwatchFile() when possible.

fs.utimes(path, atime, mtime, callback)#

Added in: v0.4.2

Change file timestamps of the file referenced by the supplied path.

Note: the arguments atime and mtime of the following related functions follow these rules:

  • The value should be a Unix timestamp in seconds. For example, Date.now() returns milliseconds, so it should be divided by 1000 before passing it in.
  • If the value is a numeric string like '123456789', the value will get converted to the corresponding number.
  • If the value is NaN or Infinity, the value will get converted to Date.now() / 1000.

fs.utimesSync(path, atime, mtime)#

Added in: v0.4.2

Synchronous version of fs.utimes(). Returns undefined.

fs.watch(filename[, options][, listener])#

Added in: v0.5.10
  • filename <String> | <Buffer>
  • options <String> | <Object>
    • persistent <Boolean> Indicates whether the process should continue to run as long as files are being watched. default = true
    • recursive <Boolean> Indicates whether all subdirectories should be watched, or only the current directory. The applies when a directory is specified, and only on supported platforms (See Caveats). default = false
    • encoding <String> Specifies the character encoding to be used for the filename passed to the listener. default = 'utf8'
  • listener <Function>

Watch for changes on filename, where filename is either a file or a directory. The returned object is a fs.FSWatcher.

The second argument is optional. If options is provided as a string, it specifies the encoding. Otherwise options should be passed as an object.

The listener callback gets two arguments (eventType, filename). eventType is either 'rename' or 'change', and filename is the name of the file which triggered the event.

Note that on most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

Also note the listener callback is attached to the 'change' event fired by fs.FSWatcher, but it is not the same thing as the 'change' value of eventType.

Caveats#

The fs.watch API is not 100% consistent across platforms, and is unavailable in some situations.

The recursive option is only supported on OS X and Windows.

Availability#

This feature depends on the underlying operating system providing a way to be notified of filesystem changes.

  • On Linux systems, this uses inotify
  • On BSD systems, this uses kqueue
  • On OS X, this uses kqueue for files and FSEvents for directories.
  • On SunOS systems (including Solaris and SmartOS), this uses event ports.
  • On Windows systems, this feature depends on ReadDirectoryChangesW.
  • On Aix systems, this feature depends on AHAFS, which must be enabled.

If the underlying functionality is not available for some reason, then fs.watch will not be able to function. For example, watching files or directories can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc), or host file systems when using virtualization software such as Vagrant, Docker, etc.

You can still use fs.watchFile, which uses stat polling, but it is slower and less reliable.

Inodes#

On Linux and OS X systems, fs.watch() resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior.

Filename Argument#

Providing filename argument in the callback is only supported on Linux and Windows. Even on supported platforms, filename is not always guaranteed to be provided. Therefore, don't assume that filename argument is always provided in the callback, and have some fallback logic if it is null.

fs.watch('somedir', (eventType, filename) => {
  console.log(`event type is: ${eventType}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

fs.watchFile(filename[, options], listener)#

Added in: v0.1.31

Watch for changes on filename. The callback listener will be called each time the file is accessed.

The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds. The default is { persistent: true, interval: 5007 }.

The listener gets two arguments the current stat object and the previous stat object:

fs.watchFile('message.text', (curr, prev) => {
  console.log(`the current mtime is: ${curr.mtime}`);
  console.log(`the previous mtime was: ${prev.mtime}`);
});

These stat objects are instances of fs.Stat.

If you want to be notified when the file was modified, not just accessed, you need to compare curr.mtime and prev.mtime.

Note: when an fs.watchFile operation results in an ENOENT error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). In Windows, blksize and blocks fields will be undefined, instead of zero. If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.

Note: fs.watch() is more efficient than fs.watchFile and fs.unwatchFile. fs.watch should be used instead of fs.watchFile and fs.unwatchFile when possible.

fs.write(fd, buffer[, offset[, length[, position]]], callback)#

Added in: v0.0.2

Write buffer to the file specified by fd.

offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position. See pwrite(2).

The callback will be given three arguments (err, written, buffer) where written specifies how many bytes were written from buffer.

Note that it is unsafe to use fs.write multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.write(fd, string[, position[, encoding]], callback)#

Added in: v0.11.5

Write string to the file specified by fd. If string is not a string, then the value will be coerced to one.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' the data will be written at the current position. See pwrite(2).

encoding is the expected string encoding.

The callback will receive the arguments (err, written, string) where written specifies how many bytes the passed string required to be written. Note that bytes written is not the same as string characters. See Buffer.byteLength.

Unlike when writing buffer, the entire string must be written. No substring may be specified. This is because the byte offset of the resulting data may not be the same as the string offset.

Note that it is unsafe to use fs.write multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.writeFile(file, data[, options], callback)#

Added in: v0.1.29

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer.

The encoding option is ignored if data is a buffer. It defaults to 'utf8'.

Example:

fs.writeFile('message.txt', 'Hello Node.js', (err) => {
  if (err) throw err;
  console.log('It\'s saved!');
});

If options is a string, then it specifies the encoding. Example:

fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);

Any specified file descriptor has to support writing.

Note that it is unsafe to use fs.writeFile multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

Note: If a file descriptor is specified as the file, it will not be closed automatically.

fs.writeFileSync(file, data[, options])#

Added in: v0.1.29

The synchronous version of fs.writeFile(). Returns undefined.

fs.writeSync(fd, buffer[, offset[, length[, position]]])#

Added in: v0.1.21

fs.writeSync(fd, string[, position[, encoding]])#

Added in: v0.11.5

Synchronous versions of fs.write(). Returns the number of bytes written.

FS Constants#

The following constants are exported by fs.constants. Note: Not every constant will be available on every operating system.

File Access Constants#

The following constants are meant for use with fs.access().

Constant Description
F_OK Flag indicating that the file is visible to the calling process.
R_OK Flag indicating that the file can be read by the calling process.
W_OK Flag indicating that the file can be written by the calling process.
X_OK Flag indicating that the file can be executed by the calling process.

File Open Constants#

The following constants are meant for use with fs.open().

Constant Description
O_RDONLY Flag indicating to open a file for read-only access.
O_WRONLY Flag indicating to open a file for write-only access.
O_RDWR Flag indicating to open a file for read-write access.
O_CREAT Flag indicating to create the file if it does not already exist.
O_EXCL Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists.
O_NOCTTY Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one).
O_TRUNC Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero.
O_APPEND Flag indicating that data will be appended to the end of the file.
O_DIRECTORY Flag indicating that the open should fail if the path is not a directory.
O_NOATIME Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only.
O_NOFOLLOW Flag indicating that the open should fail if the path is a symbolic link.
O_SYNC Flag indicating that the file is opened for synchronous I/O.
O_SYMLINK Flag indicating to open the symbolic link itself rather than the resource it is pointing to.
O_DIRECT When set, an attempt will be made to minimize caching effects of file I/O.
O_NONBLOCK Flag indicating to open the file in nonblocking mode when possible.

File Type Constants#

The following constants are meant for use with the fs.Stats object's mode property for determining a file's type.

Constant Description
S_IFMT Bit mask used to extract the file type code.
S_IFREG File type constant for a regular file.
S_IFDIR File type constant for a directory.
S_IFCHR File type constant for a character-oriented device file.
S_IFBLK File type constant for a block-oriented device file.
S_IFIFO File type constant for a FIFO/pipe.
S_IFLNK File type constant for a symbolic link.
S_IFSOCK File type constant for a socket.

File Mode Constants#

The following constants are meant for use with the fs.Stats object's mode property for determining the access permissions for a file.

Constant Description
S_IRWXU File mode indicating readable, writable and executable by owner.
S_IRUSR File mode indicating readable by owner.
S_IWUSR File mode indicating writable by owner.
S_IXUSR File mode indicating executable by owner.
S_IRWXG File mode indicating readable, writable and executable by group.
S_IRGRP File mode indicating readable by group.
S_IWGRP File mode indicating writable by group.
S_IXGRP File mode indicating executable by group.
S_IRWXO File mode indicating readable, writable and executable by others.
S_IROTH File mode indicating readable by others.
S_IWOTH File mode indicating writable by others.
S_IXOTH File mode indicating executable by others.

Global Objects#

These objects are available in all modules. Some of these objects aren't actually in the global scope but in the module scope - this will be noted.

The objects listed here are specific to Node.js. There are a number of built-in objects that are part of the JavaScript language itself, which are also globally accessible.

Class: Buffer#

Added in: v0.1.103

Used to handle binary data. See the buffer section.

__dirname#

Added in: v0.1.27

The directory name of the current module. This the same as the path.dirname() of the __filename.

__dirname is not actually a global but rather local to each module.

Example: running node example.js from /Users/mjr

console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr

__filename#

Added in: v0.0.1

The file name of the current module. This is the resolved absolute path of the current module file.

For a main program this is not necessarily the same as the file name used in the command line.

See __dirname for the directory name of the current module.

__filename is not actually a global but rather local to each module.

Examples:

Running node example.js from /Users/mjr

console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr

Given two modules: a and b, where b is a dependency of a and there is a directory structure of:

  • /Users/mjr/app/a.js
  • /Users/mjr/app/node_modules/b/b.js

References to __filename within b.js will return /Users/mjr/app/node_modules/b/b.js while references to __filename within a.js will return /Users/mjr/app/a.js.

clearImmediate(immediateObject)#

Added in: v0.9.1

clearImmediate is described in the timers section.

clearInterval(intervalObject)#

Added in: v0.0.1

clearInterval is described in the timers section.

clearTimeout(timeoutObject)#

Added in: v0.0.1

clearTimeout is described in the timers section.

console#

Added in: v0.1.100

Used to print to stdout and stderr. See the console section.

exports#

Added in: v0.1.12

A reference to the module.exports that is shorter to type. See module system documentation for details on when to use exports and when to use module.exports.

exports is not actually a global but rather local to each module.

See the module system documentation for more information.

global#

Added in: v0.1.27

In browsers, the top-level scope is the global scope. That means that in browsers if you're in the global scope var something will define a global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside an Node.js module will be local to that module.

module#

Added in: v0.1.16

A reference to the current module. In particular module.exports is used for defining what a module exports and makes available through require().

module is not actually a global but rather local to each module.

See the module system documentation for more information.

process#

Added in: v0.1.7

The process object. See the process object section.

require()#

Added in: v0.1.13

To require modules. See the Modules section. require is not actually a global but rather local to each module.

require.cache#

Added in: v0.3.0

Modules are cached in this object when they are required. By deleting a key value from this object, the next require will reload the module. Note that this does not apply to native addons, for which reloading will result in an Error.

require.extensions#

Added in: v0.3.0 Deprecated since: v0.10.6 
Stability: 0 - Deprecated

Instruct require on how to handle certain file extensions.

Process files with the extension .sjs as .js:

require.extensions['.sjs'] = require.extensions['.js'];

Deprecated In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.

Since the module system is locked, this feature will probably never go away. However, it may have subtle bugs and complexities that are best left untouched.

Note that the number of file system operations that the module system has to perform in order to resolve a require(...) statement to a filename scales linearly with the number of registered extensions.

In other words, adding extensions slows down the module loader and should be discouraged.

require.resolve()#

Added in: v0.3.0

Use the internal require() machinery to look up the location of a module, but rather than loading the module, just return the resolved filename.

setImmediate(callback[, ...args])#

Added in: v0.9.1

setImmediate is described in the timers section.

setInterval(callback, delay[, ...args])#

Added in: v0.0.1

setInterval is described in the timers section.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1

setTimeout is described in the timers section.

HTTP#

Stability: 2 - Stable

To use the HTTP server and client one must require('http').

The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses--the user is able to stream data.

HTTP message headers are represented by an object like this:

{ 'content-length': '123',
  'content-type': 'text/plain',
  'connection': 'keep-alive',
  'host': 'mysite.com',
  'accept': '*/*' }

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, Node.js's HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.

See message.headers for details on how duplicate headers are handled.

The raw headers as they were received are retained in the rawHeaders property, which is an array of [key, value, key2, value2, ...]. For example, the previous message header object might have a rawHeaders list like the following:

[ 'ConTent-Length', '123456',
  'content-LENGTH', '123',
  'content-type', 'text/plain',
  'CONNECTION', 'keep-alive',
  'Host', 'mysite.com',
  'accepT', '*/*' ]

Class: http.Agent#

Added in: v0.3.4

An Agent is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the keepAlive option.

Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection.

When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).

It is good practice, to destroy() an Agent instance when it is no longer in use, because unused sockets consume OS resources.

Sockets are removed from an agent's pool when the socket emits either a 'close' event or an 'agentRemove' event. This means that if you intend to keep one HTTP request open for a long time and don't want it to stay in the pool you can do something along the lines of:

http.get(options, (res) => {
  // Do stuff
}).on('socket', (socket) => {
  socket.emit('agentRemove');
});

You may also use an agent for an individual request. By providing {agent: false} as an option to the http.get() or http.request() functions, a one-time use Agent with default options will be used for the client connection.

agent:false:

http.get({
  hostname: 'localhost',
  port: 80,
  path: '/',
  agent: false  // create a new agent just for this one request
}, (res) => {
  // Do stuff with response
});

new Agent(options)#

Added in: v0.3.4
  • options <Object> Set of configurable options to set on the agent. Can have the following fields:
    • keepAlive <Boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Default = false
    • keepAliveMsecs <Integer> When using the keepAlive option, specifies the initial delay for TCP Keep-Alive packets. Ignored when the keepAlive option is false or undefined. Default = 1000.
    • maxSockets <Number> Maximum number of sockets to allow per host. Default = Infinity.
    • maxFreeSockets <Number> Maximum number of sockets to leave open in a free state. Only relevant if keepAlive is set to true. Default = 256.

The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults.

To configure any of them, you must create your own http.Agent instance.

const http = require('http');
var keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);

agent.createConnection(options[, callback])#

Added in: v0.11.4

Produces a socket/stream to be used for HTTP requests.

By default, this function is the same as net.createConnection(). However, custom agents may override this method in case greater flexibility is desired.

A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function, or by passing the socket/stream to callback.

callback has a signature of (err, stream).

agent.destroy()#

Added in: v0.11.4

Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if you are using an agent with keepAlive enabled, then it is best to explicitly shut down the agent when you know that it will no longer be used. Otherwise, sockets may hang open for quite a long time before the server terminates them.

agent.freeSockets#

Added in: v0.11.4

An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.

agent.getName(options)#

Added in: v0.11.4
  • options <Object> A set of options providing information for name generation
    • host <String> A domain name or IP address of the server to issue the request to
    • port <Number> Port of remote server
    • localAddress <String> Local interface to bind for network connections when issuing the request
  • Returns: <String>

Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns host:port:localAddress. For an HTTPS agent, the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options that determine socket reusability.

agent.maxFreeSockets#

Added in: v0.11.7

By default set to 256. For agents with keepAlive enabled, this sets the maximum number of sockets that will be left open in the free state.

agent.maxSockets#

Added in: v0.3.6

By default set to Infinity. Determines how many concurrent sockets the agent can have open per origin. Origin is either a 'host:port' or 'host:port:localAddress' combination.

agent.requests#

Added in: v0.5.9

An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.

agent.sockets#

Added in: v0.3.6

An object which contains arrays of sockets currently in use by the agent. Do not modify.

Class: http.ClientRequest#

Added in: v0.1.17

This object is created internally and returned from http.request(). It represents an in-progress request whose header has already been queued. The header is still mutable using the setHeader(name, value), getHeader(name), removeHeader(name) API. The actual header will be sent along with the first data chunk or when closing the connection.

To get the response, add a listener for 'response' to the request object. 'response' will be emitted from the request object when the response headers have been received. The 'response' event is executed with one argument which is an instance of http.IncomingMessage.

During the 'response' event, one can add listeners to the response object; particularly to listen for the 'data' event.

If no 'response' handler is added, then the response will be entirely discarded. However, if you add a 'response' event handler, then you must consume the data from the response object, either by calling response.read() whenever there is a 'readable' event, or by adding a 'data' handler, or by calling the .resume() method. Until the data is consumed, the 'end' event will not fire. Also, until the data is read it will consume memory that can eventually lead to a 'process out of memory' error.

Note: Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

The request implements the Writable Stream interface. This is an EventEmitter with the following events:

Event: 'abort'#

Added in: v1.4.1

Emitted when the request has been aborted by the client. This event is only emitted on the first call to abort().

Event: 'aborted'#

Added in: v0.3.8

Emitted when the request has been aborted by the server and the network socket has closed.

Event: 'connect'#

Added in: v0.7.0

Emitted each time a server responds to a request with a CONNECT method. If this event is not being listened for, clients receiving a CONNECT method will have their connections closed.

A client and server pair that shows you how to listen for the 'connect' event:

const http = require('http');
const net = require('net');
const url = require('url');

// Create an HTTP tunneling proxy
var proxy = http.createServer( (req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('okay');
});
proxy.on('connect', (req, cltSocket, head) => {
  // connect to an origin server
  var srvUrl = url.parse(`http://${req.url}`);
  var srvSocket = net.connect(srvUrl.port, srvUrl.hostname, () => {
    cltSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                    'Proxy-agent: Node.js-Proxy\r\n' +
                    '\r\n');
    srvSocket.write(head);
    srvSocket.pipe(cltSocket);
    cltSocket.pipe(srvSocket);
  });
});

// now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {

  // make a request to a tunneling proxy
  var options = {
    port: 1337,
    hostname: '127.0.0.1',
    method: 'CONNECT',
    path: 'www.google.com:80'
  };

  var req = http.request(options);
  req.end();

  req.on('connect', (res, socket, head) => {
    console.log('got connected!');

    // make a request over an HTTP tunnel
    socket.write('GET / HTTP/1.1\r\n' +
                 'Host: www.google.com:80\r\n' +
                 'Connection: close\r\n' +
                 '\r\n');
    socket.on('data', (chunk) => {
      console.log(chunk.toString());
    });
    socket.on('end', () => {
      proxy.close();
    });
  });
});

Event: 'continue'#

Added in: v0.3.2

Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.

Event: 'response'#

Added in: v0.1.0

Emitted when a response is received to this request. This event is emitted only once.

Event: 'socket'#

Added in: v0.5.3

Emitted after a socket is assigned to this request.

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a server responds to a request with an upgrade. If this event is not being listened for, clients receiving an upgrade header will have their connections closed.

A client server pair that show you how to listen for the 'upgrade' event.

const http = require('http');

// Create an HTTP server
var srv = http.createServer( (req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('okay');
});
srv.on('upgrade', (req, socket, head) => {
  socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
               'Upgrade: WebSocket\r\n' +
               'Connection: Upgrade\r\n' +
               '\r\n');

  socket.pipe(socket); // echo back
});

// now that server is running
srv.listen(1337, '127.0.0.1', () => {

  // make a request
  var options = {
    port: 1337,
    hostname: '127.0.0.1',
    headers: {
      'Connection': 'Upgrade',
      'Upgrade': 'websocket'
    }
  };

  var req = http.request(options);
  req.end();

  req.on('upgrade', (res, socket, upgradeHead) => {
    console.log('got upgraded!');
    socket.end();
    process.exit(0);
  });
});

request.abort()#

Added in: v0.3.8

Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

request.end([data][, encoding][, callback])#

Added in: v0.1.90

Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating '0\r\n\r\n'.

If data is specified, it is equivalent to calling response.write(data, encoding) followed by request.end(callback).

If callback is specified, it will be called when the request stream is finished.

request.flushHeaders()#

Added in: v1.6.0

Flush the request headers.

For efficiency reasons, Node.js normally buffers the request headers until you call request.end() or write the first chunk of request data. It then tries hard to pack the request headers and data into a single TCP packet.

That's usually what you want (it saves a TCP round-trip) but not when the first data is not sent until possibly much later. request.flushHeaders() lets you bypass the optimization and kickstart the request.

request.setNoDelay([noDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setNoDelay() will be called.

request.setSocketKeepAlive([enable][, initialDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setKeepAlive() will be called.

request.setTimeout(timeout[, callback])#

Added in: v0.5.9
  • timeout <Number> Milliseconds before a request is considered to be timed out.
  • callback <Function> Optional function to be called when a timeout occurs. Same as binding to the timeout event.

Once a socket is assigned to this request and is connected socket.setTimeout() will be called.

Returns request.

request.write(chunk[, encoding][, callback])#

Added in: v0.1.29

Sends a chunk of the body. By calling this method many times, the user can stream a request body to a server--in that case it is suggested to use the ['Transfer-Encoding', 'chunked'] header line when creating the request.

The encoding argument is optional and only applies when chunk is a string. Defaults to 'utf8'.

The callback argument is optional and will be called when this chunk of data is flushed.

Returns request.

Class: http.Server#

Added in: v0.1.17

This class inherits from net.Server and has the following additional events:

Event: 'checkContinue'#

Added in: v0.3.0

Emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'checkExpectation'#

Added in: v5.5.0

Emitted each time a request with an HTTP Expect header is received, where the value is not 100-continue. If this event is not listened for, the server will automatically respond with a 417 Expectation Failed as appropriate.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'clientError'#

Added in: v0.1.94

If a client connection emits an 'error' event, it will be forwarded here. Listener of this event is responsible for closing/destroying the underlying socket. For example, one may wish to more gracefully close the socket with an HTTP '400 Bad Request' response instead of abruptly severing the connection.

Default behavior is to destroy the socket immediately on malformed request.

socket is the net.Socket object that the error originated from.

const http = require('http');

const server = http.createServer((req, res) => {
  res.end();
});
server.on('clientError', (err, socket) => {
  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);

When the 'clientError' event occurs, there is no request or response object, so any HTTP response sent, including response headers and payload, must be written directly to the socket object. Care must be taken to ensure the response is a properly formatted HTTP response message.

Event: 'close'#

Added in: v0.1.4

Emitted when the server closes.

Event: 'connect'#

Added in: v0.7.0

Emitted each time a client requests an HTTP CONNECT method. If this event is not listened for, then clients requesting a CONNECT method will have their connections closed.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning you will need to bind to it in order to handle data sent to the server on that socket.

Event: 'connection'#

Added in: v0.1.0

When a new TCP stream is established. socket is an object of type net.Socket. Usually users will not want to access this event. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket can also be accessed at request.connection.

Event: 'request'#

Added in: v0.1.0

Emitted each time there is a request. Note that there may be multiple requests per connection (in the case of HTTP Keep-Alive connections).

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a client requests an HTTP upgrade. If this event is not listened for, then clients requesting an upgrade will have their connections closed.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning you will need to bind to it in order to handle data sent to the server on that socket.

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections. See net.Server.close().

server.listen(handle[, callback])#

Added in: v0.5.10

The handle object can be set to either a server or socket (anything with an underlying _handle member), or a {fd: <n>} object.

This will cause the server to accept connections on the specified handle, but it is presumed that the file descriptor or handle has already been bound to a port or domain socket.

Listening on a file descriptor is not supported on Windows.

This function is asynchronous. callback will be added as a listener for the 'listening' event. See also net.Server.listen().

Returns server.

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listen(path[, callback])#

Added in: v0.1.90

Start a UNIX socket server listening for connections on the given path.

This function is asynchronous. callback will be added as a listener for the 'listening' event. See also net.Server.listen(path).

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listen([port][, hostname][, backlog][, callback])#

Added in: v0.1.90

Begin accepting connections on the specified port and hostname. If the hostname is omitted, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise. Omit the port argument, or use a port value of 0, to have the operating system assign a random port, which can be retrieved by using server.address().port after the 'listening' event has been emitted.

To listen to a unix socket, supply a filename instead of port and hostname.

backlog is the maximum length of the queue of pending connections. The actual length will be determined by your OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on linux. The default value of this parameter is 511 (not 512).

This function is asynchronous. callback will be added as a listener for the 'listening' event. See also net.Server.listen(port).

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listening#

Added in: v5.7.0

A Boolean indicating whether or not the server is listening for connections.

server.maxHeadersCount#

Added in: v0.7.0

Limits maximum incoming headers count, equal to 1000 by default. If set to 0 - no limit will be applied.

server.setTimeout(msecs, callback)#

Added in: v0.9.12

Sets the timeout value for sockets, and emits a 'timeout' event on the Server object, passing the socket as an argument, if a timeout occurs.

If there is a 'timeout' event listener on the Server object, then it will be called with the timed-out socket as an argument.

By default, the Server's timeout value is 2 minutes, and sockets are destroyed automatically if they time out. However, if you assign a callback to the Server's 'timeout' event, then you are responsible for handling socket timeouts.

Returns server.

server.timeout#

Added in: v0.9.12

The number of milliseconds of inactivity before a socket is presumed to have timed out.

Note that the socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

Set to 0 to disable any kind of automatic timeout behavior on incoming connections.

Class: http.ServerResponse#

Added in: v0.1.17

This object is created internally by an HTTP server--not by the user. It is passed as the second parameter to the 'request' event.

The response implements, but does not inherit from, the Writable Stream interface. This is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.6.7

Indicates that the underlying connection was terminated before response.end() was called or able to flush.

Event: 'finish'#

Added in: v0.3.6

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.

After this event, no more events will be emitted on the response object.

response.addTrailers(headers)#

Added in: v0.3.0

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.

Note that HTTP requires the Trailer header to be sent if you intend to emit trailers, with a list of the header fields in its value. E.g.,

response.writeHead(200, { 'Content-Type': 'text/plain',
                          'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667'});
response.end();

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.end([data][, encoding][, callback])#

Added in: v0.1.90

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end(), MUST be called on each response.

If data is specified, it is equivalent to calling response.write(data, encoding) followed by response.end(callback).

If callback is specified, it will be called when the response stream is finished.

response.finished#

Added in: v0.0.2

Boolean value that indicates whether the response has completed. Starts as false. After response.end() executes, the value will be true.

response.getHeader(name)#

Added in: v0.4.0

Reads out a header that's already been queued but not sent to the client. Note that the name is case insensitive.

Example:

var contentType = response.getHeader('content-type');

response.headersSent#

Added in: v0.9.3

Boolean (read-only). True if headers were sent, false otherwise.

response.removeHeader(name)#

Added in: v0.4.0

Removes a header that's queued for implicit sending.

Example:

response.removeHeader('Content-Encoding');

response.sendDate#

Added in: v0.7.5

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

response.setHeader(name, value)#

Added in: v0.4.0

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here if you need to send multiple headers with the same name.

Example:

response.setHeader('Content-Type', 'text/html');

or

response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// returns content-type = text/plain
const server = http.createServer((req,res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('ok');
});

response.setTimeout(msecs, callback)#

Added in: v0.9.12

Sets the Socket's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then sockets are destroyed when they time out. If you assign a handler on the request, the response, or the server's 'timeout' events, then it is your responsibility to handle timed out sockets.

Returns response.

response.statusCode#

Added in: v0.4.0

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

Example:

response.statusCode = 404;

After response header was sent to the client, this property indicates the status code which was sent out.

response.statusMessage#

Added in: v0.11.8

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left as undefined then the standard message for the status code will be used.

Example:

response.statusMessage = 'Not found';

After response header was sent to the client, this property indicates the status message which was sent out.

response.write(chunk[, encoding][, callback])#

Added in: v0.1.29

If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. By default the encoding is 'utf8'. callback will be called when this chunk of data is flushed.

Note: This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time response.write() is called, it will send the buffered header information and the first body to the client. The second time response.write() is called, Node.js assumes you're going to be streaming data, and sends that separately. That is, the response is buffered up to the first chunk of body.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

response.writeContinue()#

Added in: v0.3.0

Sends a HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the 'checkContinue' event on Server.

response.writeHead(statusCode[, statusMessage][, headers])#

Added in: v0.1.30

Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404. The last argument, headers, are the response headers. Optionally one can give a human-readable statusMessage as the second argument.

Example:

var body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });

This method must only be called once on a message and it must be called before response.end() is called.

If you call response.write() or response.end() before calling this, the implicit/mutable headers will be calculated and call this function for you.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// returns content-type = text/plain
const server = http.createServer((req,res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('ok');
});

Note that Content-Length is given in bytes not characters. The above example works because the string 'hello world' contains only single byte characters. If the body contains higher coded characters then Buffer.byteLength() should be used to determine the number of bytes in a given encoding. And Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

Class: http.IncomingMessage#

Added in: v0.1.17

An IncomingMessage object is created by http.Server or http.ClientRequest and passed as the first argument to the 'request' and 'response' event respectively. It may be used to access response status, headers and data.

It implements the Readable Stream interface, as well as the following additional events, methods, and properties.

Event: 'aborted'#

Added in: v0.3.8

Emitted when the request has been aborted by the client and the network socket has closed.

Event: 'close'#

Added in: v0.4.2

Indicates that the underlying connection was closed. Just like 'end', this event occurs only once per response.

message.destroy([error])#

Added in: v0.3.0

Calls destroy() on the socket that received the IncomingMessage. If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event.

message.headers#

Added in: v0.1.5

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased. Example:

// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);

Duplicates in raw headers are handled in the following ways, depending on the header name:

  • Duplicates of age, authorization, content-length, content-type, etag, expires, from, host, if-modified-since, if-unmodified-since, last-modified, location, max-forwards, proxy-authorization, referer, retry-after, or user-agent are discarded.
  • set-cookie is always an array. Duplicates are added to the array.
  • For all other headers, the values are joined together with ', '.

message.httpVersion#

Added in: v0.1.1

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Probably either '1.1' or '1.0'.

Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second.

message.method#

Added in: v0.1.1

Only valid for request obtained from http.Server.

The request method as a string. Read only. Example: 'GET', 'DELETE'.

message.rawHeaders#

Added in: v0.11.6

The raw request/response headers list exactly as they were received.

Note that the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);

message.rawTrailers#

Added in: v0.11.6

The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event.

message.setTimeout(msecs, callback)#

Added in: v0.5.9

Calls message.connection.setTimeout(msecs, callback).

Returns message.

message.statusCode#

Added in: v0.1.1

Only valid for response obtained from http.ClientRequest.

The 3-digit HTTP response status code. E.G. 404.

message.statusMessage#

Added in: v0.11.10

Only valid for response obtained from http.ClientRequest.

The HTTP response status message (reason phrase). E.G. OK or Internal Server Error.

message.socket#

Added in: v0.3.0

The net.Socket object associated with the connection.

With HTTPS support, use request.socket.getPeerCertificate() to obtain the client's authentication details.

message.trailers#

Added in: v0.3.0

The request/response trailers object. Only populated at the 'end' event.

message.url#

Added in: v0.1.90

Only valid for request obtained from http.Server.

Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:

GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n

Then request.url will be:

'/status?name=ryan'

If you would like to parse the URL into its parts, you can use require('url').parse(request.url). Example:

$ node
> require('url').parse('/status?name=ryan')
{
  href: '/status?name=ryan',
  search: '?name=ryan',
  query: 'name=ryan',
  pathname: '/status'
}

If you would like to extract the parameters from the query string, you can use the require('querystring').parse function, or pass true as the second argument to require('url').parse. Example:

$ node
> require('url').parse('/status?name=ryan', true)
{
  href: '/status?name=ryan',
  search: '?name=ryan',
  query: {name: 'ryan'},
  pathname: '/status'
}

http.METHODS#

Added in: v0.11.8

A list of the HTTP methods that are supported by the parser.

http.STATUS_CODES#

Added in: v0.1.22

A collection of all the standard HTTP response status codes, and the short description of each. For example, http.STATUS_CODES[404] === 'Not Found'.

http.createServer([requestListener])#

Added in: v0.1.13

Returns a new instance of http.Server.

The requestListener is a function which is automatically added to the 'request' event.

http.get(options[, callback])#

Added in: v0.3.6

Since most requests are GET requests without bodies, Node.js provides this convenience method. The only difference between this method and http.request() is that it sets the method to GET and calls req.end() automatically. Note that response data must be consumed in the callback for reasons stated in http.ClientRequest section.

The callback is invoked with a single argument that is an instance of http.IncomingMessage

JSON Fetching Example:

http.get('http://nodejs.org/dist/index.json', (res) => {
  const statusCode = res.statusCode;
  const contentType = res.headers['content-type'];

  let error;
  if (statusCode !== 200) {
    error = new Error(`Request Failed.\n` +
                      `Status Code: ${statusCode}`);
  } else if (!/^application\/json/.test(contentType)) {
    error = new Error(`Invalid content-type.\n` +
                      `Expected application/json but received ${contentType}`);
  }
  if (error) {
    console.log(error.message);
    // consume response data to free up memory
    res.resume();
    return;
  }

  res.setEncoding('utf8');
  let rawData = '';
  res.on('data', (chunk) => rawData += chunk);
  res.on('end', () => {
    try {
      let parsedData = JSON.parse(rawData);
      console.log(parsedData);
    } catch (e) {
      console.log(e.message);
    }
  });
}).on('error', (e) => {
  console.log(`Got error: ${e.message}`);
});

http.globalAgent#

Added in: v0.5.9

Global instance of Agent which is used as the default for all HTTP client requests.

http.request(options[, callback])#

Added in: v0.3.6
  • options <Object>
    • protocol <String> Protocol to use. Defaults to 'http:'.
    • host <String> A domain name or IP address of the server to issue the request to. Defaults to 'localhost'.
    • hostname <String> Alias for host. To support url.parse(), hostname is preferred over host.
    • family <Number> IP address family to use when resolving host and hostname. Valid values are 4 or 6. When unspecified, both IP v4 and v6 will be used.
    • port <Number> Port of remote server. Defaults to 80.
    • localAddress <String> Local interface to bind for network connections.
    • socketPath <String> Unix Domain Socket (use one of host:port or socketPath).
    • method <String> A string specifying the HTTP request method. Defaults to 'GET'.
    • path <String> Request path. Defaults to '/'. Should include query string if any. E.G. '/index.html?page=12'. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future.
    • headers <Object> An object containing request headers.
    • auth <String> Basic authentication i.e. 'user:password' to compute an Authorization header.
    • agent <http.Agent> | <Boolean> Controls Agent behavior. Possible values:
      • undefined (default): use http.globalAgent for this host and port.
      • Agent object: explicitly use the passed in Agent.
      • false: causes a new Agent with default values to be used.
    • createConnection <Function> A function that produces a socket/stream to use for the request when the agent option is not used. This can be used to avoid creating a custom Agent class just to override the default createConnection function. See agent.createConnection() for more details.
    • timeout <Integer>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
  • callback <Function>
  • Returns: <http.ClientRequest>

Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.

options can be an object or a string. If options is a string, it is automatically parsed with url.parse().

The optional callback parameter will be added as a one time listener for the 'response' event.

http.request() returns an instance of the http.ClientRequest class. The ClientRequest instance is a writable stream. If one needs to upload a file with a POST request, then write to the ClientRequest object.

Example:

var postData = querystring.stringify({
  'msg' : 'Hello World!'
});

var options = {
  hostname: 'www.google.com',
  port: 80,
  path: '/upload',
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded',
    'Content-Length': Buffer.byteLength(postData)
  }
};

var req = http.request(options, (res) => {
  console.log(`STATUS: ${res.statusCode}`);
  console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
  res.setEncoding('utf8');
  res.on('data', (chunk) => {
    console.log(`BODY: ${chunk}`);
  });
  res.on('end', () => {
    console.log('No more data in response.');
  });
});

req.on('error', (e) => {
  console.log(`problem with request: ${e.message}`);
});

// write data to request body
req.write(postData);
req.end();

Note that in the example req.end() was called. With http.request() one must always call req.end() to signify that you're done with the request - even if there is no data being written to the request body.

If any error is encountered during the request (be that with DNS resolution, TCP level errors, or actual HTTP parse errors) an 'error' event is emitted on the returned request object. As with all 'error' events, if no listeners are registered the error will be thrown.

There are a few special headers that should be noted.

  • Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.

  • Sending a 'Content-Length' header will disable the default chunked encoding.

  • Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', you should both set a timeout and listen for the 'continue' event. See RFC2616 Section 8.2.3 for more information.

  • Sending an Authorization header will override using the auth option to compute basic authentication.

HTTPS#

Stability: 2 - Stable

HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.

