- Assertion Testing
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- Process
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- Utilities
- V8
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- Worker Threads
- ZLIB
Node.js v11.0.0-test2018080119aa41c6fc Documentation
Table of Contents
-
- File Descriptors
- Threadpool Usage
-
- stats.isBlockDevice()
- stats.isCharacterDevice()
- stats.isDirectory()
- stats.isFIFO()
- stats.isFile()
- stats.isSocket()
- stats.isSymbolicLink()
- stats.dev
- stats.ino
- stats.mode
- stats.nlink
- stats.uid
- stats.gid
- stats.rdev
- stats.size
- stats.blksize
- stats.blocks
- stats.atimeMs
- stats.mtimeMs
- stats.ctimeMs
- stats.birthtimeMs
- stats.atime
- stats.mtime
- stats.ctime
- stats.birthtime
- Stat Time Values
- fs.access(path[, mode], callback)
- fs.accessSync(path[, mode])
- fs.appendFile(path, data[, options], callback)
- fs.appendFileSync(path, data[, options])
- fs.chmodSync(path, mode)
- fs.chown(path, uid, gid, callback)
- fs.chownSync(path, uid, gid)
- fs.close(fd, callback)
- fs.closeSync(fd)
- fs.constants
- fs.copyFile(src, dest[, flags], callback)
- fs.copyFileSync(src, dest[, flags])
- fs.createReadStream(path[, options])
- fs.createWriteStream(path[, options])
- fs.exists(path, callback)
- fs.existsSync(path)
- fs.fchmod(fd, mode, callback)
- fs.fchmodSync(fd, mode)
- fs.fchown(fd, uid, gid, callback)
- fs.fchownSync(fd, uid, gid)
- fs.fdatasync(fd, callback)
- fs.fdatasyncSync(fd)
- fs.fstat(fd[, options], callback)
- fs.fstatSync(fd[, options])
- fs.fsync(fd, callback)
- fs.fsyncSync(fd)
- fs.ftruncate(fd[, len], callback)
- fs.ftruncateSync(fd[, len])
- fs.futimes(fd, atime, mtime, callback)
- fs.futimesSync(fd, atime, mtime)
- fs.lchmod(path, mode, callback)
- fs.lchmodSync(path, mode)
- fs.lchown(path, uid, gid, callback)
- fs.lchownSync(path, uid, gid)
- fs.link(existingPath, newPath, callback)
- fs.linkSync(existingPath, newPath)
- fs.lstat(path[, options], callback)
- fs.lstatSync(path[, options])
- fs.mkdir(path[, mode], callback)
- fs.mkdirSync(path[, mode])
- fs.mkdtemp(prefix[, options], callback)
- fs.mkdtempSync(prefix[, options])
- fs.open(path, flags[, mode], callback)
- fs.openSync(path, flags[, mode])
- fs.read(fd, buffer, offset, length, position, callback)
- fs.readdir(path[, options], callback)
- fs.readdirSync(path[, options])
- fs.readFile(path[, options], callback)
- fs.readFileSync(path[, options])
- fs.readlink(path[, options], callback)
- fs.readlinkSync(path[, options])
- fs.readSync(fd, buffer, offset, length, position)
- fs.realpath(path[, options], callback)
- fs.realpath.native(path[, options], callback)
- fs.realpathSync(path[, options])
- fs.realpathSync.native(path[, options])
- fs.rename(oldPath, newPath, callback)
- fs.renameSync(oldPath, newPath)
- fs.rmdir(path, callback)
- fs.rmdirSync(path)
- fs.stat(path[, options], callback)
- fs.statSync(path[, options])
- fs.symlink(target, path[, type], callback)
- fs.symlinkSync(target, path[, type])
- fs.truncate(path[, len], callback)
- fs.truncateSync(path[, len])
- fs.unlink(path, callback)
- fs.unlinkSync(path)
- fs.unwatchFile(filename[, listener])
- fs.utimes(path, atime, mtime, callback)
- fs.utimesSync(path, atime, mtime)
- fs.watchFile(filename[, options], listener)
- fs.write(fd, buffer[, offset[, length[, position]]], callback)
- fs.write(fd, string[, position[, encoding]], callback)
- fs.writeFile(file, data[, options], callback)
- fs.writeFileSync(file, data[, options])
- fs.writeSync(fd, buffer[, offset[, length[, position]]])
- fs.writeSync(fd, string[, position[, encoding]])
-
-
- filehandle.appendFile(data, options)
- filehandle.chmod(mode)
- filehandle.chown(uid, gid)
- filehandle.close()
- filehandle.datasync()
- filehandle.fd
- filehandle.read(buffer, offset, length, position)
- filehandle.readFile(options)
- filehandle.stat([options])
- filehandle.sync()
- filehandle.truncate(len)
- filehandle.utimes(atime, mtime)
- filehandle.write(buffer, offset, length, position)
- filehandle.writeFile(data, options)
- fsPromises.access(path[, mode])
- fsPromises.appendFile(path, data[, options])
- fsPromises.chmod(path, mode)
- fsPromises.chown(path, uid, gid)
- fsPromises.copyFile(src, dest[, flags])
- fsPromises.lchmod(path, mode)
- fsPromises.lchown(path, uid, gid)
- fsPromises.link(existingPath, newPath)
- fsPromises.lstat(path[, options])
- fsPromises.mkdir(path[, mode])
- fsPromises.mkdtemp(prefix[, options])
- fsPromises.open(path, flags[, mode])
- fsPromises.readdir(path[, options])
- fsPromises.readFile(path[, options])
- fsPromises.readlink(path[, options])
- fsPromises.realpath(path[, options])
- fsPromises.rename(oldPath, newPath)
- fsPromises.rmdir(path)
- fsPromises.stat(path[, options])
- fsPromises.symlink(target, path[, type])
- fsPromises.truncate(path[, len])
- fsPromises.unlink(path)
- fsPromises.utimes(path, atime, mtime)
- fsPromises.writeFile(file, data[, options])
