Table of Contents

• REPL

  • Design and Features

    • Commands and Special Keys

    • Default Evaluation

      • JavaScript Expressions
      • Global and Local Scope
      • Accessing Core Node.js Modules
      • Global Uncaught Exceptions
      • Assignment of the _ (underscore) variable
      • await keyword

    • Custom Evaluation Functions

      • Recoverable Errors
    • Customizing REPL Output

  • Class: REPLServer

    • Event: 'exit'
    • Event: 'reset'
    • replServer.defineCommand(keyword, cmd)
    • replServer.displayPrompt([preserveCursor])
    • replServer.clearBufferedCommand()
    • replServer.parseREPLKeyword(keyword[, rest])
  • repl.start([options])

  • The Node.js REPL

    • Environment Variable Options
    • Persistent History
    • Using the Node.js REPL with advanced line-editors
    • Starting multiple REPL instances against a single running instance

REPL#

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:

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'

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');

Global Uncaught Exceptions#

The REPL uses the domain module to catch all uncaught exceptions for that REPL session.

This use of the domain module in the REPL has these side effects:

• Uncaught exceptions do not emit the 'uncaughtException' event.
• Trying to use process.setUncaughtExceptionCaptureCallback() throws an ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE error.

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

Similarly, _error will refer to the last seen error, if there was any. Explicitly setting _error to a value will disable this behavior.

> throw new Error('foo');
Error: foo
> _error.message
'foo'

`await` keyword#

With the --experimental-repl-await command line option specified, experimental support for the await keyword is enabled.

> await Promise.resolve(123)
123
> await Promise.reject(new Error('REPL await'))
Error: REPL await
    at repl:1:45
> const timeout = util.promisify(setTimeout);
undefined
> const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
1002
undefined

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');

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#

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'#

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'#

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:

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)#

• 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.clearBufferedCommand();
    console.log(`Hello, ${name}!`);
    this.displayPrompt();
  }
});
replServer.defineCommand('saybye', function 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])#

• preserveCursor <boolean>

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.

replServer.clearBufferedCommand()#

The replServer.clearBufferedCommand() method clears any command that has been buffered but not yet executed. This method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

replServer.parseREPLKeyword(keyword[, rest])#

• keyword <string> the potential keyword to parse and execute
• rest <any> any parameters to the keyword command
• Returns: <boolean>

An internal method used to parse and execute REPLServer keywords. Returns true if keyword is a valid keyword, otherwise false.

repl.start([options])[src]#

• options <Object> | <string>

  • prompt <string> The input prompt to display. Default: '> ' (with a trailing space).
  • input <stream.Readable> The Readable stream from which REPL input will be read. Default: process.stdin.
  • output <stream.Writable> The Writable stream to which REPL output will be written. Default: process.stdout.
  • terminal <boolean> If true, specifies that the output should be treated as a TTY terminal, and have ANSI/VT100 escape codes written to it. Default: 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. Default: 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. Default: 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. The node CLI REPL sets this value to true. Default: false.
  • ignoreUndefined <boolean> If true, specifies that the default writer will not output the return value of a command if it evaluates to undefined. Default: false.
  • writer <Function> The function to invoke to format the output of each command before writing to output. Default: util.inspect().
  • completer <Function> An optional function used for custom Tab auto completion. See readline.InterfaceCompleter for an example.

  • replMode <symbol> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) 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'.
  • 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. Default: 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];
undefined
> a
[ 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 - Controls how many lines of history will be persisted if history is available. Must be a positive number. Default: 1000.
• NODE_REPL_MODE - May be either 'sloppy' or 'strict'. Default: 'sloppy', which will allow non-strict mode code to be run.

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=''.

Using the Node.js REPL with advanced line-editors#

For advanced line-editors, start Node.js with the environment variable NODE_NO_READLINE=1. This will start the main and debugger REPL in canonical terminal settings, which will allow use with rlwrap.

For example, the following can be added to a .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/TooTallNate/2209310.

For an example of running a REPL instance over curl(1), see: https://gist.github.com/TooTallNate/2053342.