Class: https.Agent#

Added in: v0.4.5

An Agent object for HTTPS similar to http.Agent. See https.request() for more information.

Class: https.Server#

Added in: v0.3.4

This class is a subclass of tls.Server and emits events same as http.Server. See http.Server for more information.

server.setTimeout(msecs, callback)#

Added in: v0.11.2

See http.Server#setTimeout().

server.timeout#

Added in: v0.11.2

See http.Server#timeout.

https.createServer(options[, requestListener])#

Added in: v0.3.4

Returns a new HTTPS web server object. The options is similar to tls.createServer(). The requestListener is a function which is automatically added to the 'request' event.

Example:

// curl -k https://localhost:8000/
const https = require('https');
const fs = require('fs');

const options = {
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

Or

const https = require('https');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('server.pfx')
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

server.close([callback])#

Added in: v0.1.90

See http.close() for details.

server.listen(handle[, callback])#

server.listen(path[, callback])#

server.listen(port[, host][, backlog][, callback])#

See http.listen() for details.

https.get(options, callback)#

Added in: v0.3.6

Like http.get() but for HTTPS.

options can be an object or a string. If options is a string, it is automatically parsed with url.parse().

Example:

const https = require('https');

https.get('https://encrypted.google.com/', (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });

}).on('error', (e) => {
  console.error(e);
});

https.globalAgent#

Added in: v0.5.9

Global instance of https.Agent for all HTTPS client requests.

https.request(options, callback)#

Added in: v0.3.6

Makes a request to a secure web server.

options can be an object or a string. If options is a string, it is automatically parsed with url.parse().

All options from http.request() are valid.

Example:

const https = require('https');

var options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET'
};

var req = https.request(options, (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });
});

req.on('error', (e) => {
  console.error(e);
});
req.end();

The options argument has the following options

  • host: A domain name or IP address of the server to issue the request to. Defaults to 'localhost'.
  • hostname: Alias for host. To support url.parse() hostname is preferred over host.
  • family: IP address family to use when resolving host and hostname. Valid values are 4 or 6. When unspecified, both IP v4 and v6 will be used.
  • port: Port of remote server. Defaults to 443.
  • localAddress: Local interface to bind for network connections.
  • socketPath: Unix Domain Socket (use one of host:port or socketPath).
  • method: A string specifying the HTTP request method. Defaults to 'GET'.
  • path: Request path. Defaults to '/'. Should include query string if any. E.G. '/index.html?page=12'. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future.
  • headers: An object containing request headers.
  • auth: Basic authentication i.e. 'user:password' to compute an Authorization header.
  • agent: Controls Agent behavior. When an Agent is used request will default to Connection: keep-alive. Possible values:
    • undefined (default): use globalAgent for this host and port.
    • Agent object: explicitly use the passed in Agent.
    • false: opts out of connection pooling with an Agent, defaults request to Connection: close.

The following options from tls.connect() can also be specified:

  • pfx: Certificate, Private key and CA certificates to use for SSL. Default null.
  • key: Private key to use for SSL. Default null.
  • passphrase: A string of passphrase for the private key or pfx. Default null.
  • cert: Public x509 certificate to use. Default null.
  • ca: A string, Buffer or array of strings or Buffers of trusted certificates in PEM format. If this is omitted several well known "root" CAs will be used, like VeriSign. These are used to authorize connections.
  • ciphers: A string describing the ciphers to use or exclude. Consult https://www.openssl.org/docs/man1.0.2/apps/ciphers.html#CIPHER-LIST-FORMAT for details on the format.
  • rejectUnauthorized: If true, the server certificate is verified against the list of supplied CAs. An 'error' event is emitted if verification fails. Verification happens at the connection level, before the HTTP request is sent. Default true.
  • secureProtocol: The SSL method to use, e.g. SSLv3_method to force SSL version 3. The possible values depend on your installation of OpenSSL and are defined in the constant SSL_METHODS.
  • servername: Servername for SNI (Server Name Indication) TLS extension.

In order to specify these options, use a custom Agent.

Example:

var options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};
options.agent = new https.Agent(options);

var req = https.request(options, (res) => {
  ...
});

Alternatively, opt out of connection pooling by not using an Agent.

Example:

var options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
  agent: false
};

var req = https.request(options, (res) => {
  ...
});

Modules#

Stability: 3 - Locked

Node.js has a simple module loading system. In Node.js, files and modules are in one-to-one correspondence (each file is treated as a separate module).

As an example, consider a file named foo.js:

const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);

On the first line, foo.js loads the module circle.js that is in the same directory as foo.js.

Here are the contents of circle.js:

const PI = Math.PI;

exports.area = (r) => PI * r * r;

exports.circumference = (r) => 2 * PI * r;

The module circle.js has exported the functions area() and circumference(). To add functions and objects to the root of your module, you can add them to the special exports object.

Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable PI is private to circle.js.

If you want the root of your module's export to be a function (such as a constructor) or if you want to export a complete object in one assignment instead of building it one property at a time, assign it to module.exports instead of exports.

Below, bar.js makes use of the square module, which exports a constructor:

const square = require('./square.js');
var mySquare = square(2);
console.log(`The area of my square is ${mySquare.area()}`);

The square module is defined in square.js:

// assigning to exports will not modify module, must use module.exports
module.exports = (width) => {
  return {
    area: () => width * width
  };
}

The module system is implemented in the require("module") module.

Accessing the main module#

When a file is run directly from Node.js, require.main is set to its module. That means that you can determine whether a file has been run directly by testing

require.main === module

For a file foo.js, this will be true if run via node foo.js, but false if run by require('./foo').

Because module provides a filename property (normally equivalent to __filename), the entry point of the current application can be obtained by checking require.main.filename.

Addenda: Package Manager Tips#

The semantics of Node.js's require() function were designed to be general enough to support a number of reasonable directory structures. Package manager programs such as dpkg, rpm, and npm will hopefully find it possible to build native packages from Node.js modules without modification.

Below we give a suggested directory structure that could work:

Let's say that we wanted to have the folder at /usr/lib/node/<some-package>/<some-version> hold the contents of a specific version of a package.

Packages can depend on one another. In order to install package foo, you may have to install a specific version of package bar. The bar package may itself have dependencies, and in some cases, these dependencies may even collide or form cycles.

Since Node.js looks up the realpath of any modules it loads (that is, resolves symlinks), and then looks for their dependencies in the node_modules folders as described here, this situation is very simple to resolve with the following architecture:

  • /usr/lib/node/foo/1.2.3/ - Contents of the foo package, version 1.2.3.
  • /usr/lib/node/bar/4.3.2/ - Contents of the bar package that foo depends on.
  • /usr/lib/node/foo/1.2.3/node_modules/bar - Symbolic link to /usr/lib/node/bar/4.3.2/.
  • /usr/lib/node/bar/4.3.2/node_modules/* - Symbolic links to the packages that bar depends on.

Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.

When the code in the foo package does require('bar'), it will get the version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar. Then, when the code in the bar package calls require('quux'), it'll get the version that is symlinked into /usr/lib/node/bar/4.3.2/node_modules/quux.

Furthermore, to make the module lookup process even more optimal, rather than putting packages directly in /usr/lib/node, we could put them in /usr/lib/node_modules/<name>/<version>. Then Node.js will not bother looking for missing dependencies in /usr/node_modules or /node_modules.

In order to make modules available to the Node.js REPL, it might be useful to also add the /usr/lib/node_modules folder to the $NODE_PATH environment variable. Since the module lookups using node_modules folders are all relative, and based on the real path of the files making the calls to require(), the packages themselves can be anywhere.

All Together...#

To get the exact filename that will be loaded when require() is called, use the require.resolve() function.

Putting together all of the above, here is the high-level algorithm in pseudocode of what require.resolve does:

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
3. LOAD_NODE_MODULES(X, dirname(Y))
4. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text.  STOP
2. If X.js is a file, load X.js as JavaScript text.  STOP
3. If X.json is a file, parse X.json to a JavaScript Object.  STOP
4. If X.node is a file, load X.node as binary addon.  STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for "main" field.
   b. let M = X + (json main field)
   c. LOAD_AS_FILE(M)
2. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
3. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
4. If X/index.node is a file, load X/index.node as binary addon.  STOP

LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_AS_FILE(DIR/X)
   b. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0,
   a. if PARTS[I] = "node_modules" CONTINUE
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIRS + DIR
   d. let I = I - 1
5. return DIRS

Caching#

Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same object returned, if it would resolve to the same file.

Multiple calls to require('foo') may not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles.

If you want to have a module execute code multiple times, then export a function, and call that function.

Module Caching Caveats#

Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from node_modules folders), it is not a guarantee that require('foo') will always return the exact same object, if it would resolve to different files.

Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, require('./foo') and require('./FOO') return two different objects, irrespective of whether or not ./foo and ./FOO are the same file.

Core Modules#

Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.

The core modules are defined within Node.js's source and are located in the lib/ folder.

Core modules are always preferentially loaded if their identifier is passed to require(). For instance, require('http') will always return the built in HTTP module, even if there is a file by that name.

Cycles#

When there are circular require() calls, a module might not have finished executing when it is returned.

Consider this situation:

a.js:

console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');

b.js:

console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

main.js:

console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done=%j, b.done=%j', a.done, b.done);

When main.js loads a.js, then a.js in turn loads b.js. At that point, b.js tries to load a.js. In order to prevent an infinite loop, an unfinished copy of the a.js exports object is returned to the b.js module. b.js then finishes loading, and its exports object is provided to the a.js module.

By the time main.js has loaded both modules, they're both finished. The output of this program would thus be:

$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done=true, b.done=true

If you have cyclic module dependencies in your program, make sure to plan accordingly.

File Modules#

If the exact filename is not found, then Node.js will attempt to load the required filename with the added extensions: .js, .json, and finally .node.

.js files are interpreted as JavaScript text files, and .json files are parsed as JSON text files. .node files are interpreted as compiled addon modules loaded with dlopen.

A required module prefixed with '/' is an absolute path to the file. For example, require('/home/marco/foo.js') will load the file at /home/marco/foo.js.

A required module prefixed with './' is relative to the file calling require(). That is, circle.js must be in the same directory as foo.js for require('./circle') to find it.

Without a leading '/', './', or '../' to indicate a file, the module must either be a core module or is loaded from a node_modules folder.

If the given path does not exist, require() will throw an Error with its code property set to 'MODULE_NOT_FOUND'.

Folders as Modules#

It is convenient to organize programs and libraries into self-contained directories, and then provide a single entry point to that library. There are three ways in which a folder may be passed to require() as an argument.

The first is to create a package.json file in the root of the folder, which specifies a main module. An example package.json file might look like this:

{ "name" : "some-library",
  "main" : "./lib/some-library.js" }

If this was in a folder at ./some-library, then require('./some-library') would attempt to load ./some-library/lib/some-library.js.

This is the extent of Node.js's awareness of package.json files.

Note: If the file specified by the "main" entry of package.json is missing and can not be resolved, Node.js will report the entire module as missing with the default error:

Error: Cannot find module 'some-library'

If there is no package.json file present in the directory, then Node.js will attempt to load an index.js or index.node file out of that directory. For example, if there was no package.json file in the above example, then require('./some-library') would attempt to load:

  • ./some-library/index.js
  • ./some-library/index.node

Loading from node_modules Folders#

If the module identifier passed to require() is not a core module, and does not begin with '/', '../', or './', then Node.js starts at the parent directory of the current module, and adds /node_modules, and attempts to load the module from that location. Node will not append node_modules to a path already ending in node_modules.

If it is not found there, then it moves to the parent directory, and so on, until the root of the file system is reached.

For example, if the file at '/home/ry/projects/foo.js' called require('bar.js'), then Node.js would look in the following locations, in this order:

  • /home/ry/projects/node_modules/bar.js
  • /home/ry/node_modules/bar.js
  • /home/node_modules/bar.js
  • /node_modules/bar.js

This allows programs to localize their dependencies, so that they do not clash.

You can require specific files or sub modules distributed with a module by including a path suffix after the module name. For instance require('example-module/path/to/file') would resolve path/to/file relative to where example-module is located. The suffixed path follows the same module resolution semantics.

Loading from the global folders#

If the NODE_PATH environment variable is set to a colon-delimited list of absolute paths, then Node.js will search those paths for modules if they are not found elsewhere. (Note: On Windows, NODE_PATH is delimited by semicolons instead of colons.)

NODE_PATH was originally created to support loading modules from varying paths before the current module resolution algorithm was frozen.

NODE_PATH is still supported, but is less necessary now that the Node.js ecosystem has settled on a convention for locating dependent modules. Sometimes deployments that rely on NODE_PATH show surprising behavior when people are unaware that NODE_PATH must be set. Sometimes a module's dependencies change, causing a different version (or even a different module) to be loaded as the NODE_PATH is searched.

Additionally, Node.js will search in the following locations:

  • 1: $HOME/.node_modules
  • 2: $HOME/.node_libraries
  • 3: $PREFIX/lib/node

Where $HOME is the user's home directory, and $PREFIX is Node.js's configured node_prefix.

These are mostly for historic reasons. You are highly encouraged to place your dependencies locally in node_modules folders. They will be loaded faster, and more reliably.

The module wrapper#

Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:

(function (exports, require, module, __filename, __dirname) {
// Your module code actually lives in here
});

By doing this, Node.js achieves a few things:

  • It keeps top-level variables (defined with var, const or let) scoped to the module rather than the global object.
  • It helps to provide some global-looking variables that are actually specific to the module, such as:
    • The module and exports objects that the implementor can use to export values from the module.
    • The convenience variables __filename and __dirname, containing the module's absolute filename and directory path.

The module Object#

Added in: v0.1.16

In each module, the module free variable is a reference to the object representing the current module. For convenience, module.exports is also accessible via the exports module-global. module is not actually a global but rather local to each module.

module.children#

Added in: v0.1.16

The module objects required by this one.

module.exports#

Added in: v0.1.16

The module.exports object is created by the Module system. Sometimes this is not acceptable; many want their module to be an instance of some class. To do this, assign the desired export object to module.exports. Note that assigning the desired object to exports will simply rebind the local exports variable, which is probably not what you want to do.

For example suppose we were making a module called a.js

const EventEmitter = require('events');

module.exports = new EventEmitter();

// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => {
  module.exports.emit('ready');
}, 1000);

Then in another file we could do

const a = require('./a');
a.on('ready', () => {
  console.log('module a is ready');
});

Note that assignment to module.exports must be done immediately. It cannot be done in any callbacks. This does not work:

x.js:

setTimeout(() => {
  module.exports = { a: 'hello' };
}, 0);

y.js:

const x = require('./x');
console.log(x.a);

exports shortcut#

Added in: v0.1.16

The exports variable is available within a module's file-level scope, and is assigned the value of module.exports before the module is evaluated.

It allows a shortcut, so that module.exports.f = ... can be written more succinctly as exports.f = .... However, be aware that like any variable, if a new value is assigned to exports, it is no longer bound to module.exports:

module.exports.hello = true; // Exported from require of module
exports = { hello: false };  // Not exported, only available in the module

When the module.exports property is being completely replaced by a new object, it is common to also reassign exports, for example:

module.exports = exports = function Constructor() {
    // ... etc.

To illustrate the behavior, imagine this hypothetical implementation of require(), which is quite similar to what is actually done by require():

function require(...) {
  var module = { exports: {} };
  ((module, exports) => {
    // Your module code here. In this example, define a function.
    function some_func() {};
    exports = some_func;
    // At this point, exports is no longer a shortcut to module.exports, and
    // this module will still export an empty default object.
    module.exports = some_func;
    // At this point, the module will now export some_func, instead of the
    // default object.
  })(module, module.exports);
  return module.exports;
}

module.filename#

Added in: v0.1.16

The fully resolved filename to the module.

module.id#

Added in: v0.1.16

The identifier for the module. Typically this is the fully resolved filename.

module.loaded#

Added in: v0.1.16

Whether or not the module is done loading, or is in the process of loading.

module.parent#

Added in: v0.1.16

The module that first required this one.

module.require(id)#

Added in: v0.5.1

The module.require method provides a way to load a module as if require() was called from the original module.

Note that in order to do this, you must get a reference to the module object. Since require() returns the module.exports, and the module is typically only available within a specific module's code, it must be explicitly exported in order to be used.

net#

Stability: 2 - Stable

The net module provides you with an asynchronous network wrapper. It contains functions for creating both servers and clients (called streams). You can include this module with require('net');.

Class: net.Server#

Added in: v0.1.90

This class is used to create a TCP or local server.

net.Server is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.5.0

Emitted when the server closes. Note that if connections exist, this event is not emitted until all connections are ended.

Event: 'connection'#

Added in: v0.1.90

Emitted when a new connection is made. socket is an instance of net.Socket.

Event: 'error'#

Added in: v0.1.90

Emitted when an error occurs. The 'close' event will be called directly following this event. See example in discussion of server.listen.

Event: 'listening'#

Added in: v0.1.90

Emitted when the server has been bound after calling server.listen.

server.address()#

Added in: v0.1.90

Returns the bound address, the address family name, and port of the server as reported by the operating system. Useful to find which port was assigned when getting an OS-assigned address. Returns an object with port, family, and address properties: { port: 12346, family: 'IPv4', address: '127.0.0.1' }

Example:

var server = net.createServer((socket) => {
  socket.end('goodbye\n');
}).on('error', (err) => {
  // handle errors here
  throw err;
});

// grab a random port.
server.listen(() => {
  console.log('opened server on', server.address());
});

Don't call server.address() until the 'listening' event has been emitted.

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. The optional callback will be called once the 'close' event occurs. Unlike that event, it will be called with an Error as its only argument if the server was not open when it was closed.

server.connections#

Added in: v0.2.0 Deprecated since: v0.9.7 
Stability: 0 - Deprecated: Use server.getConnections() instead.

The number of concurrent connections on the server.

This becomes null when sending a socket to a child with child_process.fork(). To poll forks and get current number of active connections use asynchronous server.getConnections instead.

server.getConnections(callback)#

Added in: v0.9.7

Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.

Callback should take two arguments err and count.

server.listen(handle[, backlog][, callback])#

Added in: v0.5.10

The handle object can be set to either a server or socket (anything with an underlying _handle member), or a {fd: <n>} object.

This will cause the server to accept connections on the specified handle, but it is presumed that the file descriptor or handle has already been bound to a port or domain socket.

Listening on a file descriptor is not supported on Windows.

This function is asynchronous. When the server has been bound, 'listening' event will be emitted. The last parameter callback will be added as a listener for the 'listening' event.

The parameter backlog behaves the same as in server.listen([port][, hostname][, backlog][, callback]).

server.listen(options[, callback])#

Added in: v0.11.14

The port, host, and backlog properties of options, as well as the optional callback function, behave as they do on a call to server.listen([port][, hostname][, backlog][, callback]). Alternatively, the path option can be used to specify a UNIX socket.

If exclusive is false (default), then cluster workers will use the same underlying handle, allowing connection handling duties to be shared. When exclusive is true, the handle is not shared, and attempted port sharing results in an error. An example which listens on an exclusive port is shown below.

server.listen({
  host: 'localhost',
  port: 80,
  exclusive: true
});

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listen(path[, backlog][, callback])#

Added in: v0.1.90

Start a local socket server listening for connections on the given path.

This function is asynchronous. When the server has been bound, 'listening' event will be emitted. The last parameter callback will be added as a listener for the 'listening' event.

On UNIX, the local domain is usually known as the UNIX domain. The path is a filesystem path name. It gets truncated to sizeof(sockaddr_un.sun_path) bytes, decreased by 1. It varies on different operating system between 91 and 107 bytes. The typical values are 107 on Linux and 103 on OS X. The path is subject to the same naming conventions and permissions checks as would be done on file creation, will be visible in the filesystem, and will persist until unlinked.

On Windows, the local domain is implemented using a named pipe. The path must refer to an entry in \\?\pipe\ or \\.\pipe\. Any characters are permitted, but the latter may do some processing of pipe names, such as resolving .. sequences. Despite appearances, the pipe name space is flat. Pipes will not persist, they are removed when the last reference to them is closed. Do not forget JavaScript string escaping requires paths to be specified with double-backslashes, such as:

net.createServer().listen(
    path.join('\\\\?\\pipe', process.cwd(), 'myctl'))

The parameter backlog behaves the same as in server.listen([port][, hostname][, backlog][, callback]).

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listen([port][, hostname][, backlog][, callback])#

Added in: v0.1.90

Begin accepting connections on the specified port and hostname. If the hostname is omitted, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise. Omit the port argument, or use a port value of 0, to have the operating system assign a random port, which can be retrieved by using server.address().port after the 'listening' event has been emitted.

Backlog is the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

This function is asynchronous. When the server has been bound, 'listening' event will be emitted. The last parameter callback will be added as a listener for the 'listening' event.

One issue some users run into is getting EADDRINUSE errors. This means that another server is already running on the requested port. One way of handling this would be to wait a second and then try again:

server.on('error', (e) => {
  if (e.code == 'EADDRINUSE') {
    console.log('Address in use, retrying...');
    setTimeout(() => {
      server.close();
      server.listen(PORT, HOST);
    }, 1000);
  }
});

(Note: All sockets in Node.js are set SO_REUSEADDR.)

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listening#

Added in: v5.7.0

A Boolean indicating whether or not the server is listening for connections.

server.maxConnections#

Added in: v0.2.0

Set this property to reject connections when the server's connection count gets high.

It is not recommended to use this option once a socket has been sent to a child with child_process.fork().

server.ref()#

Added in: v0.9.1

Opposite of unref, calling ref on a previously unrefd server will not let the program exit if it's the only server left (the default behavior). If the server is refd calling ref again will have no effect.

Returns server.

server.unref()#

Added in: v0.9.1

Calling unref on a server will allow the program to exit if this is the only active server in the event system. If the server is already unrefd calling unref again will have no effect.

Returns server.

Class: net.Socket#

Added in: v0.3.4

This object is an abstraction of a TCP or local socket. net.Socket instances implement a duplex Stream interface. They can be created by the user and used as a client (with connect()) or they can be created by Node.js and passed to the user through the 'connection' event of a server.

new net.Socket(options)#

Added in: v0.3.4

Construct a new socket object.

options is an object with the following defaults:

{
  fd: null,
  allowHalfOpen: false,
  readable: false,
  writable: false
}

fd allows you to specify the existing file descriptor of socket. Set readable and/or writable to true to allow reads and/or writes on this socket (NOTE: Works only when fd is passed). About allowHalfOpen, refer to createServer() and 'end' event.

net.Socket instances are EventEmitter with the following events:

Event: 'close'#

Added in: v0.1.90
  • had_error <Boolean> true if the socket had a transmission error.

Emitted once the socket is fully closed. The argument had_error is a boolean which says if the socket was closed due to a transmission error.

Event: 'connect'#

Added in: v0.1.90

Emitted when a socket connection is successfully established. See connect().

Event: 'data'#

Added in: v0.1.90

Emitted when data is received. The argument data will be a Buffer or String. Encoding of data is set by socket.setEncoding(). (See the Readable Stream section for more information.)

Note that the data will be lost if there is no listener when a Socket emits a 'data' event.

Event: 'drain'#

Added in: v0.1.90

Emitted when the write buffer becomes empty. Can be used to throttle uploads.

See also: the return values of socket.write()

Event: 'end'#

Added in: v0.1.90

Emitted when the other end of the socket sends a FIN packet.

By default (allowHalfOpen == false) the socket will destroy its file descriptor once it has written out its pending write queue. However, by setting allowHalfOpen == true the socket will not automatically end() its side allowing the user to write arbitrary amounts of data, with the caveat that the user is required to end() their side now.

Event: 'error'#

Added in: v0.1.90

Emitted when an error occurs. The 'close' event will be called directly following this event.

Event: 'lookup'#

Added in: v0.11.3

Emitted after resolving the hostname but before connecting. Not applicable to UNIX sockets.

Event: 'timeout'#

Added in: v0.1.90

Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.

See also: socket.setTimeout()

socket.address()#

Added in: v0.1.90

Returns the bound address, the address family name and port of the socket as reported by the operating system. Returns an object with three properties, e.g. { port: 12346, family: 'IPv4', address: '127.0.0.1' }

socket.bufferSize#

Added in: v0.3.8

net.Socket has the property that socket.write() always works. This is to help users get up and running quickly. The computer cannot always keep up with the amount of data that is written to a socket - the network connection simply might be too slow. Node.js will internally queue up the data written to a socket and send it out over the wire when it is possible. (Internally it is polling on the socket's file descriptor for being writable).

The consequence of this internal buffering is that memory may grow. This property shows the number of characters currently buffered to be written. (Number of characters is approximately equal to the number of bytes to be written, but the buffer may contain strings, and the strings are lazily encoded, so the exact number of bytes is not known.)

Users who experience large or growing bufferSize should attempt to "throttle" the data flows in their program with pause() and resume().

socket.bytesRead#

Added in: v0.5.3

The amount of received bytes.

socket.bytesWritten#

Added in: v0.5.3

The amount of bytes sent.

socket.connect(options[, connectListener])#

Added in: v0.1.90

Opens the connection for a given socket.

For TCP sockets, options argument should be an object which specifies:

  • port: Port the client should connect to (Required).

  • host: Host the client should connect to. Defaults to 'localhost'.

  • localAddress: Local interface to bind to for network connections.

  • localPort: Local port to bind to for network connections.

  • family : Version of IP stack. Defaults to 4.

  • hints: dns.lookup() hints. Defaults to 0.

  • lookup : Custom lookup function. Defaults to dns.lookup.

For local domain sockets, options argument should be an object which specifies:

  • path: Path the client should connect to (Required).

Normally this method is not needed, as net.createConnection opens the socket. Use this only if you are implementing a custom Socket.

This function is asynchronous. When the 'connect' event is emitted the socket is established. If there is a problem connecting, the 'connect' event will not be emitted, the 'error' event will be emitted with the exception.

The connectListener parameter will be added as a listener for the 'connect' event.

socket.connect(path[, connectListener])#

socket.connect(port[, host][, connectListener])#

Added in: v0.1.90

As socket.connect(options[, connectListener]), with options as either {port: port, host: host} or {path: path}.

socket.connecting#

Added in: v6.1.0

If true - socket.connect(options[, connectListener]) was called and haven't yet finished. Will be set to false before emitting connect event and/or calling socket.connect(options[, connectListener])'s callback.

socket.destroy([exception])#

Added in: v0.1.90

Ensures that no more I/O activity happens on this socket. Only necessary in case of errors (parse error or so).

If exception is specified, an 'error' event will be emitted and any listeners for that event will receive exception as an argument.

socket.destroyed#

A Boolean value that indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.

socket.end([data][, encoding])#

Added in: v0.1.90

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

If data is specified, it is equivalent to calling socket.write(data, encoding) followed by socket.end().

socket.localAddress#

Added in: v0.9.6

The string representation of the local IP address the remote client is connecting on. For example, if you are listening on '0.0.0.0' and the client connects on '192.168.1.1', the value would be '192.168.1.1'.

socket.localPort#

Added in: v0.9.6

The numeric representation of the local port. For example, 80 or 21.

socket.pause()#

Pauses the reading of data. That is, 'data' events will not be emitted. Useful to throttle back an upload.

socket.ref()#

Added in: v0.9.1

Opposite of unref, calling ref on a previously unrefd socket will not let the program exit if it's the only socket left (the default behavior). If the socket is refd calling ref again will have no effect.

Returns socket.

socket.remoteAddress#

Added in: v0.5.10

The string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

socket.remoteFamily#

Added in: v0.11.14

The string representation of the remote IP family. 'IPv4' or 'IPv6'.

socket.remotePort#

Added in: v0.5.10

The numeric representation of the remote port. For example, 80 or 21.

socket.resume()#

Resumes reading after a call to pause().

socket.setEncoding([encoding])#

Added in: v0.1.90

Set the encoding for the socket as a Readable Stream. See stream.setEncoding() for more information.

socket.setKeepAlive([enable][, initialDelay])#

Added in: v0.1.92

Enable/disable keep-alive functionality, and optionally set the initial delay before the first keepalive probe is sent on an idle socket. enable defaults to false.

Set initialDelay (in milliseconds) to set the delay between the last data packet received and the first keepalive probe. Setting 0 for initialDelay will leave the value unchanged from the default (or previous) setting. Defaults to 0.

Returns socket.

socket.setNoDelay([noDelay])#

Added in: v0.1.90

Disables the Nagle algorithm. By default TCP connections use the Nagle algorithm, they buffer data before sending it off. Setting true for noDelay will immediately fire off data each time socket.write() is called. noDelay defaults to true.

Returns socket.

socket.setTimeout(timeout[, callback])#

Added in: v0.1.90

Sets the socket to timeout after timeout milliseconds of inactivity on the socket. By default net.Socket do not have a timeout.

When an idle timeout is triggered the socket will receive a 'timeout' event but the connection will not be severed. The user must manually end() or destroy() the socket.

If timeout is 0, then the existing idle timeout is disabled.

The optional callback parameter will be added as a one time listener for the 'timeout' event.

Returns socket.

socket.unref()#

Added in: v0.9.1

Calling unref on a socket will allow the program to exit if this is the only active socket in the event system. If the socket is already unrefd calling unref again will have no effect.

Returns socket.

socket.write(data[, encoding][, callback])#

Added in: v0.1.90

Sends data on the socket. The second parameter specifies the encoding in the case of a string--it defaults to UTF8 encoding.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is again free.

The optional callback parameter will be executed when the data is finally written out - this may not be immediately.

net.connect(options[, connectListener])#

Added in: v0.7.0

A factory function, which returns a new net.Socket and automatically connects with the supplied options.

The options are passed to both the net.Socket constructor and the socket.connect method.

The connectListener parameter will be added as a listener for the 'connect' event once.

Here is an example of a client of the previously described echo server:

const net = require('net');
const client = net.connect({port: 8124}, () => {
  // 'connect' listener
  console.log('connected to server!');
  client.write('world!\r\n');
});
client.on('data', (data) => {
  console.log(data.toString());
  client.end();
});
client.on('end', () => {
  console.log('disconnected from server');
});

To connect on the socket /tmp/echo.sock the second line would just be changed to

const client = net.connect({path: '/tmp/echo.sock'});

net.connect(path[, connectListener])#

Added in: v0.1.90

A factory function, which returns a new unix net.Socket and automatically connects to the supplied path.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.connect(port[, host][, connectListener])#

Added in: v0.1.90

A factory function, which returns a new net.Socket and automatically connects to the supplied port and host.

If host is omitted, 'localhost' will be assumed.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.createConnection(options[, connectListener])#

Added in: v0.1.90

A factory function, which returns a new net.Socket and automatically connects with the supplied options.

The options are passed to both the net.Socket constructor and the socket.connect method.

Passing timeout as an option will call socket.setTimeout() after the socket is created, but before it is connecting.

The connectListener parameter will be added as a listener for the 'connect' event once.

Here is an example of a client of the previously described echo server:

const net = require('net');
const client = net.createConnection({port: 8124}, () => {
  //'connect' listener
  console.log('connected to server!');
  client.write('world!\r\n');
});
client.on('data', (data) => {
  console.log(data.toString());
  client.end();
});
client.on('end', () => {
  console.log('disconnected from server');
});

To connect on the socket /tmp/echo.sock the second line would just be changed to

const client = net.connect({path: '/tmp/echo.sock'});

net.createConnection(path[, connectListener])#

Added in: v0.1.90

A factory function, which returns a new unix net.Socket and automatically connects to the supplied path.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.createConnection(port[, host][, connectListener])#

Added in: v0.1.90

A factory function, which returns a new net.Socket and automatically connects to the supplied port and host.

If host is omitted, 'localhost' will be assumed.

The connectListener parameter will be added as a listener for the 'connect' event once.

net.createServer([options][, connectionListener])#

Added in: v0.5.0

Creates a new server. The connectionListener argument is automatically set as a listener for the 'connection' event.

options is an object with the following defaults:

{
  allowHalfOpen: false,
  pauseOnConnect: false
}

If allowHalfOpen is true, then the socket won't automatically send a FIN packet when the other end of the socket sends a FIN packet. The socket becomes non-readable, but still writable. You should call the end() method explicitly. See 'end' event for more information.

If pauseOnConnect is true, then the socket associated with each incoming connection will be paused, and no data will be read from its handle. This allows connections to be passed between processes without any data being read by the original process. To begin reading data from a paused socket, call resume().

Here is an example of an echo server which listens for connections on port 8124:

const net = require('net');
const server = net.createServer((c) => {
  // 'connection' listener
  console.log('client connected');
  c.on('end', () => {
    console.log('client disconnected');
  });
  c.write('hello\r\n');
  c.pipe(c);
});
server.on('error', (err) => {
  throw err;
});
server.listen(8124, () => {
  console.log('server bound');
});

Test this by using telnet:

telnet localhost 8124

To listen on the socket /tmp/echo.sock the third line from the last would just be changed to

server.listen('/tmp/echo.sock', () => {
  console.log('server bound');
});

Use nc to connect to a UNIX domain socket server:

nc -U /tmp/echo.sock

net.isIP(input)#

Added in: v0.3.0

Tests if input is an IP address. Returns 0 for invalid strings, returns 4 for IP version 4 addresses, and returns 6 for IP version 6 addresses.

net.isIPv4(input)#

Added in: v0.3.0

Returns true if input is a version 4 IP address, otherwise returns false.

net.isIPv6(input)#

Added in: v0.3.0

Returns true if input is a version 6 IP address, otherwise returns false.

OS#

Stability: 2 - Stable

The os module provides a number of operating system-related utility methods. It can be accessed using:

const os = require('os');

os.EOL#

Added in: v0.7.8

A string constant defining the operating system-specific end-of-line marker:

  • \n on POSIX
  • \r\n on Windows

os.arch()#

Added in: v0.5.0

The os.arch() method returns a string identifying the operating system CPU architecture for which the Node.js binary was compiled.

The current possible values are: 'arm', 'arm64', 'ia32', 'mips', 'mipsel', 'ppc', 'ppc64', 's390', 's390x', 'x32', 'x64', and 'x86'.

Equivalent to process.arch.

os.constants#

Added in: v6.3.0

Returns an object containing commonly used operating system specific constants for error codes, process signals, and so on. The specific constants currently defined are described in OS Constants.

os.cpus()#

Added in: v0.3.3

The os.cpus() method returns an array of objects containing information about each CPU/core installed.

The properties included on each object include:

  • model <String>
  • speed <number> (in MHz)
  • times <Object>
    • user <number> The number of milliseconds the CPU has spent in user mode.
    • nice <number> The number of milliseconds the CPU has spent in nice mode.
    • sys <number> The number of milliseconds the CPU has spent in sys mode.
    • idle <number> The number of milliseconds the CPU has spent in idle mode.
    • irq <number> The number of milliseconds the CPU has spent in irq mode.

For example:

[
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 252020,
      nice: 0,
      sys: 30340,
      idle: 1070356870,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 306960,
      nice: 0,
      sys: 26980,
      idle: 1071569080,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 248450,
      nice: 0,
      sys: 21750,
      idle: 1070919370,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 256880,
      nice: 0,
      sys: 19430,
      idle: 1070905480,
      irq: 20
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 511580,
      nice: 20,
      sys: 40900,
      idle: 1070842510,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 291660,
      nice: 0,
      sys: 34360,
      idle: 1070888000,
      irq: 10
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 308260,
      nice: 0,
      sys: 55410,
      idle: 1071129970,
      irq: 880
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 266450,
      nice: 1480,
      sys: 34920,
      idle: 1072572010,
      irq: 30
    }
  }
]

Note: Because nice values are UNIX-specific, on Windows the nice values of all processors are always 0.

os.endianness()#

Added in: v0.9.4

The os.endianness() method returns a string identifying the endianness of the CPU for which the Node.js binary was compiled.