-
- File System Flags
File System#
The fs
module provides an API for interacting with the file system in a
manner closely modeled around standard POSIX functions.
To use this module:
const fs = require('fs');
All file system operations have synchronous and asynchronous 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
.
const fs = require('fs');
fs.unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
Exceptions that occur using synchronous operations are thrown immediately and
may be handled using try
/catch
, or may be allowed to bubble up.
const fs = require('fs');
try {
fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
There is no guaranteed ordering when using asynchronous methods. So the
following is prone to error because the fs.stat()
operation may complete
before the fs.rename()
operation:
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)}`);
});
To correctly order the operations, move the fs.stat()
call into the callback
of the fs.rename()
operation:
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.
While it is not recommended, most fs functions allow the callback argument to
be omitted, in which case a default callback is used that rethrows errors. To
get a trace to the original call site, set the NODE_DEBUG
environment
variable:
Omitting the callback function on asynchronous fs functions is deprecated and may result in an error being thrown in the future.
$ 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.>
File paths#
Most fs
operations accept filepaths that may be specified in the form of
a string, a Buffer
, or a URL
object using the file:
protocol.
String form paths are interpreted as UTF-8 character sequences identifying
the absolute or relative filename. Relative paths will be resolved relative
to the current working directory as specified by process.cwd()
.
Example using an absolute path on POSIX:
const fs = require('fs');
fs.open('/open/some/file.txt', 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
Example using a relative path on POSIX (relative to process.cwd()
):
fs.open('file.txt', 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
Paths specified using a Buffer
are useful primarily on certain POSIX
operating systems that treat file paths as opaque byte sequences. On such
systems, it is possible for a single file path to contain sub-sequences that
use multiple character encodings. As with string paths, Buffer
paths may
be relative or absolute:
Example using an absolute path on POSIX:
fs.open(Buffer.from('/open/some/file.txt'), 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
On Windows, Node.js follows the concept of per-drive working directory. This
behavior can be observed when using a drive path without a backslash. For
example fs.readdirSync('c:\\')
can potentially return a different result than
fs.readdirSync('c:')
. For more information, see
this MSDN page.
URL object support#
For most fs
module functions, the path
or filename
argument may be passed
as a WHATWG URL
object. Only URL
objects using the file:
protocol
are supported.
const fs = require('fs');
const fileUrl = new URL('file:///tmp/hello');
fs.readFileSync(fileUrl);
file:
URLs are always absolute paths.
Using WHATWG URL
objects might introduce platform-specific behaviors.
On Windows, file:
URLs with a hostname convert to UNC paths, while file:
URLs with drive letters convert to local absolute paths. file:
URLs without a
hostname nor a drive letter will result in a throw:
// On Windows :
// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
// - WHATWG file URLs with drive letters convert to absolute path
// file:///C:/tmp/hello => C:\tmp\hello
fs.readFileSync(new URL('file:///C:/tmp/hello'));
// - WHATWG file URLs without hostname must have a drive letters
fs.readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
fs.readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute
file:
URLs with drive letters must use :
as a separator just after
the drive letter. Using another separator will result in a throw.
On all other platforms, file:
URLs with a hostname are unsupported and will
result in a throw:
// On other platforms:
// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute
// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
fs.readFileSync(new URL('file:///tmp/hello'));
A file:
URL having encoded slash characters will result in a throw on all
platforms:
// On Windows
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
// On POSIX
fs.readFileSync(new URL('file:///p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */
On Windows, file:
URLs having encoded backslash will result in a throw:
// On Windows
fs.readFileSync(new URL('file:///C:/path/%5C'));
fs.readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
File Descriptors#
On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the specific differences between operating systems and assigns all open files a numeric file descriptor.