Possible values are:

  • 'BE' for big endian
  • 'LE' for little endian.

os.freemem()#

Added in: v0.3.3

The os.freemem() method returns the amount of free system memory in bytes as an integer.

os.homedir()#

Added in: v2.3.0

The os.homedir() method returns the home directory of the current user as a string.

os.hostname()#

Added in: v0.3.3

The os.hostname() method returns the hostname of the operating system as a string.

os.loadavg()#

Added in: v0.3.3

The os.loadavg() method returns an array containing the 1, 5, and 15 minute load averages.

The load average is a measure of system activity, calculated by the operating system and expressed as a fractional number. As a rule of thumb, the load average should ideally be less than the number of logical CPUs in the system.

The load average is a UNIX-specific concept with no real equivalent on Windows platforms. On Windows, the return value is always [0, 0, 0].

os.networkInterfaces()#

Added in: v0.6.0

The os.networkInterfaces() method returns an object containing only network interfaces that have been assigned a network address.

Each key on the returned object identifies a network interface. The associated value is an array of objects that each describe an assigned network address.

The properties available on the assigned network address object include:

  • address <String> The assigned IPv4 or IPv6 address
  • netmask <String> The IPv4 or IPv6 network mask
  • family <String> Either IPv4 or IPv6
  • mac <String> The MAC address of the network interface
  • internal <boolean> true if the network interface is a loopback or similar interface that is not remotely accessible; otherwise false
  • scopeid <number> The numeric IPv6 scope ID (only specified when family is IPv6)
{
  lo: [
    {
      address: '127.0.0.1',
      netmask: '255.0.0.0',
      family: 'IPv4',
      mac: '00:00:00:00:00:00',
      internal: true
    },
    {
      address: '::1',
      netmask: 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff',
      family: 'IPv6',
      mac: '00:00:00:00:00:00',
      internal: true
    }
  ],
  eth0: [
    {
      address: '192.168.1.108',
      netmask: '255.255.255.0',
      family: 'IPv4',
      mac: '01:02:03:0a:0b:0c',
      internal: false
    },
    {
      address: 'fe80::a00:27ff:fe4e:66a1',
      netmask: 'ffff:ffff:ffff:ffff::',
      family: 'IPv6',
      mac: '01:02:03:0a:0b:0c',
      internal: false
    }
  ]
}

os.platform()#

Added in: v0.5.0

The os.platform() method returns a string identifying the operating system platform as set during compile time of Node.js.

Currently possible values are:

  • 'aix'
  • 'darwin'
  • 'freebsd'
  • 'linux'
  • 'openbsd'
  • 'sunos'
  • 'win32'

Equivalent to process.platform.

Note: The value 'android' may also be returned if the Node.js is built on the Android operating system. However, Android support in Node.js is considered to be experimental at this time.

os.release()#

Added in: v0.3.3

The os.release() method returns a string identifying the operating system release.

Note: On POSIX systems, the operating system release is determined by calling uname(3). On Windows, GetVersionExW() is used. Please see https://en.wikipedia.org/wiki/Uname#Examples for more information.

os.tmpdir()#

Added in: v0.9.9

The os.tmpdir() method returns a string specifying the operating system's default directory for temporary files.

os.totalmem()#

Added in: v0.3.3

The os.totalmem() method returns the total amount of system memory in bytes as an integer.

os.type()#

Added in: v0.3.3

The os.type() method returns a string identifying the operating system name as returned by uname(3). For example 'Linux' on Linux, 'Darwin' on OS X and 'Windows_NT' on Windows.

Please see https://en.wikipedia.org/wiki/Uname#Examples for additional information about the output of running uname(3) on various operating systems.

os.uptime()#

Added in: v0.3.3

The os.uptime() method returns the system uptime in number of seconds.

Note: Within Node.js' internals, this number is represented as a double. However, fractional seconds are not returned and the value can typically be treated as an integer.

os.userInfo(options)#

Added in: v6.0.0
  • options <Object>
    • encoding <String> Character encoding used to interpret resulting strings. If encoding is set to 'buffer', the username, shell, and homedir values will be Buffer instances. (Default: 'utf8')
  • Returns: <Object>

The os.userInfo() method returns information about the currently effective user -- on POSIX platforms, this is typically a subset of the password file. The returned object includes the username, uid, gid, shell, and homedir. On Windows, the uid and gid fields are -1, and shell is null.

The value of homedir returned by os.userInfo() is provided by the operating system. This differs from the result of os.homedir(), which queries several environment variables for the home directory before falling back to the operating system response.

OS Constants#

The following constants are exported by os.constants. Note: Not all constants will be available on every operating system.

Signal Constants#

The following signal constants are exported by os.constants.signals:

Constant Description
SIGHUP Sent to indicate when a controlling terminal is closed or a parent process exits.
SIGINT Sent to indicate when a user wishes to interrupt a process ((Ctrl+C)).
SIGQUIT Sent to indicate when a user wishes to terminate a process and perform a core dump.
SIGILL Sent to a process to notify that it has attempted to perform an illegal, malformed, unknown or privileged instruction.
SIGTRAP Sent to a process when an exception has occurred.
SIGABRT Sent to a process to request that it abort.
SIGIOT Synonym for SIGABRT
SIGBUS Sent to a process to notify that it has caused a bus error.
SIGFPE Sent to a process to notify that it has performed an illegal arithmetic operation.
SIGKILL Sent to a process to terminate it immediately.
SIGUSR1 SIGUSR2 Sent to a process to identify user-defined conditions.
SIGSEGV Sent to a process to notify of a segmentation fault.
SIGPIPE Sent to a process when it has attempted to write to a disconnected pipe.
SIGALRM Sent to a process when a system timer elapses.
SIGTERM Sent to a process to request termination.
SIGCHLD Sent to a process when a child process terminates.
SIGSTKFLT Sent to a process to indicate a stack fault on a coprocessor.
SIGCONT Sent to instruct the operating system to continue a paused process.
SIGSTOP Sent to instruct the operating system to halt a process.
SIGTSTP Sent to a process to request it to stop.
SIGBREAK Sent to indicate when a user wishes to interrupt a process.
SIGTTIN Sent to a process when it reads from the TTY while in the background.
SIGTTOU Sent to a process when it writes to the TTY while in the background.
SIGURG Sent to a process when a socket has urgent data to read.
SIGXCPU Sent to a process when it has exceeded its limit on CPU usage.
SIGXFSZ Sent to a process when it grows a file larger than the maximum allowed.
SIGVTALRM Sent to a process when a virtual timer has elapsed.
SIGPROF Sent to a process when a system timer has elapsed.
SIGWINCH Sent to a process when the controlling terminal has changed its size.
SIGIO Sent to a process when I/O is available.
SIGPOLL Synonym for SIGIO
SIGLOST Sent to a process when a file lock has been lost.
SIGPWR Sent to a process to notify of a power failure.
SIGINFO Synonym for SIGPWR
SIGSYS Sent to a process to notify of a bad argument.
SIGUNUSED Synonym for SIGSYS

Error Constants#

The following error constants are exported by os.constants.errno:

POSIX Error Constants#

Constant Description
E2BIG Indicates that the list of arguments is longer than expected.
EACCES Indicates that the operation did not have sufficient permissions.
EADDRINUSE Indicates that the network address is already in use.
EADDRNOTAVAIL Indicates that the network address is currently unavailable for use.
EAFNOSUPPORT Indicates that the network address family is not supported.
EAGAIN Indicates that there is currently no data available and to try the operation again later.
EALREADY Indicates that the socket already has a pending connection in progress.
EBADF Indicates that a file descriptor is not valid.
EBADMSG Indicates an invalid data message.
EBUSY Indicates that a device or resource is busy.
ECANCELED Indicates that an operation was canceled.
ECHILD Indicates that there are no child processes.
ECONNABORTED Indicates that the network connection has been aborted.
ECONNREFUSED Indicates that the network connection has been refused.
ECONNRESET Indicates that the network connection has been reset.
EDEADLK Indicates that a resource deadlock has been avoided.
EDESTADDRREQ Indicates that a destination address is required.
EDOM Indicates that an argument is out of the domain of the function.
EDQUOT Indicates that the disk quota has been exceeded.
EEXIST Indicates that the file already exists.
EFAULT Indicates an invalid pointer address.
EFBIG Indicates that the file is too large.
EHOSTUNREACH Indicates that the host is unreachable.
EIDRM Indicates that the identifier has been removed.
EILSEQ Indicates an illegal byte sequence.
EINPROGRESS Indicates that an operation is already in progress.
EINTR Indicates that a function call was interrupted.
EINVAL Indicates that an invalid argument was provided.
EIO Indicates an otherwise unspecified I/O error.
EISCONN Indicates that the socket is connected.
EISDIR Indicates that the path is a directory.
ELOOP Indicates too many levels of symbolic links in a path.
EMFILE Indicates that there are too many open files.
EMLINK Indicates that there are too many hard links to a file.
EMSGSIZE Indicates that the provided message is too long.
EMULTIHOP Indicates that a multihop was attempted.
ENAMETOOLONG Indicates that the filename is too long.
ENETDOWN Indicates that the network is down.
ENETRESET Indicates that the connection has been aborted by the network.
ENETUNREACH Indicates that the network is unreachable.
ENFILE Indicates too many open files in the system.
ENOBUFS Indicates that no buffer space is available.
ENODATA Indicates that no message is available on the stream head read queue.
ENODEV Indicates that there is no such device.
ENOENT Indicates that there is no such file or directory.
ENOEXEC Indicates an exec format error.
ENOLCK Indicates that there are no locks available.
ENOLINK Indications that a link has been severed.
ENOMEM Indicates that there is not enough space.
ENOMSG Indicates that there is no message of the desired type.
ENOPROTOOPT Indicates that a given protocol is not available.
ENOSPC Indicates that there is no space available on the device.
ENOSR Indicates that there are no stream resources available.
ENOSTR Indicates that a given resource is not a stream.
ENOSYS Indicates that a function has not been implemented.
ENOTCONN Indicates that the socket is not connected.
ENOTDIR Indicates that the path is not a directory.
ENOTEMPTY Indicates that the directory is not empty.
ENOTSOCK Indicates that the given item is not a socket.
ENOTSUP Indicates that a given operation is not supported.
ENOTTY Indicates an inappropriate I/O control operation.
ENXIO Indicates no such device or address.
EOPNOTSUPP Indicates that an operation is not supported on the socket. Note that while ENOTSUP and EOPNOTSUPP have the same value on Linux, according to POSIX.1 these error values should be distinct.)
EOVERFLOW Indicates that a value is too large to be stored in a given data type.
EPERM Indicates that the operation is not permitted.
EPIPE Indicates a broken pipe.
EPROTO Indicates a protocol error.
EPROTONOSUPPORT Indicates that a protocol is not supported.
EPROTOTYPE Indicates the wrong type of protocol for a socket.
ERANGE Indicates that the results are too large.
EROFS Indicates that the file system is read only.
ESPIPE Indicates an invalid seek operation.
ESRCH Indicates that there is no such process.
ESTALE Indicates that the file handle is stale.
ETIME Indicates an expired timer.
ETIMEDOUT Indicates that the connection timed out.
ETXTBSY Indicates that a text file is busy.
EWOULDBLOCK Indicates that the operation would block.
EXDEV Indicates an improper link.

Windows Specific Error Constants#

The following error codes are specific to the Windows operating system:

Constant Description
WSAEINTR Indicates an interrupted function call.
WSAEBADF Indicates an invalid file handle.
WSAEACCES Indicates insufficient permissions to complete the operation.
WSAEFAULT Indicates an invalid pointer address.
WSAEINVAL Indicates that an invalid argument was passed.
WSAEMFILE Indicates that there are too many open files.
WSAEWOULDBLOCK Indicates that a resource is temporarily unavailable.
WSAEINPROGRESS Indicates that an operation is currently in progress.
WSAEALREADY Indicates that an operation is already in progress.
WSAENOTSOCK Indicates that the resource is not a socket.
WSAEDESTADDRREQ Indicates that a destination address is required.
WSAEMSGSIZE Indicates that the message size is too long.
WSAEPROTOTYPE Indicates the wrong protocol type for the socket.
WSAENOPROTOOPT Indicates a bad protocol option.
WSAEPROTONOSUPPORT Indicates that the protocol is not supported.
WSAESOCKTNOSUPPORT Indicates that the socket type is not supported.
WSAEOPNOTSUPP Indicates that the operation is not supported.
WSAEPFNOSUPPORT Indicates that the protocol family is not supported.
WSAEAFNOSUPPORT Indicates that the address family is not supported.
WSAEADDRINUSE Indicates that the network address is already in use.
WSAEADDRNOTAVAIL Indicates that the network address is not available.
WSAENETDOWN Indicates that the network is down.
WSAENETUNREACH Indicates that the network is unreachable.
WSAENETRESET Indicates that the network connection has been reset.
WSAECONNABORTED Indicates that the connection has been aborted.
WSAECONNRESET Indicates that the connection has been reset by the peer.
WSAENOBUFS Indicates that there is no buffer space available.
WSAEISCONN Indicates that the socket is already connected.
WSAENOTCONN Indicates that the socket is not connected.
WSAESHUTDOWN Indicates that data cannot be sent after the socket has been shutdown.
WSAETOOMANYREFS Indicates that there are too many references.
WSAETIMEDOUT Indicates that the connection has timed out.
WSAECONNREFUSED Indicates that the connection has been refused.
WSAELOOP Indicates that a name cannot be translated.
WSAENAMETOOLONG Indicates that a name was too long.
WSAEHOSTDOWN Indicates that a network host is down.
WSAEHOSTUNREACH Indicates that there is no route to a network host.
WSAENOTEMPTY Indicates that the directory is not empty.
WSAEPROCLIM Indicates that there are too many processes.
WSAEUSERS Indicates that the user quota has been exceeded.
WSAEDQUOT Indicates that the disk quota has been exceeded.
WSAESTALE Indicates a stale file handle reference.
WSAEREMOTE Indicates that the item is remote.
WSASYSNOTREADY Indicates that the network subsystem is not ready.
WSAVERNOTSUPPORTED Indicates that the winsock.dll version is out of range.
WSANOTINITIALISED Indicates that successful WSAStartup has not yet been performed.
WSAEDISCON Indicates that a graceful shutdown is in progress.
WSAENOMORE Indicates that there are no more results.
WSAECANCELLED Indicates that an operation has been canceled.
WSAEINVALIDPROCTABLE Indicates that the procedure call table is invalid.
WSAEINVALIDPROVIDER Indicates an invalid service provider.
WSAEPROVIDERFAILEDINIT Indicates that the service provider failed to initialized.
WSASYSCALLFAILURE Indicates a system call failure.
WSASERVICE_NOT_FOUND Indicates that a service was not found.
WSATYPE_NOT_FOUND Indicates that a class type was not found.
WSA_E_NO_MORE Indicates that there are no more results.
WSA_E_CANCELLED Indicates that the call was canceled.
WSAEREFUSED Indicates that a database query was refused.

libuv Constants#

Constant Description
UV_UDP_REUSEADDR

Path#

Stability: 2 - Stable

The path module provides utilities for working with file and directory paths. It can be accessed using:

const path = require('path');

Windows vs. POSIX#

The default operation of the path module varies based on the operating system on which a Node.js application is running. Specifically, when running on a Windows operating system, the path module will assume that Windows-style paths are being used.

For example, using the path.basename() function with the Windows file path C:\temp\myfile.html, will yield different results when running on POSIX than when run on Windows:

On POSIX:

path.basename('C:\\temp\\myfile.html');
// Returns: 'C:\\temp\\myfile.html'

On Windows:

path.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'

To achieve consistent results when working with Windows file paths on any operating system, use path.win32:

On POSIX and Windows:

path.win32.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'

To achieve consistent results when working with POSIX file paths on any operating system, use path.posix:

On POSIX and Windows:

path.posix.basename('/tmp/myfile.html');
// Returns: 'myfile.html'

path.basename(path[, ext])#

Added in: v0.1.25

The path.basename() methods returns the last portion of a path, similar to the Unix basename command.

For example:

path.basename('/foo/bar/baz/asdf/quux.html')
// Returns: 'quux.html'

path.basename('/foo/bar/baz/asdf/quux.html', '.html')
// Returns: 'quux'

A TypeError is thrown if path is not a string or if ext is given and is not a string.

path.delimiter#

Added in: v0.9.3

Provides the platform-specific path delimiter:

  • ; for Windows
  • : for POSIX

For example, on POSIX:

console.log(process.env.PATH)
// Prints: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin'

process.env.PATH.split(path.delimiter)
// Returns: ['/usr/bin', '/bin', '/usr/sbin', '/sbin', '/usr/local/bin']

On Windows:

console.log(process.env.PATH)
// Prints: 'C:\Windows\system32;C:\Windows;C:\Program Files\node\'

process.env.PATH.split(path.delimiter)
// Returns: ['C:\\Windows\\system32', 'C:\\Windows', 'C:\\Program Files\\node\\']

path.dirname(path)#

Added in: v0.1.16

The path.dirname() method returns the directory name of a path, similar to the Unix dirname command.

For example:

path.dirname('/foo/bar/baz/asdf/quux')
// Returns: '/foo/bar/baz/asdf'

A TypeError is thrown if path is not a string.

path.extname(path)#

Added in: v0.1.25

The path.extname() method returns the extension of the path, from the last occurrence of the . (period) character to end of string in the last portion of the path. If there is no . in the last portion of the path, or if the first character of the basename of path (see path.basename()) is ., then an empty string is returned.

For example:

path.extname('index.html')
// Returns: '.html'

path.extname('index.coffee.md')
// Returns: '.md'

path.extname('index.')
// Returns: '.'

path.extname('index')
// Returns: ''

path.extname('.index')
// Returns: ''

A TypeError is thrown if path is not a string.

path.format(pathObject)#

Added in: v0.11.15

The path.format() method returns a path string from an object. This is the opposite of path.parse().

When providing properties to the pathObject remember that there are combinations where one property has priority over another:

  • pathObject.root is ignored if pathObject.dir is provided
  • pathObject.ext and pathObject.name are ignored if pathObject.base exists

For example, on POSIX:

// If `dir`, `root` and `base` are provided,
// `${dir}${path.sep}${base}`
// will be returned. `root` is ignored.
path.format({
  root: '/ignored',
  dir: '/home/user/dir',
  base: 'file.txt'
});
// Returns: '/home/user/dir/file.txt'

// `root` will be used if `dir` is not specified.
// If only `root` is provided or `dir` is equal to `root` then the
// platform separator will not be included. `ext` will be ignored.
path.format({
  root: '/',
  base: 'file.txt',
  ext: 'ignored'
});
// Returns: '/file.txt'

// `name` + `ext` will be used if `base` is not specified.
path.format({
  root: '/',
  name: 'file',
  ext: '.txt'
});
// Returns: '/file.txt'

On Windows:

path.format({
  dir : "C:\\path\\dir",
  base : "file.txt"
});
// Returns: 'C:\\path\\dir\\file.txt'

path.isAbsolute(path)#

Added in: v0.11.2

The path.isAbsolute() method determines if path is an absolute path.

If the given path is a zero-length string, false will be returned.

For example on POSIX:

path.isAbsolute('/foo/bar') // true
path.isAbsolute('/baz/..')  // true
path.isAbsolute('qux/')     // false
path.isAbsolute('.')        // false

On Windows:

path.isAbsolute('//server')    // true
path.isAbsolute('\\\\server')  // true
path.isAbsolute('C:/foo/..')   // true
path.isAbsolute('C:\\foo\\..') // true
path.isAbsolute('bar\\baz')    // false
path.isAbsolute('bar/baz')     // false
path.isAbsolute('.')           // false

A TypeError is thrown if path is not a string.

path.join([...paths])#

Added in: v0.1.16

The path.join() method joins all given path segments together using the platform specific separator as a delimiter, then normalizes the resulting path.

Zero-length path segments are ignored. If the joined path string is a zero-length string then '.' will be returned, representing the current working directory.

For example:

path.join('/foo', 'bar', 'baz/asdf', 'quux', '..')
// Returns: '/foo/bar/baz/asdf'

path.join('foo', {}, 'bar')
// throws TypeError: Arguments to path.join must be strings

A TypeError is thrown if any of the path segments is not a string.

path.normalize(path)#

Added in: v0.1.23

The path.normalize() method normalizes the given path, resolving '..' and '.' segments.

When multiple, sequential path segment separation characters are found (e.g. / on POSIX and \ on Windows), they are replaced by a single instance of the platform specific path segment separator. Trailing separators are preserved.

If the path is a zero-length string, '.' is returned, representing the current working directory.

For example on POSIX:

path.normalize('/foo/bar//baz/asdf/quux/..')
// Returns: '/foo/bar/baz/asdf'

On Windows:

path.normalize('C:\\temp\\\\foo\\bar\\..\\');
// Returns: 'C:\\temp\\foo\\'

A TypeError is thrown if path is not a string.

path.parse(path)#

Added in: v0.11.15

The path.parse() method returns an object whose properties represent significant elements of the path.

The returned object will have the following properties:

For example on POSIX:

path.parse('/home/user/dir/file.txt')
// Returns:
// {
//    root : "/",
//    dir : "/home/user/dir",
//    base : "file.txt",
//    ext : ".txt",
//    name : "file"
// }

┌─────────────────────┬────────────┐
│          dir        │    base    │
├──────┬              ├──────┬─────┤
│ root │              │ name │ ext │
"  /    home/user/dir / file  .txt "
└──────┴──────────────┴──────┴─────┘
(all spaces in the "" line should be ignored -- they are purely for formatting)

On Windows:

path.parse('C:\\path\\dir\\file.txt')
// Returns:
// {
//    root : "C:\\",
//    dir : "C:\\path\\dir",
//    base : "file.txt",
//    ext : ".txt",
//    name : "file"
// }

┌─────────────────────┬────────────┐
│          dir        │    base    │
├──────┬              ├──────┬─────┤
│ root │              │ name │ ext │
" C:\      path\dir   \ file  .txt "
└──────┴──────────────┴──────┴─────┘
(all spaces in the "" line should be ignored -- they are purely for formatting)

A TypeError is thrown if path is not a string.

path.posix#

Added in: v0.11.15

The path.posix property provides access to POSIX specific implementations of the path methods.

path.relative(from, to)#

Added in: v0.5.0

The path.relative() method returns the relative path from from to to. If from and to each resolve to the same path (after calling path.resolve() on each), a zero-length string is returned.

If a zero-length string is passed as from or to, the current working directory will be used instead of the zero-length strings.

For example on POSIX:

path.relative('/data/orandea/test/aaa', '/data/orandea/impl/bbb')
// Returns: '../../impl/bbb'

On Windows:

path.relative('C:\\orandea\\test\\aaa', 'C:\\orandea\\impl\\bbb')
// Returns: '..\\..\\impl\\bbb'

A TypeError is thrown if neither from nor to is a string.

path.resolve([...paths])#

Added in: v0.3.4

The path.resolve() method resolves a sequence of paths or path segments into an absolute path.

The given sequence of paths is processed from right to left, with each subsequent path prepended until an absolute path is constructed. For instance, given the sequence of path segments: /foo, /bar, baz, calling path.resolve('/foo', '/bar', 'baz') would return /bar/baz.

If after processing all given path segments an absolute path has not yet been generated, the current working directory is used.

The resulting path is normalized and trailing slashes are removed unless the path is resolved to the root directory.

Zero-length path segments are ignored.

If no path segments are passed, path.resolve() will return the absolute path of the current working directory.

For example:

path.resolve('/foo/bar', './baz')
// Returns: '/foo/bar/baz'

path.resolve('/foo/bar', '/tmp/file/')
// Returns: '/tmp/file'

path.resolve('wwwroot', 'static_files/png/', '../gif/image.gif')
// if the current working directory is /home/myself/node,
// this returns '/home/myself/node/wwwroot/static_files/gif/image.gif'

A TypeError is thrown if any of the arguments is not a string.

path.sep#

Added in: v0.7.9

Provides the platform-specific path segment separator:

  • \ on Windows
  • / on POSIX

For example on POSIX:

'foo/bar/baz'.split(path.sep)
// Returns: ['foo', 'bar', 'baz']

On Windows:

'foo\\bar\\baz'.split(path.sep)
// Returns: ['foo', 'bar', 'baz']

path.win32#

Added in: v0.11.15

The path.win32 property provides access to Windows-specific implementations of the path methods.

Note: On Windows, both the forward slash (/) and backward slash (\) characters are accepted as path delimiters; however, only the backward slash (\) will be used in return values.

process#

The process object is a global that provides information about, and control over, the current Node.js process. As a global, it is always available to Node.js applications without using require().

Process Events#

The process object is an instance of EventEmitter.

Event: 'beforeExit'#

Added in: v0.11.12

The 'beforeExit' event is emitted when Node.js empties its event loop and has no additional work to schedule. Normally, the Node.js process will exit when there is no work scheduled, but a listener registered on the 'beforeExit' event can make asynchronous calls, and thereby cause the Node.js process to continue.

The listener callback function is invoked with the value of process.exitCode passed as the only argument.

The 'beforeExit' event is not emitted for conditions causing explicit termination, such as calling process.exit() or uncaught exceptions.

The 'beforeExit' should not be used as an alternative to the 'exit' event unless the intention is to schedule additional work.

Event: 'disconnect'#

Added in: v0.7.7

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the 'disconnect' event will be emitted when the IPC channel is closed.

Event: 'exit'#

Added in: v0.1.7

The 'exit' event is emitted when the Node.js process is about to exit as a result of either:

  • The process.exit() method being called explicitly;
  • The Node.js event loop no longer having any additional work to perform.

There is no way to prevent the exiting of the event loop at this point, and once all 'exit' listeners have finished running the Node.js process will terminate.

The listener callback function is invoked with the exit code specified either by the process.exitCode property, or the exitCode argument passed to the process.exit() method, as the only argument.

For example:

process.on('exit', (code) => {
  console.log(`About to exit with code: ${code}`);
});

Listener functions must only perform synchronous operations. The Node.js process will exit immediately after calling the 'exit' event listeners causing any additional work still queued in the event loop to be abandoned. In the following example, for instance, the timeout will never occur:

process.on('exit', (code) => {
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
});

Event: 'message'#

Added in: v0.5.10

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the 'message' event is emitted whenever a message sent by a parent process using childprocess.send() is received by the child process.

The listener callback is invoked with the following arguments:

Event: 'rejectionHandled'#

Added in: v1.4.1

The 'rejectionHandled' event is emitted whenever a Promise has been rejected and an error handler was attached to it (using promise.catch(), for example) later than one turn of the Node.js event loop.

The listener callback is invoked with a reference to the rejected Promise as the only argument.

The Promise object would have previously been emitted in an 'unhandledRejection' event, but during the course of processing gained a rejection handler.

There is no notion of a top level for a Promise chain at which rejections can always be handled. Being inherently asynchronous in nature, a Promise rejection can be handled at a future point in time — possibly much later than the event loop turn it takes for the 'unhandledRejection' event to be emitted.

Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.

In synchronous code, the 'uncaughtException' event is emitted when the list of unhandled exceptions grows.

In asynchronous code, the 'unhandledRejection' event is emitted when the list of unhandled rejections grows, and the 'rejectionHandled' event is emitted when the list of unhandled rejections shrinks.

For example:

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, p) => {
  unhandledRejections.set(p, reason);
});
process.on('rejectionHandled', (p) => {
  unhandledRejections.delete(p);
});

In this example, the unhandledRejections Map will grow and shrink over time, reflecting rejections that start unhandled and then become handled. It is possible to record such errors in an error log, either periodically (which is likely best for long-running application) or upon process exit (which is likely most convenient for scripts).

Event: 'uncaughtException'#

Added in: v0.1.18

The 'uncaughtException' event is emitted when an uncaught JavaScript exception bubbles all the way back to the event loop. By default, Node.js handles such exceptions by printing the stack trace to stderr and exiting. Adding a handler for the 'uncaughtException' event overrides this default behavior.

The listener function is called with the Error object passed as the only argument.

For example:

process.on('uncaughtException', (err) => {
  fs.writeSync(1, `Caught exception: ${err}`);
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

Warning: Using 'uncaughtException' correctly#

Note that 'uncaughtException' is a crude mechanism for exception handling intended to be used only as a last resort. The event should not be used as an equivalent to On Error Resume Next. Unhandled exceptions inherently mean that an application is in an undefined state. Attempting to resume application code without properly recovering from the exception can cause additional unforeseen and unpredictable issues.

Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.

Attempting to resume normally after an uncaught exception can be similar to pulling out of the power cord when upgrading a computer -- nine out of ten times nothing happens - but the 10th time, the system becomes corrupted.

The correct use of 'uncaughtException' is to perform synchronous cleanup of allocated resources (e.g. file descriptors, handles, etc) before shutting down the process. It is not safe to resume normal operation after 'uncaughtException'.

To restart a crashed application in a more reliable way, whether uncaughtException is emitted or not, an external monitor should be employed in a separate process to detect application failures and recover or restart as needed.

Event: 'unhandledRejection'#

Added in: v1.4.1

The 'unhandledRejection' event is emitted whenever a Promise is rejected and no error handler is attached to the promise within a turn of the event loop. When programming with Promises, exceptions are encapsulated as "rejected promises". Rejections can be caught and handled using promise.catch() and are propagated through a Promise chain. The 'unhandledRejection' event is useful for detecting and keeping track of promises that were rejected whose rejections have not yet been handled.

The listener function is called with the following arguments:

  • reason <Error> | <any> The object with which the promise was rejected (typically an Error object).
  • p the Promise that was rejected.

For example:

process.on('unhandledRejection', (reason, p) => {
  console.log('Unhandled Rejection at: Promise', p, 'reason:', reason);
  // application specific logging, throwing an error, or other logic here
});

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // note the typo (`pasre`)
}); // no `.catch` or `.then`

The following will also trigger the 'unhandledRejection' event to be emitted:

function SomeResource() {
  // Initially set the loaded status to a rejected promise
  this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}

var resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn

In this example case, it is possible to track the rejection as a developer error as would typically be the case for other 'unhandledRejection' events. To address such failures, a non-operational .catch(() => { }) handler may be attached to resource.loaded, which would prevent the 'unhandledRejection' event from being emitted. Alternatively, the 'rejectionHandled' event may be used.

Event: 'warning'#

Added in: v6.0.0

The 'warning' event is emitted whenever Node.js emits a process warning.

A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs or security vulnerabilities.

The listener function is called with a single warning argument whose value is an Error object. There are three key properties that describe the warning:

  • name <String> The name of the warning (currently Warning by default).
  • message <String> A system-provided description of the warning.
  • stack <String> A stack trace to the location in the code where the warning was issued.
process.on('warning', (warning) => {
  console.warn(warning.name);    // Print the warning name
  console.warn(warning.message); // Print the warning message
  console.warn(warning.stack);   // Print the stack trace
});

By default, Node.js will print process warnings to stderr. The --no-warnings command-line option can be used to suppress the default console output but the 'warning' event will still be emitted by the process object.

The following example illustrates the warning that is printed to stderr when too many listeners have been added to an event

$ node
> event.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) Warning: Possible EventEmitter memory leak detected. 2 foo
... listeners added. Use emitter.setMaxListeners() to increase limit

In contrast, the following example turns off the default warning output and adds a custom handler to the 'warning' event:

$ node --no-warnings
> var p = process.on('warning', (warning) => console.warn('Do not do that!'));
> event.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!

The --trace-warnings command-line option can be used to have the default console output for warnings include the full stack trace of the warning.

Launching Node.js using the --throw-deprecation command line flag will cause custom deprecation warnings to be thrown as exceptions.

Using the --trace-deprecation command line flag will cause the custom deprecation to be printed to stderr along with the stack trace.

Using the --no-deprecation command line flag will suppress all reporting of the custom deprecation.

The *-deprecation command line flags only affect warnings that use the name DeprecationWarning.

Emitting custom warnings#

See the process.emitWarning() method for issuing custom or application-specific warnings.

Signal Events#

Signal events will be emitted when the Node.js process receives a signal. Please refer to signal(7) for a listing of standard POSIX signal names such as SIGINT, SIGHUP, etc.

The name of each event will be the uppercase common name for the signal (e.g. 'SIGINT' for SIGINT signals).

For example:

// Begin reading from stdin so the process does not exit.
process.stdin.resume();

process.on('SIGINT', () => {
  console.log('Received SIGINT.  Press Control-D to exit.');
});

Note: An easy way to send the SIGINT signal is with <Ctrl>-C in most terminal programs.

It is important to take note of the following:

  • SIGUSR1 is reserved by Node.js to start the debugger. It's possible to install a listener but doing so will not stop the debugger from starting.
  • SIGTERM and SIGINT have default handlers on non-Windows platforms that resets the terminal mode before exiting with code 128 + signal number. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).
  • SIGPIPE is ignored by default. It can have a listener installed.
  • SIGHUP is generated on Windows when the console window is closed, and on other platforms under various similar conditions, see signal(7). It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior of SIGHUP is to terminate Node.js, but once a listener has been installed its default behavior will be removed.
  • SIGTERM is not supported on Windows, it can be listened on.
  • SIGINT from the terminal is supported on all platforms, and can usually be generated with CTRL+C (though this may be configurable). It is not generated when terminal raw mode is enabled.
  • SIGBREAK is delivered on Windows when <Ctrl>+<Break> is pressed, on non-Windows platforms it can be listened on, but there is no way to send or generate it.
  • SIGWINCH is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.
  • SIGKILL cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.
  • SIGSTOP cannot have a listener installed.
  • SIGBUS, SIGFPE, SIGSEGV and SIGILL, when not raised artificially using kill(2), inherently leave the process in a state from which it is not safe to attempt to call JS listeners. Doing so might lead to the process hanging in an endless loop, since listeners attached using process.on() are called asynchronously and therefore unable to correct the underlying problem.

Note: Windows does not support sending signals, but Node.js offers some emulation with process.kill(), and ChildProcess.kill(). Sending signal 0 can be used to test for the existence of a process. Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional termination of the target process.

process.abort()#

Added in: v0.7.0

The process.abort() method causes the Node.js process to exit immediately and generate a core file.

process.arch#

Added in: v0.5.0

The process.arch property returns a String identifying the processor architecture that the Node.js process is currently running on. For instance 'arm', 'ia32', or 'x64'.

console.log(`This processor architecture is ${process.arch}`);

process.argv#

Added in: v0.1.27

The process.argv property returns an array containing the command line arguments passed when the Node.js process was launched. The first element will be process.execPath. See process.argv0 if access to the original value of argv[0] is needed. The second element will be the path to the JavaScript file being executed. The remaining elements will be any additional command line arguments.

For example, assuming the following script for process-args.js:

// print process.argv
process.argv.forEach((val, index) => {
  console.log(`${index}: ${val}`);
});

Launching the Node.js process as:

$ node process-2.js one two=three four

Would generate the output:

0: /usr/local/bin/node
1: /Users/mjr/work/node/process-2.js
2: one
3: two=three
4: four

process.argv0#

Added in: 6.4.0

The process.argv0 property stores a read-only copy of the original value of argv[0] passed when Node.js starts.

$ bash -c 'exec -a customArgv0 ./node'
> process.argv[0]
'/Volumes/code/external/node/out/Release/node'
> process.argv0
'customArgv0'

process.channel#

Added in: v7.1.0

If the Node.js process was spawned with an IPC channel (see the Child Process documentation), the process.channel property is a reference to the IPC channel. If no IPC channel exists, this property is undefined.

process.chdir(directory)#

Added in: v0.1.17

The process.chdir() method changes the current working directory of the Node.js process or throws an exception if doing so fails (for instance, if the specified directory does not exist).

console.log(`Starting directory: ${process.cwd()}`);
try {
  process.chdir('/tmp');
  console.log(`New directory: ${process.cwd()}`);
}
catch (err) {
  console.log(`chdir: ${err}`);
}

process.config#

Added in: v0.7.7

The process.config property returns an Object containing the JavaScript representation of the configure options used to compile the current Node.js executable. This is the same as the config.gypi file that was produced when running the ./configure script.

An example of the possible output looks like:

{
  target_defaults:
   { cflags: [],
     default_configuration: 'Release',
     defines: [],
     include_dirs: [],
     libraries: [] },
  variables:
   {
     host_arch: 'x64',
     node_install_npm: 'true',
     node_prefix: '',
     node_shared_cares: 'false',
     node_shared_http_parser: 'false',
     node_shared_libuv: 'false',
     node_shared_zlib: 'false',
     node_use_dtrace: 'false',
     node_use_openssl: 'true',
     node_shared_openssl: 'false',
     strict_aliasing: 'true',
     target_arch: 'x64',
     v8_use_snapshot: 'true'
   }
}

Note: The process.config property is not read-only and there are existing modules in the ecosystem that are known to extend, modify, or entirely replace the value of process.config.

process.connected#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the process.connected property will return true so long as the IPC channel is connected and will return false after process.disconnect() is called.

Once process.connected is false, it is no longer possible to send messages over the IPC channel using process.send().

process.cpuUsage([previousValue])#

Added in: v6.1.0

The process.cpuUsage() method returns the user and system CPU time usage of the current process, in an object with properties user and system, whose values are microsecond values (millionth of a second). These values measure time spent in user and system code respectively, and may end up being greater than actual elapsed time if multiple CPU cores are performing work for this process.

The result of a previous call to process.cpuUsage() can be passed as the argument to the function, to get a diff reading.

const startUsage = process.cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(process.cpuUsage(startUsage));
// { user: 514883, system: 11226 }

process.cwd()#

Added in: v0.1.8

The process.cwd() method returns the current working directory of the Node.js process.

console.log(`Current directory: ${process.cwd()}`);

process.disconnect()#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the process.disconnect() method will close the IPC channel to the parent process, allowing the child process to exit gracefully once there are no other connections keeping it alive.

The effect of calling process.disconnect() is that same as calling the parent process's ChildProcess.disconnect().

If the Node.js process was not spawned with an IPC channel, process.disconnect() will be undefined.

process.env#

Added in: v0.1.27

The process.env property returns an object containing the user environment. See environ(7).

An example of this object looks like:

{
  TERM: 'xterm-256color',
  SHELL: '/usr/local/bin/bash',
  USER: 'maciej',
  PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
  PWD: '/Users/maciej',
  EDITOR: 'vim',
  SHLVL: '1',
  HOME: '/Users/maciej',
  LOGNAME: 'maciej',
  _: '/usr/local/bin/node'
}

It is possible to modify this object, but such modifications will not be reflected outside the Node.js process. In other words, the following example would not work:

$ node -e 'process.env.foo = "bar"' && echo $foo

While the following will:

process.env.foo = 'bar';
console.log(process.env.foo);

Assigning a property on process.env will implicitly convert the value to a string.

Example:

process.env.test = null;
console.log(process.env.test);
// => 'null'
process.env.test = undefined;
console.log(process.env.test);
// => 'undefined'

Use delete to delete a property from process.env.

Example:

process.env.TEST = 1;
delete process.env.TEST;
console.log(process.env.TEST);
// => undefined

On Windows operating systems, environment variables are case-insensitive.

Example:

process.env.TEST = 1;
console.log(process.env.test);
// => 1

process.emitWarning(warning[, type[, code]][, ctor])#

Added in: v6.0.0
  • warning <String> | <Error> The warning to emit.
  • type <String> When warning is a String, type is the name to use for the type of warning being emitted. Default: Warning.
  • code <String> A unique identifier for the warning instance being emitted.
  • ctor <Function> When warning is a String, ctor is an optional function used to limit the generated stack trace. Default process.emitWarning

The process.emitWarning() method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the process.on('warning') event.

// Emit a warning using a string...
process.emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!

// Emit a warning using a string and a type...
process.emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!

process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) CustomWarning [WARN001]: Something Happened!

In each of the previous examples, an Error object is generated internally by process.emitWarning() and passed through to the process.on('warning') event.

process.on('warning', (warning) => {
  console.warn(warning.name);
  console.warn(warning.message);
  console.warn(warning.code);
  console.warn(warning.stack);
});

If warning is passed as an Error object, it will be passed through to the process.on('warning') event handler unmodified (and the optional type, code and ctor arguments will be ignored):

// Emit a warning using an Error object...
const myWarning = new Error('Warning! Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';

process.emitWarning(myWarning);
// Emits: (node:56338) CustomWarning [WARN001]: Warning! Something Happened!

A TypeError is thrown if warning is anything other than a string or Error object.

Note that while process warnings use Error objects, the process warning mechanism is not a replacement for normal error handling mechanisms.

The following additional handling is implemented if the warning type is DeprecationWarning:

  • If the --throw-deprecation command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event.
  • If the --no-deprecation command-line flag is used, the deprecation warning is suppressed.
  • If the --trace-deprecation command-line flag is used, the deprecation warning is printed to stderr along with the full stack trace.

Avoiding duplicate warnings#

As a best practice, warnings should be emitted only once per process. To do so, it is recommended to place the emitWarning() behind a simple boolean flag as illustrated in the example below:

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    process.emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing

process.execArgv#

Added in: v0.7.7

The process.execArgv property returns the set of Node.js-specific command-line options passed when the Node.js process was launched. These options do not appear in the array returned by the process.argv property, and do not include the Node.js executable, the name of the script, or any options following the script name. These options are useful in order to spawn child processes with the same execution environment as the parent.

For example:

$ node --harmony script.js --version

Results in process.execArgv:

['--harmony']

And process.argv:

['/usr/local/bin/node', 'script.js', '--version']

process.execPath#

Added in: v0.1.100

The process.execPath property returns the absolute pathname of the executable that started the Node.js process.

For example:

'/usr/local/bin/node'

process.exit([code])#

Added in: v0.1.13
  • code <Integer> The exit code. Defaults to 0.

The process.exit() method instructs Node.js to terminate the process as quickly as possible with the specified exit code. If the code is omitted, exit uses either the 'success' code 0 or the value of process.exitCode if specified.

To exit with a 'failure' code:

process.exit(1);

The shell that executed Node.js should see the exit code as 1.

It is important to note that calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

In most situations, it is not actually necessary to call process.exit() explicitly. The Node.js process will exit on its own if there is no additional work pending in the event loop. The process.exitCode property can be set to tell the process which exit code to use when the process exits gracefully.

For instance, the following example illustrates a misuse of the process.exit() method that could lead to data printed to stdout being truncated and lost:

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exit(1);
}

The reason this is problematic is because writes to process.stdout in Node.js are sometimes non-blocking and may occur over multiple ticks of the Node.js event loop. Calling process.exit(), however, forces the process to exit before those additional writes to stdout can be performed.

Rather than calling process.exit() directly, the code should set the process.exitCode and allow the process to exit naturally by avoiding scheduling any additional work for the event loop:

// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exitCode = 1;
}

If it is necessary to terminate the Node.js process due to an error condition, throwing an uncaught error and allowing the process to terminate accordingly is safer than calling process.exit().

process.exitCode#

Added in: v0.11.8

A number which will be the process exit code, when the process either exits gracefully, or is exited via process.exit() without specifying a code.

Specifying a code to process.exit(code) will override any previous setting of process.exitCode.

process.getegid()#

Added in: v2.0.0

The process.getegid() method returns the numerical effective group identity of the Node.js process. (See getegid(2).)

if (process.getegid) {
  console.log(`Current gid: ${process.getegid()}`);
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.geteuid()#

Added in: v2.0.0

The process.geteuid() method returns the numerical effective user identity of the process. (See geteuid(2).)

if (process.geteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.getgid()#

Added in: v0.1.31

The process.getgid() method returns the numerical group identity of the process. (See getgid(2).)

if (process.getgid) {
  console.log(`Current gid: ${process.getgid()}`);
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.getgroups()#

Added in: v0.9.4

The process.getgroups() method returns an array with the supplementary group IDs. POSIX leaves it unspecified if the effective group ID is included but Node.js ensures it always is.

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.getuid()#

Added in: v0.1.28

The process.getuid() method returns the numeric user identity of the process. (See getuid(2).)

if (process.getuid) {
  console.log(`Current uid: ${process.getuid()}`);
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.hrtime([time])#

Added in: v0.7.6
  • time <Array> The result of a previous call to process.hrtime()
  • Returns: <Array>

The process.hrtime() method returns the current high-resolution real time in a [seconds, nanoseconds] tuple Array, where nanoseconds is the remaining part of the real time that can't be represented in second precision.

time is an optional parameter that must be the result of a previous process.hrtime() call to diff with the current time. If the parameter passed in is not a tuple Array, a TypeError will be thrown. Passing in a user-defined array instead of the result of a previous call to process.hrtime() will lead to undefined behavior.

These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals:

const NS_PER_SEC = 1e9;
var time = process.hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  var diff = process.hrtime(time);
  // [ 1, 552 ]

  console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
  // benchmark took 1000000527 nanoseconds
}, 1000);

process.initgroups(user, extra_group)#

Added in: v0.9.4
  • user <String> | <number> The user name or numeric identifier.
  • extra_group <String> | <number> A group name or numeric identifier.

The process.initgroups() method reads the /etc/group file and initializes the group access list, using all groups of which the user is a member. This is a privileged operation that requires that the Node.js process either have root access or the CAP_SETGID capability.

Note that care must be taken when dropping privileges. Example:

console.log(process.getgroups());         // [ 0 ]
process.initgroups('bnoordhuis', 1000);   // switch user
console.log(process.getgroups());         // [ 27, 30, 46, 1000, 0 ]
process.setgid(1000);                     // drop root gid
console.log(process.getgroups());         // [ 27, 30, 46, 1000 ]

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.kill(pid[, signal])#

Added in: v0.0.6
  • pid <number> A process ID
  • signal <String> | <number> The signal to send, either as a string or number. Defaults to 'SIGTERM'.

The process.kill() method sends the signal to the process identified by pid.

Signal names are strings such as 'SIGINT' or 'SIGHUP'. See Signal Events and kill(2) for more information.

This method will throw an error if the target pid does not exist. As a special case, a signal of 0 can be used to test for the existence of a process. Windows platforms will throw an error if the pid is used to kill a process group.

Note:Even though the name of this function is process.kill(), it is really just a signal sender, like the kill system call. The signal sent may do something other than kill the target process.

For example:

process.on('SIGHUP', () => {
  console.log('Got SIGHUP signal.');
});

setTimeout(() => {
  console.log('Exiting.');
  process.exit(0);
}, 100);

process.kill(process.pid, 'SIGHUP');

Note: When SIGUSR1 is received by a Node.js process, Node.js will start the debugger, see Signal Events.

process.mainModule#

Added in: v0.1.17

The process.mainModule property provides an alternative way of retrieving require.main. The difference is that if the main module changes at runtime, require.main may still refer to the original main module in modules that were required before the change occurred. Generally, it's safe to assume that the two refer to the same module.

As with require.main, process.mainModule will be undefined if there is no entry script.

process.memoryUsage()#

Added in: v0.1.16

The process.memoryUsage() method returns an object describing the memory usage of the Node.js process measured in bytes.

For example, the code:

console.log(process.memoryUsage());

Will generate:

{
  rss: 4935680,
  heapTotal: 1826816,
  heapUsed: 650472,
  external: 49879
}

heapTotal and heapUsed refer to V8's memory usage. external refers to the memory usage of C++ objects bound to JavaScript objects managed by V8.

process.nextTick(callback[, ...args])#

Added in: v0.1.26
  • callback <Function>
  • ...args <any> Additional arguments to pass when invoking the callback

The process.nextTick() method adds the callback to the "next tick queue". Once the current turn of the event loop turn runs to completion, all callbacks currently in the next tick queue will be called.

This is not a simple alias to setTimeout(fn, 0). It is much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

console.log('start');
process.nextTick(() => {
  console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback

This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:

function MyThing(options) {
  this.setupOptions(options);

  process.nextTick(() => {
    this.startDoingStuff();
  });
}

var thing = new MyThing();
thing.getReadyForStuff();

// thing.startDoingStuff() gets called now, not before.

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

// WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
  if (arg) {
    cb();
    return;
  }

  fs.stat('file', cb);
}

This API is hazardous because in the following case:

maybeSync(true, () => {
  foo();
});
bar();

It is not clear whether foo() or bar() will be called first.

The following approach is much better:

function definitelyAsync(arg, cb) {
  if (arg) {
    process.nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}

Note: the next tick queue is completely drained on each pass of the event loop before additional I/O is processed. As a result, recursively setting nextTick callbacks will block any I/O from happening, just like a while(true); loop.

process.pid#

Added in: v0.1.15

The process.pid property returns the PID of the process.

console.log(`This process is pid ${process.pid}`);

process.platform#

Added in: v0.1.16

The process.platform property returns a string identifying the operating system platform on which the Node.js process is running. For instance 'darwin', 'freebsd', 'linux', 'sunos' or 'win32'

console.log(`This platform is ${process.platform}`);

process.release#

Added in: v3.0.0

The process.release property returns an Object containing metadata related to the current release, including URLs for the source tarball and headers-only tarball.

process.release contains the following properties:

  • name <String> A value that will always be 'node' for Node.js. For legacy io.js releases, this will be 'io.js'.
  • sourceUrl <String> an absolute URL pointing to a .tar.gz file containing the source code of the current release.
  • headersUrl<String> an absolute URL pointing to a .tar.gz file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.
  • libUrl <String> an absolute URL pointing to a node.lib file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.
  • lts <String> a string label identifying the LTS label for this release. If the Node.js release is not an LTS release, this will be undefined.

For example:

{
  name: 'node',
  lts: 'Argon',
  sourceUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5.tar.gz',
  headersUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5-headers.tar.gz',
  libUrl: 'https://nodejs.org/download/release/v4.4.5/win-x64/node.lib'
}

In custom builds from non-release versions of the source tree, only the name property may be present. The additional properties should not be relied upon to exist.

process.send(message[, sendHandle[, options]][, callback])#

Added in: v0.5.9

If Node.js is spawned with an IPC channel, the process.send() method can be used to send messages to the parent process. Messages will be received as a 'message' event on the parent's ChildProcess object.

If Node.js was not spawned with an IPC channel, process.send() will be undefined.

Note: This function uses JSON.stringify() internally to serialize the message.*

process.setegid(id)#

Added in: v2.0.0
  • id <String> | <number> A group name or ID

The process.setegid() method sets the effective group identity of the process. (See setegid(2).) The id can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated a numeric ID.

if (process.getegid && process.setegid) {
  console.log(`Current gid: ${process.getegid()}`);
  try {
    process.setegid(501);
    console.log(`New gid: ${process.getegid()}`);
  }
  catch (err) {
    console.log(`Failed to set gid: ${err}`);
  }
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.seteuid(id)#

Added in: v2.0.0
  • id <String> | <number> A user name or ID

The process.seteuid() method sets the effective user identity of the process. (See seteuid(2).) The id can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

if (process.geteuid && process.seteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
  try {
    process.seteuid(501);
    console.log(`New uid: ${process.geteuid()}`);
  }
  catch (err) {
    console.log(`Failed to set uid: ${err}`);
  }
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.setgid(id)#

Added in: v0.1.31
  • id <String> | <number> The group name or ID

The process.setgid() method sets the group identity of the process. (See setgid(2).) The id can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated numeric ID.

if (process.getgid && process.setgid) {
  console.log(`Current gid: ${process.getgid()}`);
  try {
    process.setgid(501);
    console.log(`New gid: ${process.getgid()}`);
  }
  catch (err) {
    console.log(`Failed to set gid: ${err}`);
  }
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.setgroups(groups)#

Added in: v0.9.4

The process.setgroups() method sets the supplementary group IDs for the Node.js process. This is a privileged operation that requires the Node.js process to have root or the CAP_SETGID capability.

The groups array can contain numeric group IDs, group names or both.

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.setuid(id)#

Added in: v0.1.28

The process.setuid(id) method sets the user identity of the process. (See setuid(2).) The id can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

if (process.getuid && process.setuid) {
  console.log(`Current uid: ${process.getuid()}`);
  try {
    process.setuid(501);
    console.log(`New uid: ${process.getuid()}`);
  }
  catch (err) {
    console.log(`Failed to set uid: ${err}`);
  }
}

Note: This function is only available on POSIX platforms (i.e. not Windows or Android)

process.stderr#

The process.stderr property returns a Writable stream equivalent to or associated with stderr (fd 2).

Note: process.stderr and process.stdout differ from other Node.js streams in several ways:

  1. They cannot be closed (end() will throw).
  2. They never emit the 'finish' event.
  3. Writes can block when output is redirected to a file.
    • Note that disks are fast and operating systems normally employ write-back caching so this is very uncommon.
  4. Writes on UNIX will block by default if output is going to a TTY (a terminal).
  5. Windows functionality differs. Writes block except when output is going to a TTY.

To check if Node.js is being run in a TTY context, read the isTTY property on process.stderr, process.stdout, or process.stdin:

process.stdin#

The process.stdin property returns a Readable stream equivalent to or associated with stdin (fd 0).

For example:

process.stdin.setEncoding('utf8');

process.stdin.on('readable', () => {
  var chunk = process.stdin.read();
  if (chunk !== null) {
    process.stdout.write(`data: ${chunk}`);
  }
});

process.stdin.on('end', () => {
  process.stdout.write('end');
});

As a Readable stream, process.stdin can also be used in "old" mode that is compatible with scripts written for Node.js prior to v0.10. For more information see Stream compatibility.

Note: In "old" streams mode the stdin stream is paused by default, so one must call process.stdin.resume() to read from it. Note also that calling process.stdin.resume() itself would switch stream to "old" mode.

process.stdout#

The process.stdout property returns a Writable stream equivalent to or associated with stdout (fd 1).

For example:

console.log = (msg) => {
  process.stdout.write(`${msg}\n`);
};

Note: process.stderr and process.stdout differ from other Node.js streams in several ways:

  1. They cannot be closed (end() will throw).
  2. They never emit the 'finish' event.
  3. Writes can block when output is redirected to a file.
    • Note that disks are fast and operating systems normally employ write-back caching so this is very uncommon.
  4. Writes on UNIX will block by default if output is going to a TTY (a terminal).
  5. Windows functionality differs. Writes block except when output is going to a TTY.

To check if Node.js is being run in a TTY context, read the isTTY property on process.stderr, process.stdout, or process.stdin:

TTY Terminals and process.stdout#

The process.stderr and process.stdout streams are blocking when outputting to TTYs (terminals) on OS X as a workaround for the operating system's small, 1kb buffer size. This is to prevent interleaving between stdout and stderr.

To check if Node.js is being run in a TTY context, check the isTTY property on process.stderr, process.stdout, or process.stdin.

For instance:

$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false

$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false

See the TTY documentation for more information.

process.title#

Added in: v0.1.104

The process.title property returns the current process title (i.e. returns the current value of ps). Assigning a new value to process.title modifies the current value of ps.

Note: When a new value is assigned, different platforms will impose different maximum length restrictions on the title. Usually such restrictions are quite limited. For instance, on Linux and OS X, process.title is limited to the size of the binary name plus the length of the command line arguments because setting the process.title overwrites the argv memory of the process. Node.js v0.8 allowed for longer process title strings by also overwriting the environ memory but that was potentially insecure and confusing in some (rather obscure) cases.

process.umask([mask])#

Added in: v0.1.19
  • mask <number>

The process.umask() method sets or returns the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process. The old mask is return if the mask argument is given, otherwise returns the current mask.

const newmask = 0o022;
const oldmask = process.umask(newmask);
console.log(
  `Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`
);

process.uptime()#

Added in: v0.5.0

The process.uptime() method returns the number of seconds the current Node.js process has been running.

process.version#

Added in: v0.1.3

The process.version property returns the Node.js version string.

console.log(`Version: ${process.version}`);

process.versions#

Added in: v0.2.0

The process.versions property returns an object listing the version strings of Node.js and its dependencies. process.versions.modules indicates the current ABI version, which is increased whenever a C++ API changes. Node.js will refuse to load modules that were compiled against a different module ABI version.

console.log(process.versions);

Will generate output similar to:

{
  http_parser: '2.3.0',
  node: '1.1.1',
  v8: '4.1.0.14',
  uv: '1.3.0',
  zlib: '1.2.8',
  ares: '1.10.0-DEV',
  modules: '43',
  icu: '55.1',
  openssl: '1.0.1k',
  unicode: '8.0',
  cldr: '29.0',
  tz: '2016b' }

Exit Codes#

Node.js will normally exit with a 0 status code when no more async operations are pending. The following status codes are used in other cases:

  • 1 Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an 'uncaughtException' event handler.
  • 2 - Unused (reserved by Bash for builtin misuse)
  • 3 Internal JavaScript Parse Error - The JavaScript source code internal in Node.js's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 4 Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node.js's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 5 Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix FATAL ERROR.
  • 6 Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
  • 7 Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a 'uncaughtException' or domain.on('error') handler throws an error.
  • 8 - Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.
  • 9 - Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.
  • 10 Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node.js's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 12 Invalid Debug Argument - The --debug, --inspect and/or --debug-brk options were set, but the port number chosen was invalid or unavailable.
  • >128 Signal Exits - If Node.js receives a fatal signal such as SIGKILL or SIGHUP, then its exit code will be 128 plus the value of the signal code. This is a standard Unix practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code.

punycode#

Stability: 0 - Deprecated

The version of the punycode module bundled in Node.js is being deprecated. In a future major version of Node.js this module will be removed. Users currently depending on the punycode module should switch to using the userland-provided Punycode.js module instead.

The punycode module is a bundled version of the Punycode.js module. It can be accessed using:

const punycode = require('punycode');

Punycode is a character encoding scheme defined by RFC 3492 that is primarily intended for use in Internationalized Domain Names. Because host names in URLs are limited to ASCII characters only, Domain Names that contain non-ASCII characters must be converted into ASCII using the Punycode scheme. For instance, the Japanese character that translates into the English word, 'example' is '例'. The Internationalized Domain Name, '例.com' (equivalent to 'example.com') is represented by Punycode as the ASCII string 'xn--fsq.com'.

The punycode module provides a simple implementation of the Punycode standard.

Note: The punycode module is a third-party dependency used by Node.js and made available to developers as a convenience. Fixes or other modifications to the module must be directed to the Punycode.js project.

punycode.decode(string)#

Added in: v0.5.1

The punycode.decode() method converts a Punycode string of ASCII-only characters to the equivalent string of Unicode codepoints.

punycode.decode('maana-pta'); // 'mañana'
punycode.decode('--dqo34k'); // '☃-⌘'

punycode.encode(string)#

Added in: v0.5.1

The punycode.encode() method converts a string of Unicode codepoints to a Punycode string of ASCII-only characters.

punycode.encode('mañana'); // 'maana-pta'
punycode.encode('☃-⌘'); // '--dqo34k'

punycode.toASCII(domain)#

Added in: v0.6.1

The punycode.toASCII() method converts a Unicode string representing an Internationalized Domain Name to Punycode. Only the non-ASCII parts of the domain name will be converted. Calling punycode.toASCII() on a string that already only contains ASCII characters will have no effect.

// encode domain names
punycode.toASCII('mañana.com');  // 'xn--maana-pta.com'
punycode.toASCII('☃-⌘.com');   // 'xn----dqo34k.com'
punycode.toASCII('example.com'); // 'example.com'

punycode.toUnicode(domain)#

Added in: v0.6.1

The punycode.toUnicode() method converts a string representing a domain name containing Punycode encoded characters into Unicode. Only the Punycode encoded parts of the domain name are be converted.

// decode domain names
punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
punycode.toUnicode('xn----dqo34k.com');  // '☃-⌘.com'
punycode.toUnicode('example.com');       // 'example.com'

punycode.ucs2#

Added in: v0.7.0

punycode.ucs2.decode(string)#

Added in: v0.7.0

The punycode.ucs2.decode() method returns an array containing the numeric codepoint values of each Unicode symbol in the string.

punycode.ucs2.decode('abc'); // [0x61, 0x62, 0x63]
// surrogate pair for U+1D306 tetragram for centre:
punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]

punycode.ucs2.encode(codePoints)#

Added in: v0.7.0

The punycode.ucs2.encode() method returns a string based on an array of numeric code point values.

punycode.ucs2.encode([0x61, 0x62, 0x63]); // 'abc'
punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'

punycode.version#

Added in: v0.6.1

Returns a string identifying the current Punycode.js version number.

Query String#

Stability: 2 - Stable

The querystring module provides utilities for parsing and formatting URL query strings. It can be accessed using:

const querystring = require('querystring');

querystring.escape(str)#

Added in: v0.1.25

The querystring.escape() method performs URL percent-encoding on the given str in a manner that is optimized for the specific requirements of URL query strings.

The querystring.escape() method is used by querystring.stringify() and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement percent-encoding implementation if necessary by assigning querystring.escape to an alternative function.

querystring.parse(str[, sep[, eq[, options]]])#

Added in: v0.1.25
  • str <String> The URL query string to parse
  • sep <String> The substring used to delimit key and value pairs in the query string. Defaults to '&'.
  • eq <String>. The substring used to delimit keys and values in the query string. Defaults to '='.
  • options <Object>
    • decodeURIComponent <Function> The function to use when decoding percent-encoded characters in the query string. Defaults to querystring.unescape().
    • maxKeys <number> Specifies the maximum number of keys to parse. Defaults to 1000. Specify 0 to remove key counting limitations.

The querystring.parse() method parses a URL query string (str) into a collection of key and value pairs.

For example, the query string 'foo=bar&abc=xyz&abc=123' is parsed into:

{
  foo: 'bar',
  abc: ['xyz', '123']
}

Note: The object returned by the querystring.parse() method does not prototypically extend from the JavaScript Object. This means that the typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

By default, percent-encoded characters within the query string will be assumed to use UTF-8 encoding. If an alternative character encoding is used, then an alternative decodeURIComponent option will need to be specified as illustrated in the following example:

// Assuming gbkDecodeURIComponent function already exists...

querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
  { decodeURIComponent: gbkDecodeURIComponent })

querystring.stringify(obj[, sep[, eq[, options]]])#

Added in: v0.1.25
  • obj <Object> The object to serialize into a URL query string
  • sep <String> The substring used to delimit key and value pairs in the query string. Defaults to '&'.
  • eq <String>. The substring used to delimit keys and values in the query string. Defaults to '='.
  • options
    • encodeURIComponent <Function> The function to use when converting URL-unsafe characters to percent-encoding in the query string. Defaults to querystring.escape().

The querystring.stringify() method produces a URL query string from a given obj by iterating through the object's "own properties".

For example:

querystring.stringify({ foo: 'bar', baz: ['qux', 'quux'], corge: '' })
// returns 'foo=bar&baz=qux&baz=quux&corge='

querystring.stringify({foo: 'bar', baz: 'qux'}, ';', ':')
// returns 'foo:bar;baz:qux'

By default, characters requiring percent-encoding within the query string will be encoded as UTF-8. If an alternative encoding is required, then an alternative encodeURIComponent option will need to be specified as illustrated in the following example:

// Assuming gbkEncodeURIComponent function already exists,

querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
  { encodeURIComponent: gbkEncodeURIComponent })

querystring.unescape(str)#

Added in: v0.1.25

The querystring.unescape() method performs decoding of URL percent-encoded characters on the given str.

The querystring.unescape() method is used by querystring.parse() and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement decoding implementation if necessary by assigning querystring.unescape to an alternative function.

By default, the querystring.unescape() method will attempt to use the JavaScript built-in decodeURIComponent() method to decode. If that fails, a safer equivalent that does not throw on malformed URLs will be used.

Readline#

Stability: 2 - Stable

The readline module provides an interface for reading data from a Readable stream (such as process.stdin) one line at a time. It can be accessed using:

const readline = require('readline');

The following simple example illustrates the basic use of the readline module.

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question('What do you think of Node.js? ', (answer) => {
  // TODO: Log the answer in a database
  console.log(`Thank you for your valuable feedback: ${answer}`);

  rl.close();
});

Note Once this code is invoked, the Node.js application will not terminate until the readline.Interface is closed because the interface waits for data to be received on the input stream.

Class: Interface#

Added in: v0.1.104

Instances of the readline.Interface class are constructed using the readline.createInterface() method. Every instance is associated with a single input Readable stream and a single output Writable stream. The output stream is used to print prompts for user input that arrives on, and is read from, the input stream.

Event: 'close'#

Added in: v0.1.98

The 'close' event is emitted when one of the following occur:

  • The rl.close() method is called and the readline.Interface instance has relinquished control over the input and output streams;
  • The input stream receives its 'end' event;
  • The input stream receives <ctrl>-D to signal end-of-transmission (EOT);
  • The input stream receives <ctrl>-C to signal SIGINT and there is no SIGINT event listener registered on the readline.Interface instance.

The listener function is called without passing any arguments.

The readline.Interface instance should be considered to be "finished" once the 'close' event is emitted.

Event: 'line'#

Added in: v0.1.98

The 'line' event is emitted whenever the input stream receives an end-of-line input (\n, \r, or \r\n). This usually occurs when the user presses the <Enter>, or <Return> keys.

The listener function is called with a string containing the single line of received input.

For example:

rl.on('line', (input) => {
  console.log(`Received: ${input}`);
});

Event: 'pause'#

Added in: v0.7.5

The 'pause' event is emitted when one of the following occur:

  • The input stream is paused.
  • The input stream is not paused and receives the SIGCONT event. (See events SIGTSTP and SIGCONT)

The listener function is called without passing any arguments.

For example:

rl.on('pause', () => {
  console.log('Readline paused.');
});

Event: 'resume'#

Added in: v0.7.5

The 'resume' event is emitted whenever the input stream is resumed.

The listener function is called without passing any arguments.

rl.on('resume', () => {
  console.log('Readline resumed.');
});

Event: 'SIGCONT'#

Added in: v0.7.5

The 'SIGCONT' event is emitted when a Node.js process previously moved into the background using <ctrl>-Z (i.e. SIGTSTP) is then brought back to the foreground using fg(1).

If the input stream was paused before the SIGTSTP request, this event will not be emitted.

The listener function is invoked without passing any arguments.

For example:

rl.on('SIGCONT', () => {
  // `prompt` will automatically resume the stream
  rl.prompt();
});

Note: The 'SIGCONT' event is not supported on Windows.

Event: 'SIGINT'#

Added in: v0.3.0

The 'SIGINT' event is emitted whenever the input stream receives a <ctrl>-C input, known typically as SIGINT. If there are no 'SIGINT' event listeners registered when the input stream receives a SIGINT, the 'pause' event will be emitted.

The listener function is invoked without passing any arguments.

For example:

rl.on('SIGINT', () => {
  rl.question('Are you sure you want to exit?', (answer) => {
    if (answer.match(/^y(es)?$/i)) rl.pause();
  });
});

Event: 'SIGTSTP'#

Added in: v0.7.5

The 'SIGTSTP' event is emitted when the input stream receives a <ctrl>-Z input, typically known as SIGTSTP. If there are no SIGTSTP event listeners registered when the input stream receives a SIGTSTP, the Node.js process will be sent to the background.

When the program is resumed using fg(1), the 'pause' and SIGCONT events will be emitted. These can be used to resume the input stream.

The 'pause' and 'SIGCONT' events will not be emitted if the input was paused before the process was sent to the background.

The listener function is invoked without passing any arguments.

For example:

rl.on('SIGTSTP', () => {
  // This will override SIGTSTP and prevent the program from going to the
  // background.
  console.log('Caught SIGTSTP.');
});

Note: The 'SIGTSTP' event is not supported on Windows.

rl.close()#

Added in: v0.1.98

The rl.close() method closes the readline.Interface instance and relinquishes control over the input and output streams. When called, the 'close' event will be emitted.

rl.pause()#

Added in: v0.3.4

The rl.pause() method pauses the input stream, allowing it to be resumed later if necessary.

Calling rl.pause() does not immediately pause other events (including 'line') from being emitted by the readline.Interface instance.

rl.prompt([preserveCursor])#

Added in: v0.1.98
  • preserveCursor <boolean> If true, prevents the cursor placement from being reset to 0.

The rl.prompt() method writes the readline.Interface instances configured prompt to a new line in output in order to provide a user with a new location at which to provide input.

When called, rl.prompt() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the prompt is not written.

rl.question(query, callback)#

Added in: v0.3.3
  • query <String> A statement or query to write to output, prepended to the prompt.
  • callback <Function> A callback function that is invoked with the user's input in response to the query.

The rl.question() method displays the query by writing it to the output, waits for user input to be provided on input, then invokes the callback function passing the provided input as the first argument.

When called, rl.question() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the query is not written.

Example usage:

rl.question('What is your favorite food?', (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
});

Note: The callback function passed to rl.question() does not follow the typical pattern of accepting an Error object or null as the first argument. The callback is called with the provided answer as the only argument.

rl.resume()#

Added in: v0.3.4

The rl.resume() method resumes the input stream if it has been paused.

rl.setPrompt(prompt)#

Added in: v0.1.98

The rl.setPrompt() method sets the prompt that will be written to output whenever rl.prompt() is called.

rl.write(data[, key])#

Added in: v0.1.98
  • data <String>
  • key <Object>
    • ctrl <boolean> true to indicate the <ctrl> key.
    • meta <boolean> true to indicate the <Meta> key.
    • shift <boolean> true to indicate the <Shift> key.
    • name <String> The name of the a key.

The rl.write() method will write either data or a key sequence identified by key to the output. The key argument is supported only if output is a TTY text terminal.

If key is specified, data is ignored.

When called, rl.write() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the data and key are not written.

For example:

rl.write('Delete this!');
// Simulate Ctrl+u to delete the line written previously
rl.write(null, {ctrl: true, name: 'u'});

Note: The rl.write() method will write the data to the readline Interface's input as if it were provided by the user.

readline.clearLine(stream, dir)#

Added in: v0.7.7
  • stream <Writable>
  • dir <number>
    • -1 - to the left from cursor
    • 1 - to the right from cursor
    • 0 - the entire line

The readline.clearLine() method clears current line of given TTY stream in a specified direction identified by dir.

readline.clearScreenDown(stream)#

Added in: v0.7.7
  • stream <Writable>

The readline.clearScreenDown() method clears the given TTY stream from the current position of the cursor down.

readline.createInterface(options)#

Added in: v0.1.98
  • options <Object>
    • input <Readable> The Readable stream to listen to. This option is required.
    • output <Writable> The Writable stream to write readline data to.
    • completer <Function> An optional function used for Tab autocompletion.
    • terminal <boolean> true if the input and output streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Defaults to checking isTTY on the output stream upon instantiation.
    • historySize <number> maximum number of history lines retained. To disable the history set this value to 0. Defaults to 30. This option makes sense only if terminal is set to true by the user or by an internal output check, otherwise the history caching mechanism is not initialized at all.
    • prompt - the prompt string to use. Default: '> '
    • crlfDelay <number> If the delay between \r and \n exceeds crlfDelay milliseconds, both \r and \n will be treated as separate end-of-line input. Default to 100 milliseconds. crlfDelay will be coerced to [100, 2000] range.

The readline.createInterface() method creates a new readline.Interface instance.

For example:

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

Once the readline.Interface instance is created, the most common case is to listen for the 'line' event:

rl.on('line', (line) => {
  console.log(`Received: ${line}`);
});

If terminal is true for this instance then the output stream will get the best compatibility if it defines an output.columns property and emits a 'resize' event on the output if or when the columns ever change (process.stdout does this automatically when it is a TTY).

Use of the completer Function#

When called, the completer function is provided the current line entered by the user, and is expected to return an Array with 2 entries:

  • An Array with matching entries for the completion.
  • The substring that was used for the matching.