The fs.open()
method is used to allocate a new file descriptor. Once
allocated, the file descriptor may be used to read data from, write data to,
or request information about the file.
fs.open('/open/some/file.txt', 'r', (err, fd) => {
if (err) throw err;
fs.fstat(fd, (err, stat) => {
if (err) throw err;
// use stat
// always close the file descriptor!
fs.close(fd, (err) => {
if (err) throw err;
});
});
});
Most operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.
Threadpool Usage#
All file system APIs except fs.FSWatcher()
and those that are explicitly
synchronous use libuv's threadpool, which can have surprising and negative
performance implications for some applications. See the
UV_THREADPOOL_SIZE
documentation for more information.
Class: fs.FSWatcher#
A successful call to fs.watch()
method will return a new fs.FSWatcher
object.
All fs.FSWatcher
objects are EventEmitter
's that will emit a 'change'
event whenever a specific watched file is modified.
Event: 'change'#
eventType
<string> The type of change event that has occurredfilename
<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 UTF-8 string.
// Example when handled through fs.watch() listener
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
// Prints: <Buffer ...>
}
});
Event: 'close'#
Emitted when the watcher stops watching for changes. The closed
fs.FSWatcher
object is no longer usable in the event handler.
Event: 'error'#
error
<Error>
Emitted when an error occurs while watching the file. The errored
fs.FSWatcher
object is no longer usable in the event handler.
watcher.close()#
Stop watching for changes on the given fs.FSWatcher
. Once stopped, the
fs.FSWatcher
object is no longer usable.
Class: fs.ReadStream#
A successful call to fs.createReadStream()
will return a new fs.ReadStream
object.
All fs.ReadStream
objects are Readable Streams.
Event: 'close'#
Emitted when the fs.ReadStream
's underlying file descriptor has been closed.
Event: 'open'#
fd
<integer> Integer file descriptor used by theReadStream
.
Emitted when the fs.ReadStream
's file descriptor has been opened.
Event: 'ready'#
Emitted when the fs.ReadStream
is ready to be used.
Fires immediately after 'open'
.
readStream.bytesRead#
The number of bytes that have been read so far.
readStream.path#
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#
A fs.Stats
object provides information about a file.
Objects returned from fs.stat()
, fs.lstat()
and fs.fstat()
and
their synchronous counterparts are of this type.
If bigint
in the options
passed to those methods is true, the numeric values
will be bigint
instead of number
.
Stats {
dev: 2114,
ino: 48064969,
mode: 33188,
nlink: 1,
uid: 85,
gid: 100,
rdev: 0,
size: 527,
blksize: 4096,
blocks: 8,
atimeMs: 1318289051000.1,
mtimeMs: 1318289051000.1,
ctimeMs: 1318289051000.1,
birthtimeMs: 1318289051000.1,
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 }
bigint
version:
Stats {
dev: 2114n,
ino: 48064969n,
mode: 33188n,
nlink: 1n,
uid: 85n,
gid: 100n,
rdev: 0n,
size: 527n,
blksize: 4096n,
blocks: 8n,
atimeMs: 1318289051000n,
mtimeMs: 1318289051000n,
ctimeMs: 1318289051000n,
birthtimeMs: 1318289051000n,
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 }
stats.isBlockDevice()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a block device.
stats.isCharacterDevice()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a character device.
stats.isDirectory()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a file system directory.
stats.isFIFO()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a first-in-first-out (FIFO)
pipe.
stats.isFile()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a regular file.
stats.isSocket()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a socket.
stats.isSymbolicLink()#
- Returns: <boolean>
Returns true
if the fs.Stats
object describes a symbolic link.
This method is only valid when using fs.lstat()
.
stats.dev#
The numeric identifier of the device containing the file.
stats.ino#
The file system specific "Inode" number for the file.
stats.mode#
A bit-field describing the file type and mode.
stats.nlink#
The number of hard-links that exist for the file.
stats.uid#
The numeric user identifier of the user that owns the file (POSIX).
stats.gid#
The numeric group identifier of the group that owns the file (POSIX).
stats.rdev#
A numeric device identifier if the file is considered "special".
stats.size#
The size of the file in bytes.
stats.blksize#
The file system block size for i/o operations.
stats.blocks#
The number of blocks allocated for this file.
stats.atimeMs#
The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.
stats.mtimeMs#
The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.
stats.ctimeMs#
The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.
stats.birthtimeMs#
The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.
stats.atime#
The timestamp indicating the last time this file was accessed.
stats.mtime#
The timestamp indicating the last time this file was modified.
stats.ctime#
The timestamp indicating the last time the file status was changed.
stats.birthtime#
The timestamp indicating the creation time of this file.