For instance: [[substr1, substr2, ...], originalsubstring].

function completer(line) {
  const completions = '.help .error .exit .quit .q'.split(' ');
  const hits = completions.filter((c) => { return c.indexOf(line) === 0 });
  // show all completions if none found
  return [hits.length ? hits : completions, line];
}

The completer function can be called asynchronously if it accepts two arguments:

function completer(linePartial, callback) {
  callback(null, [['123'], linePartial]);
}

readline.cursorTo(stream, x, y)#

Added in: v0.7.7
  • stream <Writable>
  • x <number>
  • y <number>

The readline.cursorTo() method moves cursor to the specified position in a given TTY stream.

readline.emitKeypressEvents(stream[, interface])#

Added in: v0.7.7
  • stream <Readable>
  • interface <readline.Interface>

The readline.emitKeypressEvents() method causes the given Writable stream to begin emitting 'keypress' events corresponding to received input.

Optionally, interface specifies a readline.Interface instance for which autocompletion is disabled when copy-pasted input is detected.

If the stream is a TTY, then it must be in raw mode.

readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
  process.stdin.setRawMode(true);

readline.moveCursor(stream, dx, dy)#

Added in: v0.7.7
  • stream <Writable>
  • dx <number>
  • dy <Number>

The readline.moveCursor() method moves the cursor relative to its current position in a given TTY stream.

Example: Tiny CLI#

The following example illustrates the use of readline.Interface class to implement a small command-line interface:

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: 'OHAI> '
});

rl.prompt();

rl.on('line', (line) => {
  switch(line.trim()) {
    case 'hello':
      console.log('world!');
      break;
    default:
      console.log(`Say what? I might have heard '${line.trim()}'`);
      break;
  }
  rl.prompt();
}).on('close', () => {
  console.log('Have a great day!');
  process.exit(0);
});

Example: Read File Stream Line-by-Line#

A common use case for readline is to consume input from a filesystem Readable stream one line at a time, as illustrated in the following example:

const readline = require('readline');
const fs = require('fs');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt')
});

rl.on('line', (line) => {
  console.log(`Line from file: ${line}`);
});

REPL#

Stability: 2 - Stable

The repl module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:

const repl = require('repl');

Design and Features#

The repl module exports the repl.REPLServer class. While running, instances of repl.REPLServer will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from stdin and stdout, respectively, or may be connected to any Node.js stream.

Instances of repl.REPLServer support automatic completion of inputs, simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions.

Commands and Special Keys#

The following special commands are supported by all REPL instances:

  • .break - When in the process of inputting a multi-line expression, entering the .break command (or pressing the <ctrl>-C key combination) will abort further input or processing of that expression.
  • .clear - Resets the REPL context to an empty object and clears any multi-line expression currently being input.
  • .exit - Close the I/O stream, causing the REPL to exit.
  • .help - Show this list of special commands.
  • .save - Save the current REPL session to a file: > .save ./file/to/save.js
  • .load - Load a file into the current REPL session. > .load ./file/to/load.js
  • .editor - Enter editor mode (<ctrl>-D to finish, <ctrl>-C to cancel)
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
  return `Hello ${name}!`;
}

welcome('Node.js User');

// ^D
'Hello Node.js User!'
>

The following key combinations in the REPL have these special effects:

  • <ctrl>-C - When pressed once, has the same effect as the .break command. When pressed twice on a blank line, has the same effect as the .exit command.
  • <ctrl>-D - Has the same effect as the .exit command.
  • <tab> - When pressed on a blank line, displays global and local(scope) variables. When pressed while entering other input, displays relevant autocompletion options.

Default Evaluation#

By default, all instances of repl.REPLServer use an evaluation function that evaluates JavaScript expressions and provides access to Node.js' built-in modules. This default behavior can be overridden by passing in an alternative evaluation function when the repl.REPLServer instance is created.

JavaScript Expressions#

The default evaluator supports direct evaluation of JavaScript expressions:

> 1 + 1
2
> const m = 2
undefined
> m + 1
3

Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the const, let, or var keywords are declared at the global scope.

Global and Local Scope#

The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the context object associated with each REPLServer. For example:

const repl = require('repl');
const msg = 'message';

repl.start('> ').context.m = msg;

Properties in the context object appear as local within the REPL:

$ node repl_test.js
> m
'message'

It is important to note that context properties are not read-only by default. To specify read-only globals, context properties must be defined using Object.defineProperty():

const repl = require('repl');
const msg = 'message';

const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
  configurable: false,
  enumerable: true,
  value: msg
});

Accessing Core Node.js Modules#

The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input fs will be evaluated on-demand as global.fs = require('fs').

> fs.createReadStream('./some/file');

Assignment of the _ (underscore) variable#

The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable _ (underscore). Explicitly setting _ to a value will disable this behavior.

> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4

Custom Evaluation Functions#

When a new repl.REPLServer is created, a custom evaluation function may be provided. This can be used, for instance, to implement fully customized REPL applications.

The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:

const repl = require('repl');
const Translator = require('translator').Translator;

const myTranslator = new Translator('en', 'fr');

function myEval(cmd, context, filename, callback) {
  callback(null, myTranslator.translate(cmd));
}

repl.start({prompt: '> ', eval: myEval});

Recoverable Errors#

As a user is typing input into the REPL prompt, pressing the <enter> key will send the current line of input to the eval function. In order to support multi-line input, the eval function can return an instance of repl.Recoverable to the provided callback function:

function myEval(cmd, context, filename, callback) {
  let result;
  try {
    result = vm.runInThisContext(cmd);
  } catch (e) {
    if (isRecoverableError(e)) {
      return callback(new repl.Recoverable(e));
    }
  }
  callback(null, result);
}

function isRecoverableError(error) {
  if (error.name === 'SyntaxError') {
    return /^(Unexpected end of input|Unexpected token)/.test(error.message);
  }
  return false;
}

Customizing REPL Output#

By default, repl.REPLServer instances format output using the util.inspect() method before writing the output to the provided Writable stream (process.stdout by default). The useColors boolean option can be specified at construction to instruct the default writer to use ANSI style codes to colorize the output from the util.inspect() method.

It is possible to fully customize the output of a repl.REPLServer instance by passing a new function in using the writer option on construction. The following example, for instance, simply converts any input text to upper case:

const repl = require('repl');

const r = repl.start({prompt: '> ', eval: myEval, writer: myWriter});

function myEval(cmd, context, filename, callback) {
  callback(null, cmd);
}

function myWriter(output) {
  return output.toUpperCase();
}

Class: REPLServer#

Added in: v0.1.91

The repl.REPLServer class inherits from the readline.Interface class. Instances of repl.REPLServer are created using the repl.start() method and should not be created directly using the JavaScript new keyword.

Event: 'exit'#

Added in: v0.7.7

The 'exit' event is emitted when the REPL is exited either by receiving the .exit command as input, the user pressing <ctrl>-C twice to signal SIGINT, or by pressing <ctrl>-D to signal 'end' on the input stream. The listener callback is invoked without any arguments.

replServer.on('exit', () => {
  console.log('Received "exit" event from repl!');
  process.exit();
});

Event: 'reset'#

Added in: v0.11.0

The 'reset' event is emitted when the REPL's context is reset. This occurs whenever the .clear command is received as input unless the REPL is using the default evaluator and the repl.REPLServer instance was created with the useGlobal option set to true. The listener callback will be called with a reference to the context object as the only argument.

This can be used primarily to re-initialize REPL context to some pre-defined state as illustrated in the following simple example:

const repl = require('repl');

function initializeContext(context) {
  context.m = 'test';
}

const r = repl.start({prompt: '> '});
initializeContext(r.context);

r.on('reset', initializeContext);

When this code is executed, the global 'm' variable can be modified but then reset to its initial value using the .clear command:

$ ./node example.js
> m
'test'
> m = 1
1
> m
1
> .clear
Clearing context...
> m
'test'
>

replServer.defineCommand(keyword, cmd)#

Added in: v0.3.0
  • keyword <String> The command keyword (without a leading . character).
  • cmd <Object> | <Function> The function to invoke when the command is processed.

The replServer.defineCommand() method is used to add new .-prefixed commands to the REPL instance. Such commands are invoked by typing a . followed by the keyword. The cmd is either a Function or an object with the following properties:

  • help <String> Help text to be displayed when .help is entered (Optional).
  • action <Function> The function to execute, optionally accepting a single string argument.

The following example shows two new commands added to the REPL instance:

const repl = require('repl');

const replServer = repl.start({prompt: '> '});
replServer.defineCommand('sayhello', {
  help: 'Say hello',
  action(name) {
    this.lineParser.reset();
    this.bufferedCommand = '';
    console.log(`Hello, ${name}!`);
    this.displayPrompt();
  }
});
replServer.defineCommand('saybye', () => {
  console.log('Goodbye!');
  this.close();
});

The new commands can then be used from within the REPL instance:

> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!

replServer.displayPrompt([preserveCursor])#

Added in: v0.1.91

The replServer.displayPrompt() method readies the REPL instance for input from the user, printing the configured prompt to a new line in the output and resuming the input to accept new input.

When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.

When preserveCursor is true, the cursor placement will not be reset to 0.

The replServer.displayPrompt method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

repl.start(options)#

Added in: v0.1.91
  • options <Object> | <String>
    • prompt <String> The input prompt to display. Defaults to > (with a trailing space).
    • input <Readable> The Readable stream from which REPL input will be read. Defaults to process.stdin.
    • output <Writable> The Writable stream to which REPL output will be written. Defaults to process.stdout.
    • terminal <boolean> If true, specifies that the output should be treated as a a TTY terminal, and have ANSI/VT100 escape codes written to it. Defaults to checking the value of the isTTY property on the output stream upon instantiation.
    • eval <Function> The function to be used when evaluating each given line of input. Defaults to an async wrapper for the JavaScript eval() function. An eval function can error with repl.Recoverable to indicate the input was incomplete and prompt for additional lines.
    • useColors <boolean> If true, specifies that the default writer function should include ANSI color styling to REPL output. If a custom writer function is provided then this has no effect. Defaults to the REPL instances terminal value.
    • useGlobal <boolean> If true, specifies that the default evaluation function will use the JavaScript global as the context as opposed to creating a new separate context for the REPL instance. Defaults to false.
    • ignoreUndefined <boolean> If true, specifies that the default writer will not output the return value of a command if it evaluates to undefined. Defaults to false.
    • writer <Function> The function to invoke to format the output of each command before writing to output. Defaults to util.inspect().
    • completer <Function> An optional function used for custom Tab auto completion. See readline.InterfaceCompleter for an example.
    • replMode - A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode, default mode, or a hybrid mode ("magic" mode.) Acceptable values are:
      • repl.REPL_MODE_SLOPPY - evaluates expressions in sloppy mode.
      • repl.REPL_MODE_STRICT - evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.
      • repl.REPL_MODE_MAGIC - attempt to evaluates expressions in default mode. If expressions fail to parse, re-try in strict mode.
    • breakEvalOnSigint - Stop evaluating the current piece of code when SIGINT is received, i.e. Ctrl+C is pressed. This cannot be used together with a custom eval function. Defaults to false.

The repl.start() method creates and starts a repl.REPLServer instance.

If options is a string, then it specifies the input prompt:

const repl = require('repl');

// a Unix style prompt
repl.start('$ ');

The Node.js REPL#

Node.js itself uses the repl module to provide its own interactive interface for executing JavaScript. This can be used by executing the Node.js binary without passing any arguments (or by passing the -i argument):

$ node
> const a = [1, 2, 3];
[ 1, 2, 3 ]
> a.forEach((v) => {
...   console.log(v);
...   });
1
2
3

Environment Variable Options#

Various behaviors of the Node.js REPL can be customized using the following environment variables:

  • NODE_REPL_HISTORY - When a valid path is given, persistent REPL history will be saved to the specified file rather than .node_repl_history in the user's home directory. Setting this value to "" will disable persistent REPL history. Whitespace will be trimmed from the value.
  • NODE_REPL_HISTORY_SIZE - Defaults to 1000. Controls how many lines of history will be persisted if history is available. Must be a positive number.
  • NODE_REPL_MODE - May be any of sloppy, strict, or magic. Defaults to magic, which will automatically run "strict mode only" statements in strict mode.

Persistent History#

By default, the Node.js REPL will persist history between node REPL sessions by saving inputs to a .node_repl_history file located in the user's home directory. This can be disabled by setting the environment variable NODE_REPL_HISTORY="".

NODE_REPL_HISTORY_FILE#

Added in: v2.0.0 Deprecated since: v3.0.0 
Stability: 0 - Deprecated: Use NODE_REPL_HISTORY instead.

Previously in Node.js/io.js v2.x, REPL history was controlled by using a NODE_REPL_HISTORY_FILE environment variable, and the history was saved in JSON format. This variable has now been deprecated, and the old JSON REPL history file will be automatically converted to a simplified plain text format. This new file will be saved to either the user's home directory, or a directory defined by the NODE_REPL_HISTORY variable, as documented in the Environment Variable Options.

Using the Node.js REPL with advanced line-editors#

For advanced line-editors, start Node.js with the environmental variable NODE_NO_READLINE=1. This will start the main and debugger REPL in canonical terminal settings which will allow you to use with rlwrap.

For example, you could add this to your bashrc file:

alias node="env NODE_NO_READLINE=1 rlwrap node"

Starting multiple REPL instances against a single running instance#

It is possible to create and run multiple REPL instances against a single running instance of Node.js that share a single global object but have separate I/O interfaces.

The following example, for instance, provides separate REPLs on stdin, a Unix socket, and a TCP socket:

const net = require('net');
const repl = require('repl');
let connections = 0;

repl.start({
  prompt: 'Node.js via stdin> ',
  input: process.stdin,
  output: process.stdout
});

net.createServer((socket) => {
  connections += 1;
  repl.start({
    prompt: 'Node.js via Unix socket> ',
    input: socket,
    output: socket
  }).on('exit', () => {
    socket.end();
  });
}).listen('/tmp/node-repl-sock');

net.createServer((socket) => {
  connections += 1;
  repl.start({
    prompt: 'Node.js via TCP socket> ',
    input: socket,
    output: socket
  }).on('exit', () => {
    socket.end();
  });
}).listen(5001);

Running this application from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. telnet, for instance, is useful for connecting to TCP sockets, while socat can be used to connect to both Unix and TCP sockets.

By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.

For an example of running a "full-featured" (terminal) REPL over a net.Server and net.Socket instance, see: https://gist.github.com/2209310

For an example of running a REPL instance over curl(1), see: https://gist.github.com/2053342

Stream#

Stability: 2 - Stable

A stream is an abstract interface for working with streaming data in Node.js. The stream module provides a base API that makes it easy to build objects that implement the stream interface.

There are many stream objects provided by Node.js. For instance, a request to an HTTP server and process.stdout are both stream instances.

Streams can be readable, writable, or both. All streams are instances of EventEmitter.

The stream module can be accessed using:

const stream = require('stream');

While it is important for all Node.js users to understand how streams work, the stream module itself is most useful for developers that are creating new types of stream instances. Developer's who are primarily consuming stream objects will rarely (if ever) have need to use the stream module directly.

Organization of this Document#

This document is divided into two primary sections with a third section for additional notes. The first section explains the elements of the stream API that are required to use streams within an application. The second section explains the elements of the API that are required to implement new types of streams.

Types of Streams#

There are four fundamental stream types within Node.js:

Object Mode#

All streams created by Node.js APIs operate exclusively on strings and Buffer objects. It is possible, however, for stream implementations to work with other types of JavaScript values (with the exception of null, which serves a special purpose within streams). Such streams are considered to operate in "object mode".

Stream instances are switched into object mode using the objectMode option when the stream is created. Attempting to switch an existing stream into object mode is not safe.

Buffering#

Both Writable and Readable streams will store data in an internal buffer that can be retrieved using writable._writableState.getBuffer() or readable._readableState.buffer, respectively.

The amount of data potentially buffered depends on the highWaterMark option passed into the streams constructor. For normal streams, the highWaterMark option specifies a total number of bytes. For streams operating in object mode, the highWaterMark specifies a total number of objects.

Data is buffered in Readable streams when the implementation calls stream.push(chunk). If the consumer of the Stream does not call stream.read(), the data will sit in the internal queue until it is consumed.

Once the total size of the internal read buffer reaches the threshold specified by highWaterMark, the stream will temporarily stop reading data from the underlying resource until the data currently buffered can be consumed (that is, the stream will stop calling the internal readable._read() method that is used to fill the read buffer).

Data is buffered in Writable streams when the writable.write(chunk) method is called repeatedly. While the total size of the internal write buffer is below the threshold set by highWaterMark, calls to writable.write() will return true. Once the size of the internal buffer reaches or exceeds the highWaterMark, false will be returned.

A key goal of the stream API, particularly the stream.pipe() method, is to limit the buffering of data to acceptable levels such that sources and destinations of differing speeds will not overwhelm the available memory.

Because Duplex and Transform streams are both Readable and Writable, each maintain two separate internal buffers used for reading and writing, allowing each side to operate independently of the other while maintaining an appropriate and efficient flow of data. For example, net.Socket instances are Duplex streams whose Readable side allows consumption of data received from the socket and whose Writable side allows writing data to the socket. Because data may be written to the socket at a faster or slower rate than data is received, it is important for each side to operate (and buffer) independently of the other.

API for Stream Consumers#

Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:

const http = require('http');

const server = http.createServer( (req, res) => {
  // req is an http.IncomingMessage, which is a Readable Stream
  // res is an http.ServerResponse, which is a Writable Stream

  let body = '';
  // Get the data as utf8 strings.
  // If an encoding is not set, Buffer objects will be received.
  req.setEncoding('utf8');

  // Readable streams emit 'data' events once a listener is added
  req.on('data', (chunk) => {
    body += chunk;
  });

  // the end event indicates that the entire body has been received
  req.on('end', () => {
    try {
      const data = JSON.parse(body);
      // write back something interesting to the user:
      res.write(typeof data);
      res.end();
    } catch (er) {
      // uh oh!  bad json!
      res.statusCode = 400;
      return res.end(`error: ${er.message}`);
    }
  });
});

server.listen(1337);

// $ curl localhost:1337 -d '{}'
// object
// $ curl localhost:1337 -d '"foo"'
// string
// $ curl localhost:1337 -d 'not json'
// error: Unexpected token o

Writable streams (such as res in the example) expose methods such as write() and end() that are used to write data onto the stream.

Readable streams use the EventEmitter API for notifying application code when data is available to be read off the stream. That available data can be read from the stream in multiple ways.

Both Writable and Readable streams use the EventEmitter API in various ways to communicate the current state of the stream.

Duplex and Transform streams are both Writable and Readable.

Applications that are either writing data to or consuming data from a stream are not required to implement the stream interfaces directly and will generally have no reason to call require('stream').

Developers wishing to implement new types of streams should refer to the section API for Stream Implementers.

Writable Streams#

Writable streams are an abstraction for a destination to which data is written.

Examples of Writable streams include:

Note: Some of these examples are actually Duplex streams that implement the Writable interface.

All Writable streams implement the interface defined by the stream.Writable class.

While specific instances of Writable streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:

const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');

Class: stream.Writable#

Added in: v0.9.4
Event: 'close'#
Added in: v0.9.4

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

Not all Writable streams will emit the 'close' event.

Event: 'drain'#
Added in: v0.9.4

If a call to stream.write(chunk) returns false, the 'drain' event will be emitted when it is appropriate to resume writing data to the stream.

// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    var ok = true;
    do {
      i--;
      if (i === 0) {
        // last time!
        writer.write(data, encoding, callback);
      } else {
        // see if we should continue, or wait
        // don't pass the callback, because we're not done yet.
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // had to stop early!
      // write some more once it drains
      writer.once('drain', write);
    }
  }
}
Event: 'error'#
Added in: v0.9.4

The 'error' event is emitted if an error occurred while writing or piping data. The listener callback is passed a single Error argument when called.

Note: The stream is not closed when the 'error' event is emitted.

Event: 'finish'#
Added in: v0.9.4

The 'finish' event is emitted after the stream.end() method has been called, and all data has been flushed to the underlying system.

const writer = getWritableStreamSomehow();
for (var i = 0; i < 100; i ++) {
  writer.write('hello, #${i}!\n');
}
writer.end('This is the end\n');
writer.on('finish', () => {
  console.error('All writes are now complete.');
});
Event: 'pipe'#
Added in: v0.9.4

The 'pipe' event is emitted when the stream.pipe() method is called on a readable stream, adding this writable to its set of destinations.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.error('something is piping into the writer');
  assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'#
Added in: v0.9.4

The 'unpipe' event is emitted when the stream.unpipe() method is called on a Readable stream, removing this Writable from its set of destinations.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
  console.error('Something has stopped piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()#
Added in: v0.11.2

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the stream.uncork() or stream.end() methods are called.

The primary intent of writable.cork() is to avoid a situation where writing many small chunks of data to a stream do not cause a backup in the internal buffer that would have an adverse impact on performance. In such situations, implementations that implement the writable._writev() method can perform buffered writes in a more optimized manner.

writable.end([chunk][, encoding][, callback])#
Added in: v0.9.4
  • chunk <String> | <Buffer> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string or a Buffer. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <String> The encoding, if chunk is a String
  • callback <Function> Optional callback for when the stream is finished

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream. If provided, the optional callback function is attached as a listener for the 'finish' event.

Calling the stream.write() method after calling stream.end() will raise an error.

// write 'hello, ' and then end with 'world!'
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// writing more now is not allowed!
writable.setDefaultEncoding(encoding)#
Added in: v0.11.15
  • encoding <String> The new default encoding
  • Returns: this

The writable.setDefaultEncoding() method sets the default encoding for a Writable stream.

writable.uncork()#
Added in: v0.11.2

The writable.uncork() method flushes all data buffered since stream.cork() was called.

When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, it is recommended that calls to writable.uncork() be deferred using process.nextTick(). Doing so allows batching of all writable.write() calls that occur within a given Node.js event loop phase.

stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());

If the writable.cork() method is called multiple times on a stream, the same number of calls to writable.uncork() must be called to flush the buffered data.

stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
  stream.uncork();
  // The data will not be flushed until uncork() is called a second time.
  stream.uncork();
});
writable.write(chunk[, encoding][, callback])#
Added in: v0.9.4
  • chunk <String> | <Buffer> The data to write
  • encoding <String> The encoding, if chunk is a String
  • callback <Function> Callback for when this chunk of data is flushed
  • Returns: <Boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback may or may not be called with the error as its first argument. To reliably detect write errors, add a listener for the 'error' event.

The return value is true if the internal buffer is less than the highWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. It is recommended that once write() returns false, no more chunks be written until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or an 'data' or 'readable' event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use stream.pipe(). However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the the 'drain' event:

function write (data, cb) {
  if (!stream.write(data)) {
    stream.once('drain', cb)
  } else {
    process.nextTick(cb)
  }
}

// Wait for cb to be called before doing any other write.
write('hello', () => {
  console.log('write completed, do more writes now')
})

A Writable stream in object mode will always ignore the encoding argument.

Readable Streams#

Readable streams are an abstraction for a source from which data is consumed.

Examples of Readable streams include:

All Readable streams implement the interface defined by the stream.Readable class.

Two Modes#

Readable streams effectively operate in one of two modes: flowing and paused.

When in flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the EventEmitter interface.

In paused mode, the stream.read() method must be called explicitly to read chunks of data from the stream.

All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:

The Readable can switch back to paused mode using one of the following:

  • If there are no pipe destinations, by calling the stream.pause() method.
  • If there are pipe destinations, by removing any 'data' event handlers, and removing all pipe destinations by calling the stream.unpipe() method.

The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.

Note: For backwards compatibility reasons, removing 'data' event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling stream.pause() will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

Note: If a Readable is switched into flowing mode and there are no consumers available handle the data, that data will be lost. This can occur, for instance, when the readable.resume() method is called without a listener attached to the 'data' event, or when a 'data' event handler is removed from the stream.

Three States#

The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

  • readable._readableState.flowing = null
  • readable._readableState.flowing = false
  • readable._readableState.flowing = true

When readable._readableState.flowing is null, no mechanism for consuming the streams data is provided so the stream will not generate its data.

Attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable._readableState.flowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving "back pressure" will cause the readable._readableState.flowing to be set as false, temporarily halting the flowing of events but not halting the generation of data.

While readable._readableState.flowing is false, data may be accumulating within the streams internal buffer.

Choose One#

The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream.

Use of the readable.pipe() method is recommended for most users as it has been implemented to provide the easiest way of consuming stream data. Developers that require more fine-grained control over the transfer and generation of data can use the EventEmitter and readable.pause()/readable.resume() APIs.

Class: stream.Readable#

Added in: v0.9.4
Event: 'close'#
Added in: v0.9.4

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

Not all Readable streams will emit the 'close' event.

Event: 'data'#
Added in: v0.9.4
  • chunk <Buffer> | <String> | <any> The chunk of data. For streams that are not operating in object mode, the chunk will be either a string or Buffer. For streams that are in object mode, the chunk can be any JavaScript value other than null.

The 'data' event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling readable.pipe(), readable.resume(), or by attaching a listener callback to the 'data' event. The 'data' event will also be emitted whenever the readable.read() method is called and a chunk of data is available to be returned.

Attaching a 'data' event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the readable.setEncoding() method; otherwise the data will be passed as a Buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'#
Added in: v0.9.4

The 'end' event is emitted when there is no more data to be consumed from the stream.

Note: The 'end' event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling stream.read() repeatedly until all data has been consumed.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
  console.log('There will be no more data.');
});
Event: 'error'#
Added in: v0.9.4

The 'error' event may be emitted by a Readable implementation at any time. Typically, this may occur if the underlying stream in unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single Error object.

Event: 'readable'#
Added in: v0.9.4

The 'readable' event is emitted when there is data available to be read from the stream. In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

const readable = getReadableStreamSomehow();
readable.on('readable', () => {
  // there is some data to read now
});

The 'readable' event will also be emitted once the end of the stream data has been reached but before the 'end' event is emitted.

Effectively, the 'readable' event indicates that the stream has new information: either new data is available or the end of the stream has been reached. In the former case, stream.read() will return the available data. In the latter case, stream.read() will return null. For instance, in the following example, foo.txt is an empty file:

const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log('readable:', rr.read());
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

Note: In general, the readable.pipe() and 'data' event mechanisms are preferred over the use of the 'readable' event.

readable.isPaused()#

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.pipe() method. In most typical cases, there will be no reason to use this method directly.

const readable = new stream.Readable

readable.isPaused() // === false
readable.pause()
readable.isPaused() // === true
readable.resume()
readable.isPaused() // === false
readable.pause()#
Added in: v0.9.4
  • Returns: this

The readable.pause() method will cause a stream in flowing mode to stop emitting 'data' events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
  readable.pause();
  console.log('There will be no additional data for 1 second.');
  setTimeout(() => {
    console.log('Now data will start flowing again.');
    readable.resume();
  }, 1000);
});
readable.pipe(destination[, options])#
Added in: v0.9.4

The readable.pipe() method attaches a Writable stream to the readable, causing it to switch automatically into flowing mode and push all of its data to the attached Writable. The flow of data will be automatically managed so that the destination Writable stream is not overwhelmed by a faster Readable stream.

The following example pipes all of the data from the readable into a file named file.txt:

const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'
readable.pipe(writable);

It is possible to attach multiple Writable streams to a single Readable stream.

The readable.pipe() method returns a reference to the destination stream making it possible to set up chains of piped streams:

const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);

By default, stream.end() is called on the destination Writable stream when the source Readable stream emits 'end', so that the destination is no longer writable. To disable this default behavior, the end option can be passed as false, causing the destination stream to remain open, as illustrated in the following example:

reader.pipe(writer, { end: false });
reader.on('end', () => {
  writer.end('Goodbye\n');
});

One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.

Note: The process.stderr and process.stdout Writable streams are never closed until the Node.js process exits, regardless of the specified options.

readable.read([size])#
Added in: v0.9.4

The readable.read() method pulls some data out of the internal buffer and returns it. If no data available to be read, null is returned. By default, the data will be returned as a Buffer object unless an encoding has been specified using the readable.setEncoding() method or the stream is operating in object mode.

The optional size argument specifies a specific number of bytes to read. If size bytes are not available to be read, null will be returned unless the stream has ended, in which case all of the data remaining in the internal buffer will be returned (even if it exceeds size bytes).

If the size argument is not specified, all of the data contained in the internal buffer will be returned.

The readable.read() method should only be called on Readable streams operating in paused mode. In flowing mode, readable.read() is called automatically until the internal buffer is fully drained.

const readable = getReadableStreamSomehow();
readable.on('readable', () => {
  var chunk;
  while (null !== (chunk = readable.read())) {
    console.log(`Received ${chunk.length} bytes of data.`);
  }
});

In general, it is recommended that developers avoid the use of the 'readable' event and the readable.read() method in favor of using either readable.pipe() or the 'data' event.

A Readable stream in object mode will always return a single item from a call to readable.read(size), regardless of the value of the size argument.

Note: If the readable.read() method returns a chunk of data, a 'data' event will also be emitted.

Note: Calling stream.read([size]) after the 'end' event has been emitted will return null. No runtime error will be raised.

readable.resume()#
Added in: v0.9.4
  • Returns: this

The readable.resume() method causes an explicitly paused Readable stream to resume emitting 'data' events, switching the stream into flowing mode.

The readable.resume() method can be used to fully consume the data from a stream without actually processing any of that data as illustrated in the following example:

getReadableStreamSomehow()
  .resume()
  .on('end', () => {
    console.log('Reached the end, but did not read anything.');
  });
readable.setEncoding(encoding)#
Added in: v0.9.4
  • encoding <String> The encoding to use.
  • Returns: this

The readable.setEncoding() method sets the default character encoding for data read from the Readable stream.

Setting an encoding causes the stream data to be returned as string of the specified encoding rather than as Buffer objects. For instance, calling readable.setEncoding('utf8') will cause the output data will be interpreted as UTF-8 data, and passed as strings. Calling readable.setEncoding('hex') will cause the data to be encoded in hexadecimal string format.

The Readable stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as Buffer objects.

Encoding can be disabled by calling readable.setEncoding(null). This approach is useful when working with binary data or with large multi-byte strings spread out over multiple chunks.

const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
  assert.equal(typeof chunk, 'string');
  console.log('got %d characters of string data', chunk.length);
});
readable.unpipe([destination])#
Added in: v0.9.4

The readable.unpipe() method detaches a Writable stream previously attached using the stream.pipe() method.

If the destination is not specified, then all pipes are detached.

If the destination is specified, but no pipe is set up for it, then the method does nothing.

const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second
readable.pipe(writable);
setTimeout(() => {
  console.log('Stop writing to file.txt');
  readable.unpipe(writable);
  console.log('Manually close the file stream');
  writable.end();
}, 1000);
readable.unshift(chunk)#
Added in: v0.9.11

The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

Note: The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the API for Stream Implementers section for more information.

// Pull off a header delimited by \n\n
// use unshift() if we get too much
// Call the callback with (error, header, stream)
const StringDecoder = require('string_decoder').StringDecoder;
function parseHeader(stream, callback) {
  stream.on('error', callback);
  stream.on('readable', onReadable);
  const decoder = new StringDecoder('utf8');
  var header = '';
  function onReadable() {
    var chunk;
    while (null !== (chunk = stream.read())) {
      var str = decoder.write(chunk);
      if (str.match(/\n\n/)) {
        // found the header boundary
        var split = str.split(/\n\n/);
        header += split.shift();
        const remaining = split.join('\n\n');
        const buf = Buffer.from(remaining, 'utf8');
        stream.removeListener('error', callback);
        // set the readable listener before unshifting
        stream.removeListener('readable', onReadable);
        if (buf.length)
          stream.unshift(buf);
        // now the body of the message can be read from the stream.
        callback(null, header, stream);
      } else {
        // still reading the header.
        header += str;
      }
    }
  }
}

Note: Unlike stream.push(chunk), stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a stream._read() implementation on a custom stream). Following the call to readable.unshift() with an immediate stream.push('') will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

readable.wrap(stream)#
Added in: v0.9.4
  • stream <Stream> An "old style" readable stream

Versions of Node.js prior to v0.10 had streams that did not implement the entire stream module API as it is currently defined. (See Compatibility for more information.)

When using an older Node.js library that emits 'data' events and has a stream.pause() method that is advisory only, the readable.wrap() method can be used to create a Readable stream that uses the old stream as its data source.

It will rarely be necessary to use readable.wrap() but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

For example:

const OldReader = require('./old-api-module.js').OldReader;
const Readable = require('stream').Readable;
const oreader = new OldReader;
const myReader = new Readable().wrap(oreader);

myReader.on('readable', () => {
  myReader.read(); // etc.
});

Duplex and Transform Streams#

Class: stream.Duplex#

Added in: v0.9.4

Duplex streams are streams that implement both the Readable and Writable interfaces.

Examples of Duplex streams include:

Class: stream.Transform#

Added in: v0.9.4

Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.

Examples of Transform streams include:

API for Stream Implementers#

The stream module API has been designed to make it possible to easily implement streams using JavaScript's prototypal inheritance model.

First, a stream developer would declare a new JavaScript class that extends one of the four basic stream classes (stream.Writable, stream.Readable, stream.Duplex, or stream.Transform), making sure the call the appropriate parent class constructor:

const Writable = require('stream').Writable;

class MyWritable extends Writable {
  constructor(options) {
    super(options);
  }
}

The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:

Use-case

Class

Method(s) to implement

Reading only

Readable

_read

Writing only

Writable

_write, _writev

Reading and writing

Duplex

_read, _write, _writev

Operate on written data, then read the result

Transform

_transform, _flush

Note: The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for Stream Consumers section). Doing so may lead to adverse side effects in application code consuming the stream.

Simplified Construction#

For many simple cases, it is possible to construct a stream without relying on inheritance. This can be accomplished by directly creating instances of the stream.Writable, stream.Readable, stream.Duplex or stream.Transform objects and passing appropriate methods as constructor options.

For example:

const Writable = require('stream').Writable;

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  }
});

Implementing a Writable Stream#

The stream.Writable class is extended to implement a Writable stream.

Custom Writable streams must call the new stream.Writable([options]) constructor and implement the writable._write() method. The writable._writev() method may also be implemented.

Constructor: new stream.Writable(options)#

For example:

const Writable = require('stream').Writable;

class MyWritable extends Writable {
  constructor(options) {
    // Calls the stream.Writable() constructor
    super(options);
  }
}

Or, when using pre-ES6 style constructors:

const Writable = require('stream').Writable;
const util = require('util');

function MyWritable(options) {
  if (!(this instanceof MyWritable))
    return new MyWritable(options);
  Writable.call(this, options);
}
util.inherits(MyWritable, Writable);

Or, using the Simplified Constructor approach:

const Writable = require('stream').Writable;

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  },
  writev(chunks, callback) {
    // ...
  }
});

writable._write(chunk, encoding, callback)#

  • chunk <Buffer> | <String> The chunk to be written. Will always be a buffer unless the decodeStrings option was set to false.
  • encoding <String> If the chunk is a string, then encoding is the character encoding of that string. If chunk is a Buffer, or if the stream is operating in object mode, encoding may be ignored.
  • callback <Function> Call this function (optionally with an error argument) when processing is complete for the supplied chunk.

All Writable stream implementations must provide a writable._write() method to send data to the underlying resource.

Note: Transform streams provide their own implementation of the writable._write().

Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Writable class methods only.

The callback method must be called to signal either that the write completed successfully or failed with an error. The first argument passed to the callback must be the Error object if the call failed or null if the write succeeded.

It is important to note that all calls to writable.write() that occur between the time writable._write() is called and the callback is called will cause the written data to be buffered. Once the callback is invoked, the stream will emit a 'drain' event. If a stream implementation is capable of processing multiple chunks of data at once, the writable._writev() method should be implemented.

If the decodeStrings property is set in the constructor options, then chunk may be a string rather than a Buffer, and encoding will indicate the character encoding of the string. This is to support implementations that have an optimized handling for certain string data encodings. If the decodeStrings property is explicitly set to false, the encoding argument can be safely ignored, and chunk will remain the same object that is passed to .write().

The writable._write() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

writable._writev(chunks, callback)#

  • chunks <Array> The chunks to be written. Each chunk has following format: { chunk: ..., encoding: ... }.
  • callback <Function> A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.

Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Writable class methods only.

The writable._writev() method may be implemented in addition to writable._write() in stream implementations that are capable of processing multiple chunks of data at once. If implemented, the method will be called with all chunks of data currently buffered in the write queue.

The writable._writev() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

Errors While Writing#

It is recommended that errors occurring during the processing of the writable._write() and writable._writev() methods are reported by invoking the callback and passing the error as the first argument. This will cause an 'error' event to be emitted by the Writable. Throwing an Error from within writable._write() can result in unexpected and inconsistent behavior depending on how the stream is being used. Using the callback ensures consistent and predictable handling of errors.

const Writable = require('stream').Writable;

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
});

An Example Writable Stream#

The following illustrates a rather simplistic (and somewhat pointless) custom Writable stream implementation. While this specific Writable stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom Writable stream instance:

const Writable = require('stream').Writable;

class MyWritable extends Writable {
  constructor(options) {
    super(options);
  }

  _write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
}

Implementing a Readable Stream#

The stream.Readable class is extended to implement a Readable stream.

Custom Readable streams must call the new stream.Readable([options]) constructor and implement the readable._read() method.

new stream.Readable(options)#

  • options <Object>
    • highWaterMark <Number> The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Defaults to 16384 (16kb), or 16 for objectMode streams
    • encoding <String> If specified, then buffers will be decoded to strings using the specified encoding. Defaults to null
    • objectMode <Boolean> Whether this stream should behave as a stream of objects. Meaning that stream.read(n) returns a single value instead of a Buffer of size n. Defaults to false
    • read <Function> Implementation for the stream._read() method.

For example:

const Readable = require('stream').Readable;

class MyReadable extends Readable {
  constructor(options) {
    // Calls the stream.Readable(options) constructor
    super(options);
  }
}

Or, when using pre-ES6 style constructors:

const Readable = require('stream').Readable;
const util = require('util');

function MyReadable(options) {
  if (!(this instanceof MyReadable))
    return new MyReadable(options);
  Readable.call(this, options);
}
util.inherits(MyReadable, Readable);

Or, using the Simplified Constructor approach:

const Readable = require('stream').Readable;

const myReadable = new Readable({
  read(size) {
    // ...
  }
});

readable._read(size)#

  • size <Number> Number of bytes to read asynchronously

Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.

All Readable stream implementations must provide an implementation of the readable._read() method to fetch data from the underlying resource.

When readable._read() is called, if data is available from the resource, the implementation should begin pushing that data into the read queue using the this.push(dataChunk) method. _read() should continue reading from the resource and pushing data until readable.push() returns false. Only when _read() is called again after it has stopped should it resume pushing additional data onto the queue.

Note: Once the readable._read() method has been called, it will not be called again until the readable.push() method is called.

The size argument is advisory. For implementations where a "read" is a single operation that returns data can use the size argument to determine how much data to fetch. Other implementations may ignore this argument and simply provide data whenever it becomes available. There is no need to "wait" until size bytes are available before calling stream.push(chunk).

The readable._read() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

readable.push(chunk[, encoding])#

  • chunk <Buffer> | <Null> | <String> Chunk of data to push into the read queue
  • encoding <String> Encoding of String chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'
  • Returns <Boolean> true if additional chunks of data may continued to be pushed; false otherwise.

When chunk is a Buffer or string, the chunk of data will be added to the internal queue for users of the stream to consume. Passing chunk as null signals the end of the stream (EOF), after which no more data can be written.

When the Readable is operating in paused mode, the data added with readable.push() can be read out by calling the readable.read() method when the 'readable' event is emitted.

When the Readable is operating in flowing mode, the data added with readable.push() will be delivered by emitting a 'data' event.

The readable.push() method is designed to be as flexible as possible. For example, when wrapping a lower-level source that provides some form of pause/resume mechanism, and a data callback, the low-level source can be wrapped by the custom Readable instance as illustrated in the following example:

// source is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.

class SourceWrapper extends Readable {
  constructor(options) {
    super(options);

    this._source = getLowlevelSourceObject();

    // Every time there's data, push it into the internal buffer.
    this._source.ondata = (chunk) => {
      // if push() returns false, then stop reading from source
      if (!this.push(chunk))
        this._source.readStop();
    };

    // When the source ends, push the EOF-signaling `null` chunk
    this._source.onend = () => {
      this.push(null);
    };
  }
  // _read will be called when the stream wants to pull more data in
  // the advisory size argument is ignored in this case.
  _read(size) {
    this._source.readStart();
  }
}

Note: The readable.push() method is intended be called only by Readable Implementers, and only from within the readable._read() method.

Errors While Reading#

It is recommended that errors occurring during the processing of the readable._read() method are emitted using the 'error' event rather than being thrown. Throwing an Error from within readable._read() can result in unexpected and inconsistent behavior depending on whether the stream is operating in flowing or paused mode. Using the 'error' event ensures consistent and predictable handling of errors.

const Readable = require('stream').Readable;

const myReadable = new Readable({
  read(size) {
    if (checkSomeErrorCondition()) {
      process.nextTick(() => this.emit('error', err));
      return;
    }
    // do some work
  }
});

An Example Counting Stream#

The following is a basic example of a Readable stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.

const Readable = require('stream').Readable;

class Counter extends Readable {
  constructor(opt) {
    super(opt);
    this._max = 1000000;
    this._index = 1;
  }

  _read() {
    var i = this._index++;
    if (i > this._max)
      this.push(null);
    else {
      var str = '' + i;
      var buf = Buffer.from(str, 'ascii');
      this.push(buf);
    }
  }
}

Implementing a Duplex Stream#

A Duplex stream is one that implements both Readable and Writable, such as a TCP socket connection.

Because JavaScript does not have support for multiple inheritance, the stream.Duplex class is extended to implement a Duplex stream (as opposed to extending the stream.Readable and stream.Writable classes).

Note: The stream.Duplex class prototypically inherits from stream.Readable and parasitically from stream.Writable, but instanceof will work properly for both base classes due to overriding Symbol.hasInstance on stream.Writable.

Custom Duplex streams must call the new stream.Duplex([options]) constructor and implement both the readable._read() and writable._write() methods.

new stream.Duplex(options)#

  • options <Object> Passed to both Writable and Readable constructors. Also has the following fields:
    • allowHalfOpen <Boolean> Defaults to true. If set to false, then the stream will automatically end the readable side when the writable side ends and vice versa.
    • readableObjectMode <Boolean> Defaults to false. Sets objectMode for readable side of the stream. Has no effect if objectMode is true.
    • writableObjectMode <Boolean> Defaults to false. Sets objectMode for writable side of the stream. Has no effect if objectMode is true.

For example:

const Duplex = require('stream').Duplex;

class MyDuplex extends Duplex {
  constructor(options) {
    super(options);
  }
}

Or, when using pre-ES6 style constructors:

const Duplex = require('stream').Duplex;
const util = require('util');

function MyDuplex(options) {
  if (!(this instanceof MyDuplex))
    return new MyDuplex(options);
  Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);

Or, using the Simplified Constructor approach:

const Duplex = require('stream').Duplex;

const myDuplex = new Duplex({
  read(size) {
    // ...
  },
  write(chunk, encoding, callback) {
    // ...
  }
});

An Example Duplex Stream#

The following illustrates a simple example of a Duplex stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read, albeit using an API that is not compatible with Node.js streams. The following illustrates a simple example of a Duplex stream that buffers incoming written data via the Writable interface that is read back out via the Readable interface.

const Duplex = require('stream').Duplex;
const kSource = Symbol('source');

class MyDuplex extends Duplex {
  constructor(source, options) {
    super(options);
    this[kSource] = source;
  }

  _write(chunk, encoding, callback) {
    // The underlying source only deals with strings
    if (Buffer.isBuffer(chunk))
      chunk = chunk.toString();
    this[kSource].writeSomeData(chunk);
    callback();
  }

  _read(size) {
    this[kSource].fetchSomeData(size, (data, encoding) => {
      this.push(Buffer.from(data, encoding));
    });
  }
}

The most important aspect of a Duplex stream is that the Readable and Writable sides operate independently of one another despite co-existing within a single object instance.

Object Mode Duplex Streams#

For Duplex streams, objectMode can be set exclusively for either the Readable or Writable side using the readableObjectMode and writableObjectMode options respectively.

In the following example, for instance, a new Transform stream (which is a type of Duplex stream) is created that has an object mode Writable side that accepts JavaScript numbers that are converted to hexadecimal strings on the Readable side.

const Transform = require('stream').Transform;

// All Transform streams are also Duplex Streams
const myTransform = new Transform({
  writableObjectMode: true,

  transform(chunk, encoding, callback) {
    // Coerce the chunk to a number if necessary
    chunk |= 0;

    // Transform the chunk into something else.
    const data = chunk.toString(16);

    // Push the data onto the readable queue.
    callback(null, '0'.repeat(data.length % 2) + data);
  }
});

myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));

myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64

Implementing a Transform Stream#

A Transform stream is a Duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.

Note: There is no requirement that the output be the same size as the input, the same number of chunks, or arrive at the same time. For example, a Hash stream will only ever have a single chunk of output which is provided when the input is ended. A zlib stream will produce output that is either much smaller or much larger than its input.

The stream.Transform class is extended to implement a Transform stream.

The stream.Transform class prototypically inherits from stream.Duplex and implements its own versions of the writable._write() and readable._read() methods. Custom Transform implementations must implement the transform._transform() method and may also implement the transform._flush() method.

Note: Care must be taken when using Transform streams in that data written to the stream can cause the Writable side of the stream to become paused if the output on the Readable side is not consumed.

new stream.Transform(options)#

For example:

const Transform = require('stream').Transform;

class MyTransform extends Transform {
  constructor(options) {
    super(options);
  }
}

Or, when using pre-ES6 style constructors:

const Transform = require('stream').Transform;
const util = require('util');

function MyTransform(options) {
  if (!(this instanceof MyTransform))
    return new MyTransform(options);
  Transform.call(this, options);
}
util.inherits(MyTransform, Transform);

Or, using the Simplified Constructor approach:

const Transform = require('stream').Transform;

const myTransform = new Transform({
  transform(chunk, encoding, callback) {
    // ...
  }
});

Events: 'finish' and 'end'#

The 'finish' and 'end' events are from the stream.Writable and stream.Readable classes, respectively. The 'finish' event is emitted after stream.end() is called and all chunks have been processed by stream._transform(). The 'end' event is emitted after all data has been output, which occurs after the callback in transform._flush() has been called.

transform._flush(callback)#

  • callback <Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.

Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.

In some cases, a transform operation may need to emit an additional bit of data at the end of the stream. For example, a zlib compression stream will store an amount of internal state used to optimally compress the output. When the stream ends, however, that additional data needs to be flushed so that the compressed data will be complete.

Custom Transform implementations may implement the transform._flush() method. This will be called when there is no more written data to be consumed, but before the 'end' event is emitted signaling the end of the Readable stream.

Within the transform._flush() implementation, the readable.push() method may be called zero or more times, as appropriate. The callback function must be called when the flush operation is complete.

The transform._flush() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

transform._transform(chunk, encoding, callback)#

  • chunk <Buffer> | <String> The chunk to be transformed. Will always be a buffer unless the decodeStrings option was set to false.
  • encoding <String> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value - 'buffer', ignore it in this case.
  • callback <Function> A callback function (optionally with an error argument and data) to be called after the supplied chunk has been processed.

Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.

All Transform stream implementations must provide a _transform() method to accept input and produce output. The transform._transform() implementation handles the bytes being written, computes an output, then passes that output off to the readable portion using the readable.push() method.

The transform.push() method may be called zero or more times to generate output from a single input chunk, depending on how much is to be output as a result of the chunk.

It is possible that no output is generated from any given chunk of input data.

The callback function must be called only when the current chunk is completely consumed. The first argument passed to the callback must be an Error object if an error occurred while processing the input or null otherwise. If a second argument is passed to the callback, it will be forwarded on to the readable.push() method. In other words the following are equivalent:

transform.prototype._transform = function (data, encoding, callback) {
  this.push(data);
  callback();
};

transform.prototype._transform = function (data, encoding, callback) {
  callback(null, data);
};

The transform._transform() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

Class: stream.PassThrough#

The stream.PassThrough class is a trivial implementation of a Transform stream that simply passes the input bytes across to the output. Its purpose is primarily for examples and testing, but there are some use cases where stream.PassThrough is useful as a building block for novel sorts of streams.

Additional Notes#

Compatibility with Older Node.js Versions#

In versions of Node.js prior to v0.10, the Readable stream interface was simpler, but also less powerful and less useful.

  • Rather than waiting for calls the stream.read() method, 'data' events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost.
  • The stream.pause() method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive 'data' events even when the stream was in a paused state.

In Node.js v0.10, the Readable class was added. For backwards compatibility with older Node.js programs, Readable streams switch into "flowing mode" when a 'data' event handler is added, or when the stream.resume() method is called. The effect is that, even when not using the new stream.read() method and 'readable' event, it is no longer necessary to worry about losing 'data' chunks.

While most applications will continue to function normally, this introduces an edge case in the following conditions:

  • No 'data' event listener is added.
  • The stream.resume() method is never called.
  • The stream is not piped to any writable destination.

For example, consider the following code:

// WARNING!  BROKEN!
net.createServer((socket) => {

  // we add an 'end' method, but never consume the data
  socket.on('end', () => {
    // It will never get here.
    socket.end('The message was received but was not processed.\n');
  });

}).listen(1337);

In versions of Node.js prior to v0.10, the incoming message data would be simply discarded. However, in Node.js v0.10 and beyond, the socket remains paused forever.

The workaround in this situation is to call the stream.resume() method to begin the flow of data:

// Workaround
net.createServer((socket) => {

  socket.on('end', () => {
    socket.end('The message was received but was not processed.\n');
  });

  // start the flow of data, discarding it.
  socket.resume();

}).listen(1337);

In addition to new Readable streams switching into flowing mode, pre-v0.10 style streams can be wrapped in a Readable class using the readable.wrap() method.

readable.read(0)#

There are some cases where it is necessary to trigger a refresh of the underlying readable stream mechanisms, without actually consuming any data. In such cases, it is possible to call readable.read(0), which will always return null.

If the internal read buffer is below the highWaterMark, and the stream is not currently reading, then calling stream.read(0) will trigger a low-level stream._read() call.

While most applications will almost never need to do this, there are situations within Node.js where this is done, particularly in the Readable stream class internals.

readable.push('')#

Use of readable.push('') is not recommended.

Pushing a zero-byte string or Buffer to a stream that is not in object mode has an interesting side effect. Because it is a call to readable.push(), the call will end the reading process. However, because the argument is an empty string, no data is added to the readable buffer so there is nothing for a user to consume.

StringDecoder#

Stability: 2 - Stable

The string_decoder module provides an API for decoding Buffer objects into strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16 characters. It can be accessed using:

const StringDecoder = require('string_decoder').StringDecoder;

The following example shows the basic use of the StringDecoder class.

const StringDecoder = require('string_decoder').StringDecoder;
const decoder = new StringDecoder('utf8');

const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent));

const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro));

When a Buffer instance is written to the StringDecoder instance, an internal buffer is used to ensure that the decoded string does not contain any incomplete multibyte characters. These are held in the buffer until the next call to stringDecoder.write() or until stringDecoder.end() is called.

In the following example, the three UTF-8 encoded bytes of the European Euro symbol (€) are written over three separate operations:

const StringDecoder = require('string_decoder').StringDecoder;
const decoder = new StringDecoder('utf8');

decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC])));

Class: new StringDecoder([encoding])#

Added in: v0.1.99
  • encoding <string> The character encoding the StringDecoder will use. Defaults to 'utf8'.

Creates a new StringDecoder instance.

stringDecoder.end(buffer)#

Added in: v0.9.3
  • buffer <Buffer> A Buffer containing the bytes to decode.

Returns any remaining input stored in the internal buffer as a string. Bytes representing incomplete UTF-8 and UTF-16 characters will be replaced with substitution characters appropriate for the character encoding.

If the buffer argument is provided, one final call to stringDecoder.write() is performed before returning the remaining input.

stringDecoder.write(buffer)#

Added in: v0.1.99
  • buffer <Buffer> A Buffer containing the bytes to decode.

Returns a decoded string, ensuring that any incomplete multibyte characters at the end of the Buffer are omitted from the returned string and stored in an internal buffer for the next call to stringDecoder.write() or stringDecoder.end().

Timers#

Stability: 3 - Locked

The timer module exposes a global API for scheduling functions to be called at some future period of time. Because the timer functions are globals, there is no need to call require('timers') to use the API.

The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.

Class: Immediate#

This object is created internally and is returned from setImmediate(). It can be passed to clearImmediate() in order to cancel the scheduled actions.

Class: Timeout#

This object is created internally and is returned from setTimeout() and setInterval(). It can be passed to clearTimeout() or clearInterval() (respectively) in order to cancel the scheduled actions.

By default, when a timer is scheduled using either setTimeout() or setInterval(), the Node.js event loop will continue running as long as the timer is active. Each of the Timeout objects returned by these functions export both timeout.ref() and timeout.unref() functions that can be used to control this default behavior.

timeout.ref()#

Added in: v0.9.1

When called, requests that the Node.js event loop not exit so long as the Timeout is active. Calling timeout.ref() multiple times will have no effect.

Note: By default, all Timeout objects are "ref'd", making it normally unnecessary to call timeout.ref() unless timeout.unref() had been called previously.

Returns a reference to the Timeout.

timeout.unref()#

Added in: v0.9.1

When called, the active Timeout object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the Timeout object's callback is invoked. Calling timeout.unref() multiple times will have no effect.

Note: Calling timeout.unref() creates an internal timer that will wake the Node.js event loop. Creating too many of these can adversely impact performance of the Node.js application.

Returns a reference to the Timeout.

Scheduling Timers#

A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.

setImmediate(callback[, ...args])#

Added in: v0.9.1
  • callback <Function> The function to call at the end of this turn of the Node.js Event Loop
  • ...args <any> Optional arguments to pass when the callback is called.

Schedules the "immediate" execution of the callback after I/O events' callbacks and before timers created using setTimeout() and setInterval() are triggered. Returns an Immediate for use with clearImmediate().

When multiple calls to setImmediate() are made, the callback functions are queued for execution in the order in which they are created. The entire callback queue is processed every event loop iteration. If an immediate timer is queued from inside an executing callback, that timer will not be triggered until the next event loop iteration.

If callback is not a function, a TypeError will be thrown.

setInterval(callback, delay[, ...args])#

Added in: v0.0.1
  • callback <Function> The function to call when the timer elapses.
  • delay <number> The number of milliseconds to wait before calling the callback.
  • ...args <any> Optional arguments to pass when the callback is called.

Schedules repeated execution of callback every delay milliseconds. Returns a Timeout for use with clearInterval().

When delay is larger than 2147483647 or less than 1, the delay will be set to 1.

If callback is not a function, a TypeError will be thrown.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1
  • callback <Function> The function to call when the timer elapses.
  • delay <number> The number of milliseconds to wait before calling the callback.
  • ...args <any> Optional arguments to pass when the callback is called.

Schedules execution of a one-time callback after delay milliseconds. Returns a Timeout for use with clearTimeout().

The callback will likely not be invoked in precisely delay milliseconds. Node.js makes no guarantees about the exact timing of when callbacks will fire, nor of their ordering. The callback will be called as close as possible to the time specified.

Note: When delay is larger than 2147483647 or less than 1, the delay will be set to 1.

If callback is not a function, a TypeError will be thrown.

Cancelling Timers#

The setImmediate(), setInterval(), and setTimeout() methods each return objects that represent the scheduled timers. These can be used to cancel the timer and prevent it from triggering.

clearImmediate(immediate)#

Added in: v0.9.1
  • immediate <Immediate> An Immediate object as returned by setImmediate().

Cancels an Immediate object created by setImmediate().

clearInterval(timeout)#

Added in: v0.0.1
  • timeout <Timeout> A Timeout object as returned by setInterval().

Cancels a Timeout object created by setInterval().

clearTimeout(timeout)#

Added in: v0.0.1
  • timeout <Timeout> A Timeout object as returned by setTimeout().

Cancels a Timeout object created by setTimeout().

TLS (SSL)#

Stability: 2 - Stable

The tls module provides an implementation of the Transport Layer Security (TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL. The module can be accessed using:

const tls = require('tls');

TLS/SSL Concepts#

The TLS/SSL is a public/private key infrastructure (PKI). For most common cases, each client and server must have a private key.

Private keys can be generated in multiple ways. The example below illustrates use of the OpenSSL command-line interface to generate a 2048-bit RSA private key:

openssl genrsa -out ryans-key.pem 2048

With TLS/SSL, all servers (and some clients) must have a certificate. Certificates are public keys that correspond to a private key, and that are digitally signed either by a Certificate Authority or by the owner of the private key (such certificates are referred to as "self-signed"). The first step to obtaining a certificate is to create a Certificate Signing Request (CSR) file.

The OpenSSL command-line interface can be used to generate a CSR for a private key:

openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem

Once the CSR file is generated, it can either be sent to a Certificate Authority for signing or used to generate a self-signed certificate.

Creating a self-signed certificate using the OpenSSL command-line interface is illustrated in the example below:

openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem

Once the certificate is generated, it can be used to generate a .pfx or .p12 file:

openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
      -certfile ca-cert.pem -out ryans.pfx

Where:

  • in: is the signed certificate
  • inkey: is the associated private key
  • certfile: is a concatenation of all Certificate Authority (CA) certs into a single file, e.g. cat ca1-cert.pem ca2-cert.pem > ca-cert.pem

Perfect Forward Secrecy#

The term "Forward Secrecy" or "Perfect Forward Secrecy" describes a feature of key-agreement (i.e., key-exchange) methods. That is, the server and client keys are used to negotiate new temporary keys that are used specifically and only for the current communication session. Practically, this means that even if the server's private key is compromised, communication can only be decrypted by eavesdroppers if the attacker manages to obtain the key-pair specifically generated for the session.

Perfect Forward Secrecy is achieved by randomly generating a key pair for key-agreement on every TLS/SSL handshake (in contrast to using the same key for all sessions). Methods implementing this technique are called "ephemeral".

Currently two methods are commonly used to achieve Perfect Forward Secrecy (note the character "E" appended to the traditional abbreviations):

  • DHE - An ephemeral version of the Diffie Hellman key-agreement protocol.
  • ECDHE - An ephemeral version of the Elliptic Curve Diffie Hellman key-agreement protocol.

Ephemeral methods may have some performance drawbacks, because key generation is expensive.

To use Perfect Forward Secrecy using DHE with the tls module, it is required to generate Diffie-Hellman parameters and specify them with the dhparam option to tls.createSecureContext(). The following illustrates the use of the OpenSSL command-line interface to generate such parameters:

openssl dhparam -outform PEM -out dhparam.pem 2048

If using Perfect Forward Secrecy using ECDHE, Diffie-Hellman parameters are not required and a default ECDHE curve will be used. The ecdhCurve property can be used when creating a TLS Server to specify the name of an alternative curve to use, see tls.createServer() for more info.

ALPN, NPN and SNI#

ALPN (Application-Layer Protocol Negotiation Extension), NPN (Next Protocol Negotiation) and, SNI (Server Name Indication) are TLS handshake extensions:

  • ALPN/NPN - Allows the use of one TLS server for multiple protocols (HTTP, SPDY, HTTP/2)
  • SNI - Allows the use of one TLS server for multiple hostnames with different SSL certificates.

Note: Use of ALPN is recommended over NPN. The NPN extension has never been formally defined or documented and generally not recommended for use.

Client-initiated renegotiation attack mitigation#

The TLS protocol allows clients to renegotiate certain aspects of the TLS session. Unfortunately, session renegotiation requires a disproportionate amount of server-side resources, making it a potential vector for denial-of-service attacks.

To mitigate the risk, renegotiation is limited to three times every ten minutes. An 'error' event is emitted on the tls.TLSSocket instance when this threshold is exceeded. The limits are configurable:

  • tls.CLIENT_RENEG_LIMIT <number> Specifies the number of renegotiation requests. Defaults to 3.
  • tls.CLIENT_RENEG_WINDOW <number> Specifies the time renegotiation window in seconds. Defaults to 600 (10 minutes).

Note: The default renegotiation limits should not be modified without a full understanding of the implications and risks.

To test the renegotiation limits on a server, connect to it using the OpenSSL command-line client (openssl s_client -connect address:port) then input R<CR> (i.e., the letter R followed by a carriage return) multiple times.

Modifying the Default TLS Cipher suite#

Node.js is built with a default suite of enabled and disabled TLS ciphers. Currently, the default cipher suite is:

ECDHE-RSA-AES128-GCM-SHA256:
ECDHE-ECDSA-AES128-GCM-SHA256:
ECDHE-RSA-AES256-GCM-SHA384:
ECDHE-ECDSA-AES256-GCM-SHA384:
DHE-RSA-AES128-GCM-SHA256:
ECDHE-RSA-AES128-SHA256:
DHE-RSA-AES128-SHA256:
ECDHE-RSA-AES256-SHA384:
DHE-RSA-AES256-SHA384:
ECDHE-RSA-AES256-SHA256:
DHE-RSA-AES256-SHA256:
HIGH:
!aNULL:
!eNULL:
!EXPORT:
!DES:
!RC4:
!MD5:
!PSK:
!SRP:
!CAMELLIA

This default can be replaced entirely using the --tls-cipher-list command line switch. For instance, the following makes ECDHE-RSA-AES128-GCM-SHA256:!RC4 the default TLS cipher suite:

node --tls-cipher-list="ECDHE-RSA-AES128-GCM-SHA256:!RC4"

The default can also be replaced on a per client or server basis using the ciphers option from tls.createSecureContext(), which is also available in tls.createServer(), tls.connect(), and when creating new tls.TLSSockets.

Consult OpenSSL cipher list format documentation for details on the format.

Note: The default cipher suite included within Node.js has been carefully selected to reflect current security best practices and risk mitigation. Changing the default cipher suite can have a significant impact on the security of an application. The --tls-cipher-list switch and ciphers option should by used only if absolutely necessary.

The default cipher suite prefers GCM ciphers for Chrome's 'modern cryptography' setting and also prefers ECDHE and DHE ciphers for Perfect Forward Secrecy, while offering some backward compatibility.

128 bit AES is preferred over 192 and 256 bit AES in light of specific attacks affecting larger AES key sizes.

Old clients that rely on insecure and deprecated RC4 or DES-based ciphers (like Internet Explorer 6) cannot complete the handshaking process with the default configuration. If these clients must be supported, the TLS recommendations may offer a compatible cipher suite. For more details on the format, see the OpenSSL cipher list format documentation.

Class: tls.Server#

Added in: v0.3.2

The tls.Server class is a subclass of net.Server that accepts encrypted connections using TLS or SSL.

Event: 'tlsClientError'#

Added in: v6.0.0

The 'tlsClientError' event is emitted when an error occurs before a secure connection is established. The listener callback is passed two arguments when called:

  • exception <Error> The Error object describing the error
  • tlsSocket <tls.TLSSocket> The tls.TLSSocket instance from which the error originated.

Event: 'newSession'#

Added in: v0.9.2

The 'newSession' event is emitted upon creation of a new TLS session. This may be used to store sessions in external storage. The listener callback is passed three arguments when called:

  • sessionId - The TLS session identifier
  • sessionData - The TLS session data
  • callback <Function> A callback function taking no arguments that must be invoked in order for data to be sent or received over the secure connection.

Note: Listening for this event will have an effect only on connections established after the addition of the event listener.

Event: 'OCSPRequest'#

Added in: v0.11.13

The 'OCSPRequest' event is emitted when the client sends a certificate status request. The listener callback is passed three arguments when called:

  • certificate <Buffer> The server certificate
  • issuer <Buffer> The issuer's certificate
  • callback <Function> A callback function that must be invoked to provide the results of the OCSP request.

The server's current certificate can be parsed to obtain the OCSP URL and certificate ID; after obtaining an OCSP response, callback(null, resp) is then invoked, where resp is a Buffer instance containing the OCSP response. Both certificate and issuer are Buffer DER-representations of the primary and issuer's certificates. These can be used to obtain the OCSP certificate ID and OCSP endpoint URL.

Alternatively, callback(null, null) may be called, indicating that there was no OCSP response.

Calling callback(err) will result in a socket.destroy(err) call.

The typical flow of an OCSP Request is as follows:

  1. Client connects to the server and sends an 'OCSPRequest' (via the status info extension in ClientHello).
  2. Server receives the request and emits the 'OCSPRequest' event, calling the listener if registered.
  3. Server extracts the OCSP URL from either the certificate or issuer and performs an OCSP request to the CA.
  4. Server receives OCSPResponse from the CA and sends it back to the client via the callback argument
  5. Client validates the response and either destroys the socket or performs a handshake.

Note: The issuer can be null if the certificate is either self-signed or the issuer is not in the root certificates list. (An issuer may be provided via the ca option when establishing the TLS connection.)

Note: Listening for this event will have an effect only on connections established after the addition of the event listener.

Note: An npm module like asn1.js may be used to parse the certificates.

Event: 'resumeSession'#

Added in: v0.9.2

The 'resumeSession' event is emitted when the client requests to resume a previous TLS session. The listener callback is passed two arguments when called:

  • sessionId - The TLS/SSL session identifier
  • callback <Function> A callback function to be called when the prior session has been recovered.

When called, the event listener may perform a lookup in external storage using the given sessionId and invoke callback(null, sessionData) once finished. If the session cannot be resumed (i.e., doesn't exist in storage) the callback may be invoked as callback(null, null). Calling callback(err) will terminate the incoming connection and destroy the socket.

Note: Listening for this event will have an effect only on connections established after the addition of the event listener.

The following illustrates resuming a TLS session:

const tlsSessionStore = {};
server.on('newSession', (id, data, cb) => {
  tlsSessionStore[id.toString('hex')] = data;
  cb();
});
server.on('resumeSession', (id, cb) => {
  cb(null, tlsSessionStore[id.toString('hex')] || null);
});

Event: 'secureConnection'#

Added in: v0.3.2

The 'secureConnection' event is emitted after the handshaking process for a new connection has successfully completed. The listener callback is passed a single argument when called:

The tlsSocket.authorized property is a boolean indicating whether the client has been verified by one of the supplied Certificate Authorities for the server. If tlsSocket.authorized is false, then socket.authorizationError is set to describe how authorization failed. Note that depending on the settings of the TLS server, unauthorized connections may still be accepted.

The tlsSocket.npnProtocol and tlsSocket.alpnProtocol properties are strings that contain the selected NPN and ALPN protocols, respectively. When both NPN and ALPN extensions are received, ALPN takes precedence over NPN and the next protocol is selected by ALPN.

When ALPN has no selected protocol, tlsSocket.alpnProtocol returns false.

The tlsSocket.servername property is a string containing the server name requested via SNI.

server.addContext(hostname, context)#

Added in: v0.5.3
  • hostname <string> A SNI hostname or wildcard (e.g. '*')
  • context <Object> An object containing any of the possible properties from the tls.createSecureContext() options arguments (e.g. key, cert, ca, etc).

The server.addContext() method adds a secure context that will be used if the client request's SNI hostname matches the supplied hostname (or wildcard).

server.address()#

Added in: v0.6.0

Returns the bound address, the address family name, and port of the server as reported by the operating system. See net.Server.address() for more information.

server.close([callback])#

Added in: v0.3.2
  • callback <Function> An optional listener callback that will be registered to listen for the server instance's 'close' event.

The server.close() method stops the server from accepting new connections.

This function operates asynchronously. The 'close' event will be emitted when the server has no more open connections.

server.connections#

Added in: v0.3.2

Returns the current number of concurrent connections on the server.

server.getTicketKeys()#

Added in: v3.0.0

Returns a Buffer instance holding the keys currently used for encryption/decryption of the TLS Session Tickets

server.listen(port[, hostname][, callback])#

Added in: v0.3.2
  • port <number> The TCP/IP port on which to begin listening for connections. A value of 0 (zero) will assign a random port.
  • hostname <string> The hostname, IPv4, or IPv6 address on which to begin listening for connections. If undefined, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise.
  • callback <Function> A callback function to be invoked when the server has begun listening on the port and hostname.

The server.listen() methods instructs the server to begin accepting connections on the specified port and hostname.

This function operates asynchronously. If the callback is given, it will be called when the server has started listening.

See net.Server for more information.

server.setTicketKeys(keys)#

Added in: v3.0.0

Updates the keys for encryption/decryption of the TLS Session Tickets.

Note: The key's Buffer should be 48 bytes long. See ticketKeys option in tls.createServer for more information on how it is used.

Note: Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.

Class: tls.TLSSocket#

Added in: v0.11.4

The tls.TLSSocket is a subclass of net.Socket that performs transparent encryption of written data and all required TLS negotiation.

Instances of tls.TLSSocket implement the duplex Stream interface.

Note: Methods that return TLS connection metadata (e.g. tls.TLSSocket.getPeerCertificate() will only return data while the connection is open.

new tls.TLSSocket(socket[, options])#

Added in: v0.11.4
  • socket <net.Socket> An instance of net.Socket
  • options <Object>
    • isServer: The SSL/TLS protocol is asymetrical, TLSSockets must know if they are to behave as a server or a client. If true the TLS socket will be instantiated as a server. Defaults to false.
    • server <net.Server> An optional net.Server instance.
    • requestCert: Whether to authenticate the remote peer by requesting a certificate. Clients always request a server certificate. Servers (isServer is true) may optionally set requestCert to true to request a client certificate.
    • rejectUnauthorized: Optional, see tls.createServer()
    • NPNProtocols: Optional, see tls.createServer()
    • ALPNProtocols: Optional, see tls.createServer()
    • SNICallback: Optional, see tls.createServer()
    • session <Buffer> An optional Buffer instance containing a TLS session.
    • requestOCSP <boolean> If true, specifies that the OCSP status request extension will be added to the client hello and an 'OCSPResponse' event will be emitted on the socket before establishing a secure communication
    • secureContext: Optional TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by calling tls.createSecureContext() with no options.

Construct a new tls.TLSSocket object from an existing TCP socket.

Event: 'OCSPResponse'#

Added in: v0.11.13

The 'OCSPResponse' event is emitted if the requestOCSP option was set when the tls.TLSSocket was created and an OCSP response has been received. The listener callback is passed a single argument when called:

  • response <Buffer> The server's OCSP response

Typically, the response is a digitally signed object from the server's CA that contains information about server's certificate revocation status.

Event: 'secureConnect'#

Added in: v0.11.4

The 'secureConnect' event is emitted after the handshaking process for a new connection has successfully completed. The listener callback will be called regardless of whether or not the server's certificate has been authorized. It is the client's responsibility to check the tlsSocket.authorized property to determine if the server certificate was signed by one of the specified CAs. If tlsSocket.authorized === false, then the error can be found by examining the tlsSocket.authorizationError property. If either ALPN or NPN was used, the tlsSocket.alpnProtocol or tlsSocket.npnProtocol properties can be checked to determine the negotiated protocol.

tlsSocket.address()#

Added in: v0.11.4

Returns the bound address, the address family name, and port of the underlying socket as reported by the operating system. Returns an object with three properties, e.g., { port: 12346, family: 'IPv4', address: '127.0.0.1' }

tlsSocket.authorized#

Added in: v0.11.4

Returns true if the peer certificate was signed by one of the CAs specified when creating the tls.TLSSocket instance, otherwise false.

tlsSocket.authorizationError#

Added in: v0.11.4

Returns the reason why the peer's certificate was not been verified. This property is set only when tlsSocket.authorized === false.

tlsSocket.encrypted#

Added in: v0.11.4

Always returns true. This may be used to distinguish TLS sockets from regular net.Socket instances.

tlsSocket.getCipher()#

Added in: v0.11.4

Returns an object representing the cipher name and the SSL/TLS protocol version that first defined the cipher.

For example: { name: 'AES256-SHA', version: 'TLSv1/SSLv3' }

See SSL_CIPHER_get_name() and SSL_CIPHER_get_version() in https://www.openssl.org/docs/man1.0.2/ssl/SSL_CIPHER_get_name.html for more information.

tlsSocket.getEphemeralKeyInfo()#

Added in: v5.0.0

Returns an object representing the type, name, and size of parameter of an ephemeral key exchange in Perfect Forward Secrecy on a client connection. It returns an empty object when the key exchange is not ephemeral. As this is only supported on a client socket; null is returned if called on a server socket. The supported types are 'DH' and 'ECDH'. The name property is available only when type is 'ECDH'.

For Example: { type: 'ECDH', name: 'prime256v1', size: 256 }

tlsSocket.getPeerCertificate([ detailed ])#

Added in: v0.11.4
  • detailed <boolean> Include the full certificate chain if true, otherwise include just the peer's certificate.

Returns an object representing the peer's certificate. The returned object has some properties corresponding to the fields of the certificate.

If the full certificate chain was requested, each certificate will include a issuerCertificate property containing an object representing its issuer's certificate.

For example:

{ subject:
   { C: 'UK',
     ST: 'Acknack Ltd',
     L: 'Rhys Jones',
     O: 'node.js',
     OU: 'Test TLS Certificate',
     CN: 'localhost' },
  issuer:
   { C: 'UK',
     ST: 'Acknack Ltd',
     L: 'Rhys Jones',
     O: 'node.js',
     OU: 'Test TLS Certificate',
     CN: 'localhost' },
  issuerCertificate:
   { ... another certificate, possibly with a .issuerCertificate ... },
  raw: < RAW DER buffer >,
  valid_from: 'Nov 11 09:52:22 2009 GMT',
  valid_to: 'Nov  6 09:52:22 2029 GMT',
  fingerprint: '2A:7A:C2:DD:E5:F9:CC:53:72:35:99:7A:02:5A:71:38:52:EC:8A:DF',
  serialNumber: 'B9B0D332A1AA5635' }

If the peer does not provide a certificate, an empty object will be returned.

tlsSocket.getProtocol()#

Added in: v5.7.0

Returns a string containing the negotiated SSL/TLS protocol version of the current connection. The value 'unknown' will be returned for connected sockets that have not completed the handshaking process. The value null will be returned for server sockets or disconnected client sockets.

Example responses include:

  • SSLv3
  • TLSv1
  • TLSv1.1
  • TLSv1.2
  • unknown

See https://www.openssl.org/docs/man1.0.2/ssl/SSL_get_version.html for more information.

tlsSocket.getSession()#

Added in: v0.11.4

Returns the ASN.1 encoded TLS session or undefined if no session was negotiated. Can be used to speed up handshake establishment when reconnecting to the server.

tlsSocket.getTLSTicket()#

Added in: v0.11.4

Returns the TLS session ticket or undefined if no session was negotiated.

Note: This only works with client TLS sockets. Useful only for debugging, for session reuse provide session option to tls.connect().

tlsSocket.localAddress#

Added in: v0.11.4

Returns the string representation of the local IP address.

tlsSocket.localPort#

Added in: v0.11.4

Returns the numeric representation of the local port.

tlsSocket.remoteAddress#

Added in: v0.11.4

Returns the string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'.

tlsSocket.remoteFamily#

Added in: v0.11.4

Returns the string representation of the remote IP family. 'IPv4' or 'IPv6'.

tlsSocket.remotePort#

Added in: v0.11.4

Returns the numeric representation of the remote port. For example, 443.

tlsSocket.renegotiate(options, callback)#

Added in: v0.11.8
  • options <Object>
    • rejectUnauthorized <boolean>
    • requestCert
  • callback <Function> A function that will be called when the renegotiation request has been completed.

The tlsSocket.renegotiate() method initiates a TLS renegotiation process. Upon completion, the callback function will be passed a single argument that is either an Error (if the request failed) or null.

Note: This method can be used to request a peer's certificate after the secure connection has been established.

Note: When running as the server, the socket will be destroyed with an error after handshakeTimeout timeout.

tlsSocket.setMaxSendFragment(size)#

Added in: v0.11.11
  • size <number> The maximum TLS fragment size. Defaults to 16384. The maximum value is 16384.

The tlsSocket.setMaxSendFragment() method sets the maximum TLS fragment size. Returns true if setting the limit succeeded; false otherwise.

Smaller fragment sizes decrease the buffering latency on the client: larger fragments are buffered by the TLS layer until the entire fragment is received and its integrity is verified; large fragments can span multiple roundtrips and their processing can be delayed due to packet loss or reordering. However, smaller fragments add extra TLS framing bytes and CPU overhead, which may decrease overall server throughput.

tls.connect(port[, host][, options][, callback])#

Added in: v0.11.3

Same as tls.connect() except that port and host can be provided as arguments instead of options.

Note: A port or host option, if specified, will take precedence over any port or host argument.

tls.connect(path[, options][, callback])#

Added in: v0.11.3

Same as tls.connect() except that path can be provided as an argument instead of an option.

Note: A path option, if specified, will take precedence over the path argument.

tls.connect(options[, callback])#

Added in: v0.11.3
  • options <Object>
    • host <string> Host the client should connect to, defaults to 'localhost'.
    • port <number> Port the client should connect to.
    • path <string> Creates unix socket connection to path. If this option is specified, host and port are ignored.
    • socket <net.Socket> Establish secure connection on a given socket rather than creating a new socket. If this option is specified, path, host and port are ignored. Usually, a socket is already connected when passed to tls.connect(), but it can be connected later. Note that connection/disconnection/destruction of socket is the user's responsibility, calling tls.connect() will not cause net.connect() to be called.
    • rejectUnauthorized <boolean> If true, the server certificate is verified against the list of supplied CAs. An 'error' event is emitted if verification fails; err.code contains the OpenSSL error code. Defaults to true.
    • NPNProtocols <string[]> | <Buffer[]> An array of strings or Buffers containing supported NPN protocols. Buffers should have the format [len][name][len][name]... e.g. 0x05hello0x05world, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. ['hello', 'world'].
    • ALPNProtocols: <string[]> | <Buffer[]> An array of strings or Buffers containing the supported ALPN protocols. Buffers should have the format [len][name][len][name]... e.g. 0x05hello0x05world, where the first byte is the length of the next protocol name. Passing an array is usually much simpler: ['hello', 'world'].)
    • servername: <string> Server name for the SNI (Server Name Indication) TLS extension.
    • checkServerIdentity(servername, cert) <Function> A callback function to be used when checking the server's hostname against the certificate. This should throw an error if verification fails. The method should return undefined if the servername and cert are verified.
    • session <Buffer> A Buffer instance, containing TLS session.
    • minDHSize <number> Minimum size of the DH parameter in bits to accept a TLS connection. When a server offers a DH parameter with a size less than minDHSize, the TLS connection is destroyed and an error is thrown. Defaults to 1024.
    • secureContext: Optional TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by passing the entire options object to tls.createSecureContext(). Note: In effect, all tls.createSecureContext() options can be provided, but they will be completely ignored unless the secureContext option is missing.
    • ...: Optional tls.createSecureContext() options can be provided, see the secureContext option for more information.
  • callback <Function>

The callback function, if specified, will be added as a listener for the 'secureConnect' event.

tls.connect() returns a tls.TLSSocket object.

The following implements a simple "echo server" example:

const tls = require('tls');
const fs = require('fs');

const options = {
  // Necessary only if using the client certificate authentication
  key: fs.readFileSync('client-key.pem'),
  cert: fs.readFileSync('client-cert.pem'),

  // Necessary only if the server uses the self-signed certificate
  ca: [ fs.readFileSync('server-cert.pem') ]
};

const socket = tls.connect(8000, options, () => {
  console.log('client connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  process.stdin.pipe(socket);
  process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
  console.log(data);
});
socket.on('end', () => {
  server.close();
});

Or

const tls = require('tls');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('client.pfx')
};

const socket = tls.connect(8000, options, () => {
  console.log('client connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  process.stdin.pipe(socket);
  process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
  console.log(data);
});
socket.on('end', () => {
  server.close();
});

tls.createSecureContext(options)#

Added in: v0.11.13
  • options <Object>
    • pfx <string> | <Buffer> Optional PFX or PKCS12 encoded private key and certificate chain. pfx is an alternative to providing key and cert individually. PFX is usually encrypted, if it is, passphrase will be used to decrypt it.
    • key <string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> Optional private keys in PEM format. PEM allows the option of private keys being encrypted. Encrypted keys will be decrypted with options.passphrase. Multiple keys using different algorithms can be provided either as an array of unencrypted key strings or buffers, or an array of objects in the form {pem: <string|buffer>[, passphrase: <string>]}. The object form can only occur in an array. object.passphrase is optional. Encrypted keys will be decrypted with object.passphrase if provided, or options.passphrase if it is not.
    • passphrase <string> Optional shared passphrase used for a single private key and/or a PFX.
    • cert <string> | <string[]> | <Buffer> | <Buffer[]> Optional cert chains in PEM format. One cert chain should be provided per private key. Each cert chain should consist of the PEM formatted certificate for a provided private key, followed by the PEM formatted intermediate certificates (if any), in order, and not including the root CA (the root CA must be pre-known to the peer, see ca). When providing multiple cert chains, they do not have to be in the same order as their private keys in key. If the intermediate certificates are not provided, the peer will not be able to validate the certificate, and the handshake will fail.
    • ca <string> | <string[]> | <Buffer> | <Buffer[]> Optionally override the trusted CA certificates. Default is to trust the well-known CAs curated by Mozilla. Mozilla's CAs are completely replaced when CAs are explicitly specified using this option. The value can be a string or Buffer, or an Array of strings and/or Buffers. Any string or Buffer can contain multiple PEM CAs concatenated together. The peer's certificate must be chainable to a CA trusted by the server for the connection to be authenticated. When using certificates that are not chainable to a well-known CA, the certificate's CA must be explicitly specified as a trusted or the connection will fail to authenticate. If the peer uses a certificate that doesn't match or chain to one of the default CAs, use the ca option to provide a CA certificate that the peer's certificate can match or chain to. For self-signed certificates, the certificate is its own CA, and must be provided.
    • crl <string> | <string[]> | <Buffer> | <Buffer[]> Optional PEM formatted CRLs (Certificate Revocation Lists).
    • ciphers <string> Optional cipher suite specification, replacing the default. For more information, see modifying the default cipher suite.
    • honorCipherOrder <boolean> Attempt to use the server's cipher suite preferences instead of the client's. When true, causes SSL_OP_CIPHER_SERVER_PREFERENCE to be set in secureOptions, see OpenSSL Options for more information. Note: tls.createServer() sets the default value to true, other APIs that create secure contexts leave it unset.
    • ecdhCurve <string> A string describing a named curve to use for ECDH key agreement or false to disable ECDH. Defaults to tls.DEFAULT_ECDH_CURVE. Use crypto.getCurves() to obtain a list of available curve names. On recent releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.
    • dhparam <string> | <Buffer> Diffie Hellman parameters, required for Perfect Forward Secrecy. Use openssl dhparam to create the parameters. The key length must be greater than or equal to 1024 bits, otherwise an error will be thrown. It is strongly recommended to use 2048 bits or larger for stronger security. If omitted or invalid, the parameters are silently discarded and DHE ciphers will not be available.
    • secureProtocol <string> Optional SSL method to use, default is "SSLv23_method". The possible values are listed as SSL_METHODS, use the function names as strings. For example, "SSLv3_method" to force SSL version 3.
    • secureOptions <number> Optionally affect the OpenSSL protocol behaviour, which is not usually necessary. This should be used carefully if at all! Value is a numeric bitmask of the SSL_OP_* options from OpenSSL Options.
    • sessionIdContext <string> Optional opaque identifier used by servers to ensure session state is not shared between applications. Unused by clients. Note: tls.createServer() uses a 128 bit truncated SHA1 hash value generated from process.argv, other APIs that create secure contexts have no default value.

The tls.createSecureContext() method creates a credentials object.

A key is required for ciphers that make use of certificates. Either key or pfx can be used to provide it.

If the 'ca' option is not given, then Node.js will use the default publicly trusted list of CAs as given in http://mxr.mozilla.org/mozilla/source/security/nss/lib/ckfw/builtins/certdata.txt.

tls.createServer([options][, secureConnectionListener])#

Added in: v0.3.2
  • options <Object>
    • handshakeTimeout <number> Abort the connection if the SSL/TLS handshake does not finish in the specified number of milliseconds. Defaults to 120 seconds. A 'clientError' is emitted on the tls.Server object whenever a handshake times out.
    • requestCert <boolean> If true the server will request a certificate from clients that connect and attempt to verify that certificate. Defaults to false.
    • rejectUnauthorized <boolean> If true the server will reject any connection which is not authorized with the list of supplied CAs. This option only has an effect if requestCert is true. Defaults to false.
    • NPNProtocols <string[]> | <Buffer> An array of strings or a Buffer naming possible NPN protocols. (Protocols should be ordered by their priority.)
    • ALPNProtocols <string[]> | <Buffer> An array of strings or a Buffer naming possible ALPN protocols. (Protocols should be ordered by their priority.) When the server receives both NPN and ALPN extensions from the client, ALPN takes precedence over NPN and the server does not send an NPN extension to the client.
    • SNICallback(servername, cb) <Function> A function that will be called if the client supports SNI TLS extension. Two arguments will be passed when called: servername and cb. SNICallback should invoke cb(null, ctx), where ctx is a SecureContext instance. (tls.createSecureContext(...) can be used to get a proper SecureContext.) If SNICallback wasn't provided the default callback with high-level API will be used (see below).
    • sessionTimeout <number> An integer specifying the number of seconds after which the TLS session identifiers and TLS session tickets created by the server will time out. See SSL_CTX_set_timeout for more details.
    • ticketKeys: A 48-byte Buffer instance consisting of a 16-byte prefix, a 16-byte HMAC key, and a 16-byte AES key. This can be used to accept TLS session tickets on multiple instances of the TLS server. Note that this is automatically shared between cluster module workers.
    • ...: Any tls.createSecureContext() options can be provided. For servers, the identity options (pfx or key/cert) are usually required.
  • secureConnectionListener <Function>

Creates a new tls.Server. The secureConnectionListener, if provided, is automatically set as a listener for the 'secureConnection' event.

The following illustrates a simple echo server:

const tls = require('tls');
const fs = require('fs');

const options = {
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),

  // This is necessary only if using the client certificate authentication.
  requestCert: true,

  // This is necessary only if the client uses the self-signed certificate.
  ca: [ fs.readFileSync('client-cert.pem') ]
};

const server = tls.createServer(options, (socket) => {
  console.log('server connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  socket.write('welcome!\n');
  socket.setEncoding('utf8');
  socket.pipe(socket);
});
server.listen(8000, () => {
  console.log('server bound');
});

Or

const tls = require('tls');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('server.pfx'),

  // This is necessary only if using the client certificate authentication.
  requestCert: true,

};

const server = tls.createServer(options, (socket) => {
  console.log('server connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  socket.write('welcome!\n');
  socket.setEncoding('utf8');
  socket.pipe(socket);
});
server.listen(8000, () => {
  console.log('server bound');
});

This server can be tested by connecting to it using openssl s_client:

openssl s_client -connect 127.0.0.1:8000

tls.getCiphers()#

Added in: v0.10.2

Returns an array with the names of the supported SSL ciphers.

For example:

console.log(tls.getCiphers()); // ['AES128-SHA', 'AES256-SHA', ...]

tls.DEFAULT_ECDH_CURVE#

Added in: v0.11.13

The default curve name to use for ECDH key agreement in a tls server. The default value is 'prime256v1' (NIST P-256). Consult RFC 4492 and FIPS.186-4 for more details.

Deprecated APIs#

Class: CryptoStream#

Added in: v0.3.4 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use tls.TLSSocket instead.

The tls.CryptoStream class represents a stream of encrypted data. This class has been deprecated and should no longer be used.

cryptoStream.bytesWritten#

Added in: v0.3.4 Deprecated since: v0.11.3

The cryptoStream.bytesWritten property returns the total number of bytes written to the underlying socket including the bytes required for the implementation of the TLS protocol.

Class: SecurePair#

Added in: v0.3.2 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use tls.TLSSocket instead.

Returned by tls.createSecurePair().

Event: 'secure'#

Added in: v0.3.2 Deprecated since: v0.11.3

The 'secure' event is emitted by the SecurePair object once a secure connection has been established.

As with checking for the server secureConnection event, pair.cleartext.authorized should be inspected to confirm whether the certificate used is properly authorized.

tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])#

Added in: v0.3.2 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use tls.TLSSocket instead.
  • context <Object> A secure context object as returned by tls.createSecureContext()
  • isServer <boolean> true to specify that this TLS connection should be opened as a server.
  • requestCert <boolean> true to specify whether a server should request a certificate from a connecting client. Only applies when isServer is true.
  • rejectUnauthorized <boolean> true to specify whether a server should automatically reject clients with invalid certificates. Only applies when isServer is true.
  • options
    • secureContext: An optional TLS context object from tls.createSecureContext()
    • isServer: If true the TLS socket will be instantiated in server-mode. Defaults to false.
    • server <net.Server> An optional net.Server instance
    • requestCert: Optional, see tls.createServer()
    • rejectUnauthorized: Optional, see tls.createServer()
    • NPNProtocols: Optional, see tls.createServer()
    • ALPNProtocols: Optional, see tls.createServer()
    • SNICallback: Optional, see tls.createServer()
    • session <Buffer> An optional Buffer instance containing a TLS session.
    • requestOCSP <boolean> If true, specifies that the OCSP status request extension will be added to the client hello and an 'OCSPResponse' event will be emitted on the socket before establishing a secure communication

Creates a new secure pair object with two streams, one of which reads and writes the encrypted data and the other of which reads and writes the cleartext data. Generally, the encrypted stream is piped to/from an incoming encrypted data stream and the cleartext one is used as a replacement for the initial encrypted stream.

tls.createSecurePair() returns a tls.SecurePair object with cleartext and encrypted stream properties.

Note: cleartext has the same API as tls.TLSSocket.

Note: The tls.createSecurePair() method is now deprecated in favor of tls.TLSSocket(). For example, the code:

pair = tls.createSecurePair( ... );
pair.encrypted.pipe(socket);
socket.pipe(pair.encrypted);

can be replaced by:

secure_socket = tls.TLSSocket(socket, options);

where secure_socket has the same API as pair.cleartext.

TTY#

Stability: 2 - Stable

The tty module provides the tty.ReadStream and tty.WriteStream classes. In most cases, it will not be necessary or possible to use this module directly. However, it can be accessed using:

const tty = require('tty');

When Node.js detects that it is being run inside a text terminal ("TTY") context, the process.stdin will, by default, be initialized as an instance of tty.ReadStream and both process.stdout and process.stderr will, by default be instances of tty.WriteStream. The preferred method of determining whether Node.js is being run within a TTY context is to check that the value of the process.stdout.isTTY property is true:

$ node -p -e "Boolean(process.stdout.isTTY)"
true
$ node -p -e "Boolean(process.stdout.isTTY)" | cat
false

In most cases, there should be little to no reason for an application to create instances of the tty.ReadStream and tty.WriteStream classes.

Class: tty.ReadStream#

Added in: v0.5.8

The tty.ReadStream class is a subclass of net.Socket that represents the readable side of a TTY. In normal circumstances process.stdin will be the only tty.ReadStream instance in a Node.js process and there should be no reason to create additional instances.

readStream.isRaw#

Added in: v0.7.7

A boolean that is true if the TTY is currently configured to operate as a raw device. Defaults to false.

readStream.setRawMode(mode)#

Added in: v0.7.7

Allows configuration of tty.ReadStream so that it operates as a raw device.

When in raw mode, input is always available character-by-character, not including modifiers. Additionally, all special processing of characters by the terminal is disabled, including echoing input characters. Note that CTRL+C will no longer cause a SIGINT when in this mode.

  • mode <boolean> If true, configures the tty.ReadStream to operate as a raw device. If false, configures the tty.ReadStream to operate in its default mode. The readStream.isRaw property will be set to the resulting mode.

Class: tty.WriteStream#

Added in: v0.5.8

The tty.WriteStream class is a subclass of net.Socket that represents the writable side of a TTY. In normal circumstances, process.stdout and process.stderr will be the only tty.WriteStream instances created for a Node.js process and there should be no reason to create additional instances.

Event: 'resize'#

Added in: v0.7.7

The 'resize' event is emitted whenever either of the writeStream.columns or writeStream.rows properties have changed. No arguments are passed to the listener callback when called.

process.stdout.on('resize', () => {
  console.log('screen size has changed!');
  console.log(`${process.stdout.columns}x${process.stdout.rows}`);
});

writeStream.columns#

Added in: v0.7.7

A number specifying the number of columns the TTY currently has. This property is updated whenever the 'resize' event is emitted.

writeStream.rows#

Added in: v0.7.7

A number specifying the number of rows the TTY currently has. This property is updated whenever the 'resize' event is emitted.

tty.isatty(fd)#

Added in: v0.5.8
  • fd <number> A numeric file descriptor

The tty.isatty() method returns true if the given fd is associated with a TTY and false if is not.

URL#

Stability: 2 - Stable

The url module provides utilities for URL resolution and parsing. It can be accessed using:

const url = require('url');

URL Strings and URL Objects#

A URL string is a structured string containing multiple meaningful components. When parsed, a URL object is returned containing properties for each of these components.

The following details each of the components of a parsed URL. The example 'http://user:pass@host.com:8080/p/a/t/h?query=string#hash' is used to illustrate each.

┌─────────────────────────────────────────────────────────────────────────────┐
│                                    href                                     │
├──────────┬┬───────────┬─────────────────┬───────────────────────────┬───────┤
│ protocol ││   auth    │      host       │           path            │ hash  │
│          ││           ├──────────┬──────┼──────────┬────────────────┤       │
│          ││           │ hostname │ port │ pathname │     search     │       │
│          ││           │          │      │          ├─┬──────────────┤       │
│          ││           │          │      │          │ │    query     │       │
"  http:   // user:pass @ host.com : 8080   /p/a/t/h  ?  query=string   #hash "
│          ││           │          │      │          │ │              │       │
└──────────┴┴───────────┴──────────┴──────┴──────────┴─┴──────────────┴───────┘
(all spaces in the "" line should be ignored -- they are purely for formatting)

urlObject.href#

The href property is the full URL string that was parsed with both the protocol and host components converted to lower-case.

For example: 'http://user:pass@host.com:8080/p/a/t/h?query=string#hash'

urlObject.protocol#

The protocol property identifies the URL's lower-cased protocol scheme.

For example: 'http:'

urlObject.slashes#

The slashes property is a boolean with a value of true if two ASCII forward-slash characters (/) are required following the colon in the protocol.

urlObject.host#

The host property is the full lower-cased host portion of the URL, including the port if specified.

For example: 'host.com:8080'

urlObject.auth#

The auth property is the username and password portion of the URL, also referred to as "userinfo". This string subset follows the protocol and double slashes (if present) and precedes the host component, delimited by an ASCII "at sign" (@). The format of the string is {username}[:{password}], with the [:{password}] portion being optional.

For example: 'user:pass'

urlObject.hostname#

The hostname property is the lower-cased host name portion of the host component without the port included.

For example: 'host.com'

urlObject.port#

The port property is the numeric port portion of the host component.

For example: '8080'

urlObject.pathname#

The pathname property consists of the entire path section of the URL. This is everything following the host (including the port) and before the start of the query or hash components, delimited by either the ASCII question mark (?) or hash (#) characters.

For example '/p/a/t/h'

No decoding of the path string is performed.

urlObject.search#

The search property consists of the entire "query string" portion of the URL, including the leading ASCII question mark (?) character.

For example: '?query=string'

No decoding of the query string is performed.

urlObject.path#

The path property is a concatenation of the pathname and search components.

For example: '/p/a/t/h?query=string'

No decoding of the path is performed.

urlObject.query#

The query property is either the query string without the leading ASCII question mark (?), or an object returned by the querystring module's parse() method. Whether the query property is a string or object is determined by the parseQueryString argument passed to url.parse().

For example: 'query=string' or {'query': 'string'}

If returned as a string, no decoding of the query string is performed. If returned as an object, both keys and values are decoded.

urlObject.hash#

The hash property consists of the "fragment" portion of the URL including the leading ASCII hash (#) character.

For example: '#hash'

url.format(urlObject)#

Added in: v0.1.25
  • urlObject <Object> | <String> A URL object (as returned by url.parse() or constructed otherwise). If a string, it is converted to an object by passing it to url.parse().

The url.format() method returns a formatted URL string derived from urlObject.

If urlObject is not an object or a string, url.parse() will throw a TypeError.

The formatting process operates as follows:

  • A new empty string result is created.
  • If urlObject.protocol is a string, it is appended as-is to result.
  • Otherwise, if urlObject.protocol is not undefined and is not a string, an Error is thrown.
  • For all string values of urlObject.protocol that do not end with an ASCII colon (:) character, the literal string : will be appended to result.
  • If either of the following conditions is true, then the literal string // will be appended to result:
    • urlObject.slashes property is true;
    • urlObject.protocol begins with http, https, ftp, gopher, or file;
  • If the value of the urlObject.auth property is truthy, and either urlObject.host or urlObject.hostname are not undefined, the value of urlObject.auth will be coerced into a string and appended to result followed by the literal string @.
  • If the urlObject.host property is undefined then:
    • If the urlObject.hostname is a string, it is appended to result.
    • Otherwise, if urlObject.hostname is not undefined and is not a string, an Error is thrown.
    • If the urlObject.port property value is truthy, and urlObject.hostname is not undefined:
      • The literal string : is appended to result, and
      • The value of urlObject.port is coerced to a string and appended to result.
  • Otherwise, if the urlObject.host property value is truthy, the value of urlObject.host is coerced to a string and appended to result.
  • If the urlObject.pathname property is a string that is not an empty string:
    • If the urlObject.pathname does not start with an ASCII forward slash (/), then the literal string '/' is appended to result.
    • The value of urlObject.pathname is appended to result.
  • Otherwise, if urlObject.pathname is not undefined and is not a string, an Error is thrown.
  • If the urlObject.search property is undefined and if the urlObject.query property is an Object, the literal string ? is appended to result followed by the output of calling the querystring module's stringify() method passing the value of urlObject.query.
  • Otherwise, if urlObject.search is a string:
    • If the value of urlObject.search does not start with the ASCII question mark (?) character, the literal string ? is appended to result.
    • The value of urlObject.search is appended to result.
  • Otherwise, if urlObject.search is not undefined and is not a string, an Error is thrown.
  • If the urlObject.hash property is a string:
    • If the value of urlObject.hash does not start with the ASCII hash (#) character, the literal string # is appended to result.
    • The value of urlObject.hash is appended to result.
  • Otherwise, if the urlObject.hash property is not undefined and is not a string, an Error is thrown.
  • result is returned.

url.parse(urlString[, parseQueryString[, slashesDenoteHost]])#

Added in: v0.1.25
  • urlString <String> The URL string to parse.
  • parseQueryString <Boolean> If true, the query property will always be set to an object returned by the querystring module's parse() method. If false, the query property on the returned URL object will be an unparsed, undecoded string. Defaults to false.
  • slashesDenoteHost <Boolean> If true, the first token after the literal string // and preceding the next / will be interpreted as the host. For instance, given //foo/bar, the result would be {host: 'foo', pathname: '/bar'} rather than {pathname: '//foo/bar'}. Defaults to false.

The url.parse() method takes a URL string, parses it, and returns a URL object.

url.resolve(from, to)#

Added in: v0.1.25
  • from <String> The Base URL being resolved against.
  • to <String> The HREF URL being resolved.

The url.resolve() method resolves a target URL relative to a base URL in a manner similar to that of a Web browser resolving an anchor tag HREF.

For example:

url.resolve('/one/two/three', 'four')         // '/one/two/four'
url.resolve('http://example.com/', '/one')    // 'http://example.com/one'
url.resolve('http://example.com/one', '/two') // 'http://example.com/two'

Escaped Characters#

URLs are only permitted to contain a certain range of characters. Spaces (' ') and the following characters will be automatically escaped in the properties of URL objects:

< > " ` \r \n \t { } | \ ^ '

For example, the ASCII space character (' ') is encoded as %20. The ASCII forward slash (/) character is encoded as %3C.

The WHATWG URL API#

Stability: 1 - Experimental

The url module provides an experimental implementation of the WHATWG URL Standard as an alternative to the existing url.parse() API.

const URL = require('url').URL;
const myURL = new URL('https://example.org/foo');

console.log(myURL.href);     // https://example.org/foo
console.log(myURL.protocol); // https:
console.log(myURL.hostname); // example.org
console.log(myURL.pathname); // /foo

Note: Using the delete keyword (e.g. delete myURL.protocol, delete myURL.pathname, etc) has no effect but will still return true.

Class: URL#

Constructor: new URL(input[, base])#

  • input <String> The input URL to parse
  • base <String> | <URL> The base URL to resolve against if the input is not absolute.

Creates a new URL object by parsing the input relative to the base. If base is passed as a string, it will be parsed equivalent to new URL(base).

const myURL = new URL('/foo', 'https://example.org/');
  // https://example.org/foo

A TypeError will be thrown if the input or base are not valid URLs. Note that an effort will be made to coerce the given values into strings. For instance:

const myURL = new URL({toString: () => 'https://example.org/'});
  // https://example.org/

Unicode characters appearing within the hostname of input will be automatically converted to ASCII using the Punycode algorithm.

const myURL = new URL('https://你好你好');
  // https://xn--6qqa088eba

Additional examples of parsed URLs may be found in the WHATWG URL Standard.

url.hash#

Gets and sets the fragment portion of the URL.

const myURL = new URL('https://example.org/foo#bar');
console.log(myURL.hash);
  // Prints #bar

myURL.hash = 'baz';
console.log(myURL.href);
  // Prints https://example.org/foo#baz

Invalid URL characters included in the value assigned to the hash property are percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.host#

Gets and sets the host portion of the URL.

const myURL = new URL('https://example.org:81/foo');
console.log(myURL.host);
  // Prints example.org:81

myURL.host = 'example.com:82';
console.log(myURL.href);
  // Prints https://example.com:82/foo

Invalid host values assigned to the host property are ignored.

url.hostname#

Gets and sets the hostname portion of the URL. The key difference between url.host and url.hostname is that url.hostname does not include the port.

const myURL = new URL('https://example.org:81/foo');
console.log(myURL.hostname);
  // Prints example.org

myURL.hostname = 'example.com:82';
console.log(myURL.href);
  // Prints https://example.com:81/foo

Invalid hostname values assigned to the hostname property are ignored.

url.href#

Gets and sets the serialized URL.

const myURL = new URL('https://example.org/foo');
console.log(myURL.href);
  // Prints https://example.org/foo

myURL.href = 'https://example.com/bar'
  // Prints https://example.com/bar

Setting the value of the href property to a new value is equivalent to creating a new URL object using new URL(value). Each of the URL object's properties will be modified.

If the value assigned to the href property is not a valid URL, a TypeError will be thrown.

url.origin#

Gets the read-only serialization of the URL's origin. Unicode characters that may be contained within the hostname will be encoded as-is without Punycode encoding.

const myURL = new URL('https://example.org/foo/bar?baz');
console.log(myURL.origin);
  // Prints https://example.org

const idnURL = new URL('https://你好你好');
console.log(idnURL.origin);
  // Prints https://你好你好

console.log(idnURL.hostname);
  // Prints xn--6qqa088eba

url.password#

Gets and sets the password portion of the URL.

const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.password);
  // Prints xyz

myURL.password = '123';
console.log(myURL.href);
  // Prints https://abc:123@example.com

Invalid URL characters included in the value assigned to the password property are percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.pathname#

Gets and sets the path portion of the URL.

const myURL = new URL('https://example.org/abc/xyz?123');
console.log(myURL.pathname);
  // Prints /abc/xyz

myURL.pathname = '/abcdef';
console.log(myURL.href);
  // Prints https://example.org/abcdef?123

Invalid URL characters included in the value assigned to the pathname property are percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.port#

Gets and sets the port portion of the URL. When getting the port, the value is returned as a String.

const myURL = new URL('https://example.org:8888');
console.log(myURL.port);
  // Prints 8888

myURL.port = 1234;
console.log(myURL.href);
  // Prints https://example.org:1234

The port value may be set as either a number or as a String containing a number in the range 0 to 65535 (inclusive). Setting the value to the default port of the URL objects given protocol will result in the port value becoming the empty string ('').

Invalid URL port values assigned to the port property are ignored.

url.protocol#

Gets and sets the protocol portion of the URL.

const myURL = new URL('https://example.org');
console.log(myURL.protocol);
  // Prints http:

myURL.protocol = 'ftp';
console.log(myURL.href);
  // Prints ftp://example.org

Invalid URL protocol values assigned to the protocol property are ignored.

url.search#

Gets and sets the serialized query portion of the URL.

const myURL = new URL('https://example.org/abc?123');
console.log(myURL.search);
  // Prints ?123

myURL.search = 'abc=xyz';
console.log(myURL.href);
  // Prints https://example.org/abc?abc=xyz

Any invalid URL characters appearing in the value assigned the search property will be percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.searchParams#

Gets a URLSearchParams object representing the query parameters of the URL.

url.username#

Gets and sets the username portion of the URL.

const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.username);
  // Prints abc

myURL.username = '123';
console.log(myURL.href);
  // Prints https://123:xyz@example.com

Any invalid URL characters appearing in the value assigned the username property will be percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.toString()#

The toString() method on the URL object returns the serialized URL. The value returned is equivalent to that of url.href.

Class: URLSearchParams#

The URLSearchParams object provides read and write access to the query of a URL. The URLSearchParams class can also be used standalone with one of the four following constructors.

const { URL, URLSearchParams } = require('url');

const myURL = new URL('https://example.org/?abc=123');
console.log(myURL.searchParams.get('abc'));
  // Prints 123

myURL.searchParams.append('abc', 'xyz');
console.log(myURL.href);
  // Prints https://example.org/?abc=123&abc=xyz

myURL.searchParams.delete('abc');
myURL.searchParams.set('a', 'b');
console.log(myURL.href);
  // Prints https://example.org/?a=b

const newSearchParams = new URLSearchParams(myURL.searchParams);
// The above is equivalent to
// const newSearchParams = new URLSearchParams(myURL.search);

newSearchParams.append('a', 'c');
console.log(myURL.href);
  // Prints https://example.org/?a=b
console.log(newSearchParams.toString());
  // Prints a=b&a=c

// newSearchParams.toString() is implicitly called
myURL.search = newSearchParams;
console.log(myURL.href);
  // Prints https://example.org/?a=b&a=c
newSearchParams.delete('a');
console.log(myURL.href);
  // Prints https://example.org/?a=b&a=c

Constructor: new URLSearchParams()#

Instantiate a new empty URLSearchParams object.

Constructor: new URLSearchParams(string)#

Parse the string as a query string, and use it to instantiate a new URLSearchParams object. A leading '?', if present, is ignored.

const { URLSearchParams } = require('url');
let params;

params = new URLSearchParams('user=abc&query=xyz');
console.log(params.get('user'));
  // Prints 'abc'
console.log(params.toString());
  // Prints 'user=abc&query=xyz'

params = new URLSearchParams('?user=abc&query=xyz');
console.log(params.toString());
  // Prints 'user=abc&query=xyz'

Constructor: new URLSearchParams(obj)#

  • obj <Object> An object representing a collection of key-value pairs

Instantiate a new URLSearchParams object with a query hash map. The key and value of each property of obj are always coerced to strings.

Note: Unlike querystring module, duplicate keys in the form of array values are not allowed. Arrays are stringified using array.toString(), which simply joins all array elements with commas.

const { URLSearchParams } = require('url');
const params = new URLSearchParams({
  user: 'abc',
  query: ['first', 'second']
});
console.log(params.getAll('query'));
  // Prints ['first,second']
console.log(params.toString());
  // Prints 'user=abc&query=first%2Csecond'

Constructor: new URLSearchParams(iterable)#

  • iterable <Iterable> An iterable object whose elements are key-value pairs

Instantiate a new URLSearchParams object with an iterable map in a way that is similar to Map's constructor. iterable can be an Array or any iterable object. That means iterable can be another URLSearchParams, in which case the constructor will simply create a clone of the provided URLSearchParams. Elements of iterable are key-value pairs, and can themselves be any iterable object.

Duplicate keys are allowed.

const { URLSearchParams } = require('url');
let params;

// Using an array
params = new URLSearchParams([
  ['user', 'abc'],
  ['query', 'first'],
  ['query', 'second']
]);
console.log(params.toString());
  // Prints 'user=abc&query=first&query=second'

// Using a Map object
const map = new Map();
map.set('user', 'abc');
map.set('query', 'xyz');
params = new URLSearchParams(map);
console.log(params.toString());
  // Prints 'user=abc&query=xyz'

// Using a generator function
function* getQueryPairs() {
  yield ['user', 'abc'];
  yield ['query', 'first'];
  yield ['query', 'second'];
}
params = new URLSearchParams(getQueryPairs());
console.log(params.toString());
  // Prints 'user=abc&query=first&query=second'

// Each key-value pair must have exactly two elements
new URLSearchParams([
  ['user', 'abc', 'error']
]);
  // Throws TypeError: Each query pair must be a name/value tuple

urlSearchParams.append(name, value)#

Append a new name-value pair to the query string.

urlSearchParams.delete(name)#

Remove all name-value pairs whose name is name.

urlSearchParams.entries()#

Returns an ES6 Iterator over each of the name-value pairs in the query. Each item of the iterator is a JavaScript Array. The first item of the Array is the name, the second item of the Array is the value.

Alias for urlSearchParams\[\@\@iterator\]().

urlSearchParams.forEach(fn)#

  • fn <Function> Function invoked for each name-value pair in the query.

Iterates over each name-value pair in the query and invokes the given function.

const URL = require('url').URL;
const myURL = new URL('https://example.org/?a=b&c=d');
myURL.searchParams.forEach((value, name) => {
  console.log(name, value);
});

urlSearchParams.get(name)#

  • name <String>
  • Returns: <String> or null if there is no name-value pair with the given name.

Returns the value of the first name-value pair whose name is name.

urlSearchParams.getAll(name)#

Returns the values of all name-value pairs whose name is name.

urlSearchParams.has(name)#

Returns true if there is at least one name-value pair whose name is name.

urlSearchParams.keys()#

Returns an ES6 Iterator over the names of each name-value pair.

urlSearchParams.set(name, value)#

Remove any existing name-value pairs whose name is name and append a new name-value pair.

urlSearchParams.toString()#

Returns the search parameters serialized as a URL-encoded string.

urlSearchParams.values()#

Returns an ES6 Iterator over the values of each name-value pair.

urlSearchParams[\@\@iterator]()#

Returns an ES6 Iterator over each of the name-value pairs in the query string. Each item of the iterator is a JavaScript Array. The first item of the Array is the name, the second item of the Array is the value.

Alias for urlSearchParams.entries().

require('url').domainToAscii(domain)#

Returns the Punycode ASCII serialization of the domain.

Note: The require('url').domainToAscii() method is introduced as part of the new URL implementation but is not part of the WHATWG URL standard.

require('url').domainToUnicode(domain)#

Returns the Unicode serialization of the domain.

Note: The require('url').domainToUnicode() API is introduced as part of the the new URL implementation but is not part of the WHATWG URL standard.

Percent-Encoding in the WHATWG URL Standard#

URLs are permitted to only contain a certain range of characters. Any character falling outside of that range must be encoded. How such characters are encoded, and which characters to encode depends entirely on where the character is located within the structure of the URL. The WHATWG URL Standard uses a more selective and fine grained approach to selecting encoded characters than that used by the older url.parse() and url.format() methods.

The WHATWG algorithm defines three "encoding sets" that describe ranges of characters that must be percent-encoded:

  • The simple encode set includes code points in range U+0000 to U+001F (inclusive) and all code points greater than U+007E.

  • The default encode set includes the simple encode set and code points U+0020, U+0022, U+0023, U+003C, U+003E, U+003F, U+0060, U+007B, and U+007D.

  • The userinfo encode set includes the default encode set and code points U+002F, U+003A, U+003B, U+003D, U+0040, U+005B, U+005C, U+005D, U+005E, and U+007C.

The simple encode set is used primary for URL fragments and certain specific conditions for the path. The userinfo encode set is used specifically for username and passwords encoded within the URL. The default encode set is used for all other cases.

When non-ASCII characters appear within a hostname, the hostname is encoded using the Punycode algorithm. Note, however, that a hostname may contain both Punycode encoded and percent-encoded characters. For example:

const URL = require('url').URL;
const myURL = new URL('https://%CF%80.com/foo');
console.log(myURL.href);
  // Prints https://xn--1xa.com/foo
console.log(myURL.origin);
  // Prints https://π.com

util#

Stability: 2 - Stable

The util module is primarily designed to support the needs of Node.js' own internal APIs. However, many of the utilities are useful for application and module developers as well. It can be accessed using:

const util = require('util');

util.debuglog(section)#

Added in: v0.11.3
  • section <String> A string identifying the portion of the application for which the debuglog function is being created.
  • Returns: <Function> The logging function

The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.

For example:

const util = require('util');
const debuglog = util.debuglog('foo');

debuglog('hello from foo [%d]', 123);

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

FOO 3245: hello from foo [123]

where 3245 is the process id. If it is not run with that environment variable set, then it will not print anything.

Multiple comma-separated section names may be specified in the NODE_DEBUG environment variable. For example: NODE_DEBUG=fs,net,tls.

util.deprecate(function, string)#

Added in: v0.8.0

The util.deprecate() method wraps the given function or class in such a way that it is marked as deprecated.

const util = require('util');

exports.puts = util.deprecate(function() {
  for (var i = 0, len = arguments.length; i < len; ++i) {
    process.stdout.write(arguments[i] + '\n');
  }
}, 'util.puts: Use console.log instead');

When called, util.deprecate() will return a function that will emit a DeprecationWarning using the process.on('warning') event. By default, this warning will be emitted and printed to stderr exactly once, the first time it is called. After the warning is emitted, the wrapped function is called.

If either the --no-deprecation or --no-warnings command line flags are used, or if the process.noDeprecation property is set to true prior to the first deprecation warning, the util.deprecate() method does nothing.

If the --trace-deprecation or --trace-warnings command line flags are set, or the process.traceDeprecation property is set to true, a warning and a stack trace are printed to stderr the first time the deprecated function is called.

If the --throw-deprecation command line flag is set, or the process.throwDeprecation property is set to true, then an exception will be thrown when the deprecated function is called.

The --throw-deprecation command line flag and process.throwDeprecation property take precedence over --trace-deprecation and process.traceDeprecation.

util.format(format[, ...args])#

Added in: v0.5.3
  • format <String> A printf-like format string.

The util.format() method returns a formatted string using the first argument as a printf-like format.

The first argument is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the converted value from the corresponding argument. Supported placeholders are:

  • %s - String.
  • %d - Number (both integer and float).
  • %j - JSON. Replaced with the string '[Circular]' if the argument contains circular references.
  • %% - single percent sign ('%'). This does not consume an argument.

If the placeholder does not have a corresponding argument, the placeholder is not replaced.

util.format('%s:%s', 'foo');
// Returns: 'foo:%s'

If there are more arguments passed to the util.format() method than the number of placeholders, the extra arguments are coerced into strings (for objects and symbols, util.inspect() is used) then concatenated to the returned string, each delimited by a space.

util.format('%s:%s', 'foo', 'bar', 'baz'); // 'foo:bar baz'

If the first argument is not a format string then util.format() returns a string that is the concatenation of all arguments separated by spaces. Each argument is converted to a string using util.inspect().

util.format(1, 2, 3); // '1 2 3'

util.inherits(constructor, superConstructor)#

Added in: v0.3.0

Note: usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

const util = require('util');
const EventEmitter = require('events');

function MyStream() {
  EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
  this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"

ES6 example using class and extends

const EventEmitter = require('events');

class MyStream extends EventEmitter {
  constructor() {
    super();
  }
  write(data) {
    this.emit('data', data);
  }
}

const stream = new MyStream();

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('With ES6');

util.inspect(object[, options])#

Added in: v0.3.0
  • object <any> Any JavaScript primitive or Object.
  • options <Object>
    • showHidden <boolean> If true, the object's non-enumerable symbols and properties will be included in the formatted result. Defaults to false.
    • depth <number> Specifies the number of times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to 2. To make it recurse indefinitely pass null.
    • colors <boolean> If true, the output will be styled with ANSI color codes. Defaults to false. Colors are customizable, see Customizing util.inspect colors.
    • customInspect <boolean> If false, then custom inspect(depth, opts) functions exported on the object being inspected will not be called. Defaults to true.
    • showProxy <boolean> If true, then objects and functions that are Proxy objects will be introspected to show their target and handler objects. Defaults to false.
    • maxArrayLength <number> Specifies the maximum number of array and TypedArray elements to include when formatting. Defaults to 100. Set to null to show all array elements. Set to 0 or negative to show no array elements.
    • breakLength <number> The length at which an object's keys are split across multiple lines. Set to Infinity to format an object as a single line. Defaults to 60 for legacy compatibility.

The util.inspect() method returns a string representation of object that is primarily useful for debugging. Additional options may be passed that alter certain aspects of the formatted string.

The following example inspects all properties of the util object:

const util = require('util');

console.log(util.inspect(util, { showHidden: true, depth: null }));

Values may supply their own custom inspect(depth, opts) functions, when called these receive the current depth in the recursive inspection, as well as the options object passed to util.inspect().

Customizing util.inspect colors#

Color output (if enabled) of util.inspect is customizable globally via the util.inspect.styles and util.inspect.colors properties.

util.inspect.styles is a map associating a style name to a color from util.inspect.colors.

The default styles and associated colors are:

  • number - yellow
  • boolean - yellow
  • string - green
  • date - magenta
  • regexp - red
  • null - bold
  • undefined - grey
  • special - cyan (only applied to functions at this time)
  • name - (no styling)

The predefined color codes are: white, grey, black, blue, cyan, green, magenta, red and yellow. There are also bold, italic, underline and inverse codes.

Color styling uses ANSI control codes that may not be supported on all terminals.

Custom inspection functions on Objects#

Objects may also define their own [util.inspect.custom](depth, opts) (or, equivalently inspect(depth, opts)) function that util.inspect() will invoke and use the result of when inspecting the object:

const util = require('util');

class Box {
  constructor(value) {
    this.value = value;
  }

  inspect(depth, options) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = util.inspect(this.value, newOptions).replace(/\n/g, '\n' + padding);
    return options.stylize('Box', 'special') + '< ' + inner + ' >';
  }
}

const box = new Box(true);

util.inspect(box);
// Returns: "Box< true >"

Custom [util.inspect.custom](depth, opts) functions typically return a string but may return a value of any type that will be formatted accordingly by util.inspect().

const util = require('util');

const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = function(depth) {
  return { bar: 'baz' };
};

util.inspect(obj);
// Returns: "{ bar: 'baz' }"

A custom inspection method can alternatively be provided by exposing an inspect(depth, opts) method on the object:

const util = require('util');

const obj = { foo: 'this will not show up in the inspect() output' };
obj.inspect = function(depth) {
  return { bar: 'baz' };
};

util.inspect(obj);
// Returns: "{ bar: 'baz' }"

util.inspect.defaultOptions#

Added in: v6.4.0

The defaultOptions value allows customization of the default options used by util.inspect. This is useful for functions like console.log or util.format which implicitly call into util.inspect. It shall be set to an object containing one or more valid util.inspect() options. Setting option properties directly is also supported.

const util = require('util');
const arr = Array(101);

console.log(arr); // logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array

util.inspect.custom#

Added in: v6.6.0

A Symbol that can be used to declare custom inspect functions, see Custom inspection functions on Objects.

Deprecated APIs#

The following APIs have been deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.

util.debug(string)#

Added in: v0.3.0 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use console.error() instead.
  • string <String> The message to print to stderr

Deprecated predecessor of console.error.

util.error([...strings])#

Added in: v0.3.0 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use console.error() instead.
  • ...strings <String> The message to print to stderr

Deprecated predecessor of console.error.

util.isArray(object)#

Added in: v0.6.0 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Internal alias for Array.isArray.

Returns true if the given object is an Array. Otherwise, returns false.

const util = require('util');

util.isArray([]);
// Returns: true
util.isArray(new Array);
// Returns: true
util.isArray({});
// Returns: false

util.isBoolean(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a Boolean. Otherwise, returns false.

const util = require('util');

util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true

util.isBuffer(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated: Use Buffer.isBuffer() instead.
  • object <any>

Returns true if the given object is a Buffer. Otherwise, returns false.

const util = require('util');

util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true

util.isDate(object)#

Added in: v0.6.0 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a Date. Otherwise, returns false.

const util = require('util');

util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false

util.isError(object)#

Added in: v0.6.0 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is an Error. Otherwise, returns false.

const util = require('util');

util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false

Note that this method relies on Object.prototype.toString() behavior. It is possible to obtain an incorrect result when the object argument manipulates @@toStringTag.

const util = require('util');
const obj = { name: 'Error', message: 'an error occurred' };

util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true

util.isFunction(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a Function. Otherwise, returns false.

const util = require('util');

function Foo() {}
const Bar = function() {};

util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true

util.isNull(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is strictly null. Otherwise, returns false.

const util = require('util');

util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true

util.isNullOrUndefined(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is null or undefined. Otherwise, returns false.

const util = require('util');

util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true

util.isNumber(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a Number. Otherwise, returns false.

const util = require('util');

util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true

util.isObject(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is strictly an Object and not a Function. Otherwise, returns false.

const util = require('util');

util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(function(){});
// Returns: false

util.isPrimitive(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a primitive type. Otherwise, returns false.

const util = require('util');

util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(function() {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false

util.isRegExp(object)#

Added in: v0.6.0 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a RegExp. Otherwise, returns false.

const util = require('util');

util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false

util.isString(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a string. Otherwise, returns false.

const util = require('util');

util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false

util.isSymbol(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is a Symbol. Otherwise, returns false.

const util = require('util');

util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true

util.isUndefined(object)#

Added in: v0.11.5 Deprecated since: v4.0.0 
Stability: 0 - Deprecated
  • object <any>

Returns true if the given object is undefined. Otherwise, returns false.

const util = require('util');

const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false

util.log(string)#

Added in: v0.3.0 Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use a third party module instead.

The util.log() method prints the given string to stdout with an included timestamp.

const util = require('util');

util.log('Timestamped message.');

util.print([...strings])#

Added in: v0.3.0 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of console.log.

util.puts([...strings])#

Added in: v0.3.0 Deprecated since: v0.11.3 
Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of console.log.

util._extend(target, source)#

Added in: v0.7.5 Deprecated since: v6.0.0 
Stability: 0 - Deprecated: Use Object.assign() instead.

The util._extend() method was never intended to be used outside of internal Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very similar built-in functionality through Object.assign().

V8#

The v8 module exposes APIs that are specific to the version of V8 built into the Node.js binary. It can be accessed using:

const v8 = require('v8');

Note: The APIs and implementation are subject to change at any time.

v8.getHeapStatistics()#

Added in: v1.0.0

Returns an object with the following properties:

  • total_heap_size <number>
  • total_heap_size_executable <number>
  • total_physical_size <number>
  • total_available_size <number>
  • used_heap_size <number>
  • heap_size_limit <number>
  • malloced_memory <number>
  • peak_malloced_memory <number>
  • does_zap_garbage <number>

does_zap_garbage is a 0/1 boolean, which signifies whether the --zap_code_space option is enabled or not. This makes V8 overwrite heap garbage with a bit pattern. The RSS footprint (resident memory set) gets bigger because it continuously touches all heap pages and that makes them less likely to get swapped out by the operating system.

For example:

{
  total_heap_size: 7326976,
  total_heap_size_executable: 4194304,
  total_physical_size: 7326976,
  total_available_size: 1152656,
  used_heap_size: 3476208,
  heap_size_limit: 1535115264,
  malloced_memory: 16384,
  peak_malloced_memory: 1127496,
  does_zap_garbage: 0
}

v8.getHeapSpaceStatistics()#

Added in: v6.0.0

Returns statistics about the V8 heap spaces, i.e. the segments which make up the V8 heap. Neither the ordering of heap spaces, nor the availability of a heap space can be guaranteed as the statistics are provided via the V8 GetHeapSpaceStatistics function and may change from one V8 version to the next.

The value returned is an array of objects containing the following properties:

  • space_name <string>
  • space_size <number>
  • space_used_size <number>
  • space_available_size <number>
  • physical_space_size <number>

For example:

[
  {
    "space_name": "new_space",
    "space_size": 2063872,
    "space_used_size": 951112,
    "space_available_size": 80824,
    "physical_space_size": 2063872
  },
  {
    "space_name": "old_space",
    "space_size": 3090560,
    "space_used_size": 2493792,
    "space_available_size": 0,
    "physical_space_size": 3090560
  },
  {
    "space_name": "code_space",
    "space_size": 1260160,
    "space_used_size": 644256,
    "space_available_size": 960,
    "physical_space_size": 1260160
  },
  {
    "space_name": "map_space",
    "space_size": 1094160,
    "space_used_size": 201608,
    "space_available_size": 0,
    "physical_space_size": 1094160
  },
  {
    "space_name": "large_object_space",
    "space_size": 0,
    "space_used_size": 0,
    "space_available_size": 1490980608,
    "physical_space_size": 0
  }
]

v8.setFlagsFromString(string)#

Added in: v1.0.0

The v8.setFlagsFromString() method can be used to programmatically set V8 command line flags. This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

The V8 options available for a version of Node.js may be determined by running node --v8-options. An unofficial, community-maintained list of options and their effects is available here.

Usage:

// Print GC events to stdout for one minute.
const v8 = require('v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(function() { v8.setFlagsFromString('--notrace_gc'); }, 60e3);

Executing JavaScript#

Stability: 2 - Stable

The vm module provides APIs for compiling and running code within V8 Virtual Machine contexts. It can be accessed using:

const vm = require('vm');

JavaScript code can be compiled and run immediately or compiled, saved, and run later.

Class: vm.Script#

Added in: v0.3.1

Instances of the vm.Script class contain precompiled scripts that can be executed in specific sandboxes (or "contexts").

new vm.Script(code, options)#

Added in: v0.3.1
  • code <string> The JavaScript code to compile.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
    • cachedData <Buffer> Provides an optional Buffer with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
    • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully.

Creating a new vm.Script object compiles code but does not run it. The compiled vm.Script can be run later multiple times. It is important to note that the code is not bound to any global object; rather, it is bound before each run, just for that run.

script.runInContext(contextifiedSandbox[, options])#

Added in: v0.3.1
  • contextifiedSandbox <Object> A contextified object as returned by the vm.createContext() method.
  • options <Object>
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
    • breakOnSigint: if true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on("SIGINT") will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown.

Runs the compiled code contained by the vm.Script object within the given contextifiedSandbox and returns the result. Running code does not have access to local scope.

The following example compiles code that increments a global variable, sets the value of another global variable, then execute the code multiple times. The globals are contained in the sandbox object.

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

const script = new vm.Script('count += 1; name = "kitty";');

const context = new vm.createContext(sandbox);
for (var i = 0; i < 10; ++i) {
  script.runInContext(context);
}

console.log(util.inspect(sandbox));

// { animal: 'cat', count: 12, name: 'kitty' }

script.runInNewContext([sandbox][, options])#

Added in: v0.3.1
  • sandbox <Object> An object that will be contextified. If undefined, a new object will be created.
  • options <Object>
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

First contextifies the given sandbox, runs the compiled code contained by the vm.Script object within the created sandbox, and returns the result. Running code does not have access to local scope.

The following example compiles code that sets a global variable, then executes the code multiple times in different contexts. The globals are set on and contained within each individual sandbox.

const util = require('util');
const vm = require('vm');

const script = new vm.Script('globalVar = "set"');

const sandboxes = [{}, {}, {}];
sandboxes.forEach((sandbox) => {
  script.runInNewContext(sandbox);
});

console.log(util.inspect(sandboxes));

// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

script.runInThisContext(options)#

Added in: v0.3.1
  • options <Object>
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

Runs the compiled code contained by the vm.Script within the context of the current global object. Running code does not have access to local scope, but does have access to the current global object.

The following example compiles code that increments a global variable then executes that code multiple times:

const vm = require('vm');

global.globalVar = 0;

const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });

for (var i = 0; i < 1000; ++i) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

vm.createContext([sandbox])#

Added in: v0.3.1

If given a sandbox object, the vm.createContext() method will prepare that sandbox so that it can be used in calls to vm.runInContext() or script.runInContext(). Inside such scripts, the sandbox object will be the global object, retaining all of its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, global variables will remain unchanged.

const util = require('util');
const vm = require('vm');

var globalVar = 3;

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

vm.runInContext('globalVar *= 2;', sandbox);

console.log(util.inspect(sandbox)); // 2

console.log(util.inspect(globalVar)); // 3

If sandbox is omitted (or passed explicitly as undefined), a new, empty contextified sandbox object will be returned.

The vm.createContext() method is primarily useful for creating a single sandbox that can be used to run multiple scripts. For instance, if emulating a web browser, the method can be used to create a single sandbox representing a window's global object, then run all <script> tags together within the context of that sandbox.

vm.isContext(sandbox)#

Added in: v0.11.7

Returns true if the given sandbox object has been contextified using vm.createContext().

vm.runInContext(code, contextifiedSandbox[, options])#

  • code <string> The JavaScript code to compile and run.
  • contextifiedSandbox <Object> The contextified object that will be used as the global when the code is compiled and run.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

The vm.runInContext() method compiles code, runs it within the context of the contextifiedSandbox, then returns the result. Running code does not have access to the local scope. The contextifiedSandbox object must have been previously contextified using the vm.createContext() method.

The following example compiles and executes different scripts using a single contextified object:

const util = require('util');
const vm = require('vm');

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

for (var i = 0; i < 10; ++i) {
  vm.runInContext('globalVar *= 2;', sandbox);
}
console.log(util.inspect(sandbox));

// { globalVar: 1024 }

vm.runInDebugContext(code)#

Added in: v0.11.14
  • code <string> The JavaScript code to compile and run.

The vm.runInDebugContext() method compiles and executes code inside the V8 debug context. The primary use case is to gain access to the V8 Debug object:

const vm = require('vm');
const Debug = vm.runInDebugContext('Debug');
console.log(Debug.findScript(process.emit).name);  // 'events.js'
console.log(Debug.findScript(process.exit).name);  // 'internal/process.js'

Note: The debug context and object are intrinsically tied to V8's debugger implementation and may change (or even be removed) without prior warning.

The Debug object can also be made available using the V8-specific --expose_debug_as= command line option.

vm.runInNewContext(code[, sandbox][, options])#

Added in: v0.3.1
  • code <string> The JavaScript code to compile and run.
  • sandbox <Object> An object that will be contextified. If undefined, a new object will be created.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

The vm.runInNewContext() first contextifies the given sandbox object (or creates a new sandbox if passed as undefined), compiles the code, runs it within the context of the created context, then returns the result. Running code does not have access to the local scope.

The following example compiles and executes code that increments a global variable and sets a new one. These globals are contained in the sandbox.

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

vm.runInNewContext('count += 1; name = "kitty"', sandbox);
console.log(util.inspect(sandbox));

// { animal: 'cat', count: 3, name: 'kitty' }

vm.runInThisContext(code[, options])#

Added in: v0.3.1
  • code <string> The JavaScript code to compile and run.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

vm.runInThisContext() compiles code, runs it within the context of the current global and returns the result. Running code does not have access to local scope, but does have access to the current global object.

The following example illustrates using both vm.runInThisContext() and the JavaScript eval() function to run the same code:

const vm = require('vm');
var localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log('vmResult:', vmResult);
console.log('localVar:', localVar);

const evalResult = eval('localVar = "eval";');
console.log('evalResult:', evalResult);
console.log('localVar:', localVar);

// vmResult: 'vm', localVar: 'initial value'
// evalResult: 'eval', localVar: 'eval'

Because vm.runInThisContext() does not have access to the local scope, localVar is unchanged. In contrast, eval() does have access to the local scope, so the value localVar is changed. In this way vm.runInThisContext() is much like an indirect eval() call, e.g. (0,eval)('code').

Example: Running an HTTP Server within a VM#

When using either script.runInThisContext() or vm.runInThisContext(), the code is executed within the current V8 global context. The code passed to this VM context will have its own isolated scope.

In order to run a simple web server using the http module the code passed to the context must either call require('http') on its own, or have a reference to the http module passed to it. For instance:

'use strict';
const vm = require('vm');

let code =
`(function(require) {

   const http = require('http');

   http.createServer( (request, response) => {
     response.writeHead(200, {'Content-Type': 'text/plain'});
     response.end('Hello World\\n');
   }).listen(8124);

   console.log('Server running at http://127.0.0.1:8124/');
 })`;

 vm.runInThisContext(code)(require);

Note: The require() in the above case shares the state with the context it is passed from. This may introduce risks when untrusted code is executed, e.g. altering objects in the context in unwanted ways.

What does it mean to "contextify" an object?#

All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:

In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

When the method vm.createContext() is called, the sandbox object that is passed in (or a newly created object if sandbox is undefined) is associated internally with a new instance of a V8 Context. This V8 Context provides the code run using the vm modules methods with an isolated global environment within which it can operate. The process of creating the V8 Context and associating it with the sandbox object is what this document refers to as "contextifying" the sandbox.

Zlib#

Stability: 2 - Stable

The zlib module provides compression functionality implemented using Gzip and Deflate/Inflate. It can be accessed using:

const zlib = require('zlib');

Compressing or decompressing a stream (such as a file) can be accomplished by piping the source stream data through a zlib stream into a destination stream:

const gzip = zlib.createGzip();
const fs = require('fs');
const inp = fs.createReadStream('input.txt');
const out = fs.createWriteStream('input.txt.gz');

inp.pipe(gzip).pipe(out);

It is also possible to compress or decompress data in a single step:

const input = '.................................';
zlib.deflate(input, (err, buffer) => {
  if (!err) {
    console.log(buffer.toString('base64'));
  } else {
    // handle error
  }
});

const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
zlib.unzip(buffer, (err, buffer) => {
  if (!err) {
    console.log(buffer.toString());
  } else {
    // handle error
  }
});

Compressing HTTP requests and responses#

The zlib module can be used to implement support for the gzip and deflate content-encoding mechanisms defined by HTTP.

The HTTP Accept-Encoding header is used within an http request to identify the compression encodings accepted by the client. The Content-Encoding header is used to identify the compression encodings actually applied to a message.

Note: the examples given below are drastically simplified to show the basic concept. Using zlib encoding can be expensive, and the results ought to be cached. See Memory Usage Tuning for more information on the speed/memory/compression tradeoffs involved in zlib usage.

// client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const request = http.get({ host: 'example.com',
                         path: '/',
                         port: 80,
                         headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => {
  var output = fs.createWriteStream('example.com_index.html');

  switch (response.headers['content-encoding']) {
    // or, just use zlib.createUnzip() to handle both cases
    case 'gzip':
      response.pipe(zlib.createGunzip()).pipe(output);
      break;
    case 'deflate':
      response.pipe(zlib.createInflate()).pipe(output);
      break;
    default:
      response.pipe(output);
      break;
  }
});

// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
http.createServer((request, response) => {
  var raw = fs.createReadStream('index.html');
  var acceptEncoding = request.headers['accept-encoding'];
  if (!acceptEncoding) {
    acceptEncoding = '';
  }

  // Note: this is not a conformant accept-encoding parser.
  // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
  if (acceptEncoding.match(/\bdeflate\b/)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
    raw.pipe(zlib.createDeflate()).pipe(response);
  } else if (acceptEncoding.match(/\bgzip\b/)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
    raw.pipe(zlib.createGzip()).pipe(response);
  } else {
    response.writeHead(200, {});
    raw.pipe(response);
  }
}).listen(1337);

By default, the zlib methods will throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to compressed the last chunk of input data:

// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');

zlib.unzip(buffer,
           {finishFlush: zlib.constants.Z_SYNC_FLUSH},
           (err, buffer) => {
  if (!err) {
    console.log(buffer.toString());
  } else {
    // handle error
  }
});

This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.

Memory Usage Tuning#

From zlib/zconf.h, modified to node.js's usage:

The memory requirements for deflate are (in bytes):

(1 << (windowBits+2)) +  (1 << (memLevel+9))

That is: 128K for windowBits=15 + 128K for memLevel = 8 (default values) plus a few kilobytes for small objects.

For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:

{ windowBits: 14, memLevel: 7 }

This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes)

1 << windowBits

That is, 32K for windowBits=15 (default value) plus a few kilobytes for small objects.

This is in addition to a single internal output slab buffer of size chunkSize, which defaults to 16K.

The speed of zlib compression is affected most dramatically by the level setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster.

In general, greater memory usage options will mean that Node.js has to make fewer calls to zlib because it will be able to process more data on each write operation. So, this is another factor that affects the speed, at the cost of memory usage.

Flushing#

Calling .flush() on a compression stream will make zlib return as much output as currently possible. This may come at the cost of degraded compression quality, but can be useful when data needs to be available as soon as possible.

In the following example, flush() is used to write a compressed partial HTTP response to the client:

const zlib = require('zlib');
const http = require('http');

http.createServer((request, response) => {
  // For the sake of simplicity, the Accept-Encoding checks are omitted.
  response.writeHead(200, { 'content-encoding': 'gzip' });
  const output = zlib.createGzip();
  output.pipe(response);

  setInterval(() => {
    output.write(`The current time is ${Date()}\n`, () => {
      // The data has been passed to zlib, but the compression algorithm may
      // have decided to buffer the data for more efficient compression.
      // Calling .flush() will make the data available as soon as the client
      // is ready to receive it.
      output.flush();
    });
  }, 1000);
}).listen(1337);

Constants#

Added in: v0.5.8

All of the constants defined in zlib.h are also defined on require('zlib').constants. In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation. See http://zlib.net/manual.html#Constants for more details.

Note: Previously, the constants were available directly from require('zlib'), for instance zlib.Z_NO_FLUSH. Accessing the constants directly from the module is currently still possible but should be considered deprecated.

Allowed flush values.

  • zlib.constants.Z_NO_FLUSH
  • zlib.constants.Z_PARTIAL_FLUSH
  • zlib.constants.Z_SYNC_FLUSH
  • zlib.constants.Z_FULL_FLUSH
  • zlib.constants.Z_FINISH
  • zlib.constants.Z_BLOCK
  • zlib.constants.Z_TREES

Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.

  • zlib.constants.Z_OK
  • zlib.constants.Z_STREAM_END
  • zlib.constants.Z_NEED_DICT
  • zlib.constants.Z_ERRNO
  • zlib.constants.Z_STREAM_ERROR
  • zlib.constants.Z_DATA_ERROR
  • zlib.constants.Z_MEM_ERROR
  • zlib.constants.Z_BUF_ERROR
  • zlib.constants.Z_VERSION_ERROR

Compression levels.

  • zlib.constants.Z_NO_COMPRESSION
  • zlib.constants.Z_BEST_SPEED
  • zlib.constants.Z_BEST_COMPRESSION
  • zlib.constants.Z_DEFAULT_COMPRESSION

Compression strategy.

  • zlib.constants.Z_FILTERED
  • zlib.constants.Z_HUFFMAN_ONLY
  • zlib.constants.Z_RLE
  • zlib.constants.Z_FIXED
  • zlib.constants.Z_DEFAULT_STRATEGY

Class Options#

Added in: v0.11.1

Each class takes an options object. All options are optional.

Note that some options are only relevant when compressing, and are ignored by the decompression classes.

  • flush (default: zlib.constants.Z_NO_FLUSH)
  • finishFlush (default: zlib.constants.Z_FINISH)
  • chunkSize (default: 16*1024)
  • windowBits
  • level (compression only)
  • memLevel (compression only)
  • strategy (compression only)
  • dictionary (deflate/inflate only, empty dictionary by default)

See the description of deflateInit2 and inflateInit2 at http://zlib.net/manual.html#Advanced for more information on these.

Class: zlib.Deflate#

Added in: v0.5.8

Compress data using deflate.

Class: zlib.DeflateRaw#

Added in: v0.5.8

Compress data using deflate, and do not append a zlib header.

Class: zlib.Gunzip#

Added in: v0.5.8

Decompress a gzip stream.

Class: zlib.Gzip#

Added in: v0.5.8

Compress data using gzip.

Class: zlib.Inflate#

Added in: v0.5.8

Decompress a deflate stream.

Class: zlib.InflateRaw#

Added in: v0.5.8

Decompress a raw deflate stream.

Class: zlib.Unzip#

Added in: v0.5.8

Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.

Class: zlib.Zlib#

Added in: v0.5.8

Not exported by the zlib module. It is documented here because it is the base class of the compressor/decompressor classes.

zlib.flush([kind], callback)#

Added in: v0.5.8

kind defaults to zlib.constants.Z_FULL_FLUSH.

Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.

Calling this only flushes data from the internal zlib state, and does not perform flushing of any kind on the streams level. Rather, it behaves like a normal call to .write(), i.e. it will be queued up behind other pending writes and will only produce output when data is being read from the stream.

zlib.params(level, strategy, callback)#

Added in: v0.11.4

Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.

zlib.reset()#

Added in: v0.7.0

Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.

zlib.constants#

Added in: v7.0.0

Provides an object enumerating Zlib-related constants.

zlib.createDeflate(options)#

Added in: v0.5.8

Returns a new Deflate object with an options.

zlib.createDeflateRaw(options)#

Added in: v0.5.8

Returns a new DeflateRaw object with an options.

zlib.createGunzip(options)#

Added in: v0.5.8

Returns a new Gunzip object with an options.

zlib.createGzip(options)#

Added in: v0.5.8

Returns a new Gzip object with an options.

zlib.createInflate(options)#

Added in: v0.5.8

Returns a new Inflate object with an options.

zlib.createInflateRaw(options)#

Added in: v0.5.8

Returns a new InflateRaw object with an options.

zlib.createUnzip(options)#

Added in: v0.5.8

Returns a new Unzip object with an options.

Convenience Methods#

All of these take a Buffer or string as the first argument, an optional second argument to supply options to the zlib classes and will call the supplied callback with callback(error, result).

Every method has a *Sync counterpart, which accept the same arguments, but without a callback.

zlib.deflate(buf[, options], callback)#

Added in: v0.6.0

zlib.deflateSync(buf[, options])#

Added in: v0.11.12

Compress a Buffer or string with Deflate.

zlib.deflateRaw(buf[, options], callback)#

Added in: v0.6.0

zlib.deflateRawSync(buf[, options])#

Added in: v0.11.12

Compress a Buffer or string with DeflateRaw.

zlib.gunzip(buf[, options], callback)#

Added in: v0.6.0

zlib.gunzipSync(buf[, options])#

Added in: v0.11.12

Decompress a Buffer or string with Gunzip.

zlib.gzip(buf[, options], callback)#

Added in: v0.6.0

zlib.gzipSync(buf[, options])#

Added in: v0.11.12

Compress a Buffer or string with Gzip.

zlib.inflate(buf[, options], callback)#

Added in: v0.6.0

zlib.inflateSync(buf[, options])#

Added in: v0.11.12

Decompress a Buffer or string with Inflate.

zlib.inflateRaw(buf[, options], callback)#

Added in: v0.6.0

zlib.inflateRawSync(buf[, options])#

Added in: v0.11.12

Decompress a Buffer or string with InflateRaw.

zlib.unzip(buf[, options], callback)#

Added in: v0.6.0

zlib.unzipSync(buf[, options])#

Added in: v0.11.12

Decompress a Buffer or string with Unzip.