Stat Time Values#
The atimeMs
, mtimeMs
, ctimeMs
, birthtimeMs
properties are
numbers that hold the corresponding times in milliseconds. Their
precision is platform specific. atime
, mtime
, ctime
, and birthtime
are
Date
object alternate representations of the various times. The
Date
and number values are not connected. Assigning a new number value, or
mutating the Date
value, will not be reflected in the corresponding alternate
representation.
The times in the stat object have the following semantics:
atime
"Access Time" - Time when file data last accessed. Changed by themknod(2)
,utimes(2)
, andread(2)
system calls.mtime
"Modified Time" - Time when file data last modified. Changed by themknod(2)
,utimes(2)
, andwrite(2)
system calls.ctime
"Change Time" - Time when file status was last changed (inode data modification). Changed by thechmod(2)
,chown(2)
,link(2)
,mknod(2)
,rename(2)
,unlink(2)
,utimes(2)
,read(2)
, andwrite(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 thectime
or1970-01-01T00:00Z
(ie, unix epoch timestamp0
). This value may be greater thanatime
ormtime
in this case. On Darwin and other FreeBSD variants, also set if theatime
is explicitly set to an earlier value than the currentbirthtime
using theutimes(2)
system call.
Prior to Node.js 0.12, the ctime
held the birthtime
on Windows systems. As
of 0.12, ctime
is not "creation time", and on Unix systems, it never was.
Class: fs.WriteStream#
WriteStream
is a Writable Stream.
Event: 'close'#
Emitted when the WriteStream
's underlying file descriptor has been closed.
Event: 'open'#
fd
<integer> Integer file descriptor used by theWriteStream
.
Emitted when the WriteStream
's file is opened.
Event: 'ready'#
Emitted when the fs.WriteStream
is ready to be used.
Fires immediately after 'open'
.
writeStream.bytesWritten#
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.path#
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)#
path
<string> | <Buffer> | <URL>mode
<integer> Default:fs.constants.F_OK
-
callback
<Function>err
<Error>
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. Check File Access Constants for possible values
of mode
. It is possible to create a mask consisting of the bitwise OR of
two or more values (e.g. fs.constants.W_OK | fs.constants.R_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 an Error
object. The following examples check if
package.json
exists, and if it is readable or writable.
const file = 'package.json';
// Check if the file exists in the current directory.
fs.access(file, fs.constants.F_OK, (err) => {
console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});
// Check if the file is readable.
fs.access(file, fs.constants.R_OK, (err) => {
console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});
// Check if the file is writable.
fs.access(file, fs.constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});
// Check if the file exists in the current directory, and if it is writable.
fs.access(file, fs.constants.F_OK | fs.constants.W_OK, (err) => {
if (err) {
console.error(
`${file} ${err.code === 'ENOENT' ? 'does not exist' : 'is read-only'}`);
} else {
console.log(`${file} exists, and it is writable`);
}
});
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.
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;
}
throw err;
}
writeMyData(fd);
});
read (NOT RECOMMENDED)
fs.access('myfile', (err) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
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;
}
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 will not be used directly, for example when its accessibility is a signal from another process.
fs.accessSync(path[, mode])#
Synchronously 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. Check File Access Constants for
possible values of mode
. It is possible to create a mask consisting of
the bitwise OR of two or more values
(e.g. fs.constants.W_OK | fs.constants.R_OK
).
If any of the accessibility checks fail, an Error
will be thrown. Otherwise,
the method will return undefined
.
try {
fs.accessSync('etc/passwd', fs.constants.R_OK | fs.constants.W_OK);
console.log('can read/write');
} catch (err) {
console.error('no access!');
}
fs.appendFile(path, data[, options], callback)#
path
<string> | <Buffer> | <URL> | <number> filename or file descriptordata
<string> | <Buffer>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
-
callback
<Function>err
<Error>
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);
The path
may be specified as a numeric file descriptor that has been opened
for appending (using fs.open()
or fs.openSync()
). The file descriptor will
not be closed automatically.
fs.open('message.txt', 'a', (err, fd) => {
if (err) throw err;
fs.appendFile(fd, 'data to append', 'utf8', (err) => {
fs.close(fd, (err) => {
if (err) throw err;
});
if (err) throw err;
});
});
fs.appendFileSync(path, data[, options])#
path
<string> | <Buffer> | <URL> | <number> filename or file descriptordata
<string> | <Buffer>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
Synchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a Buffer
.
Example:
try {
fs.appendFileSync('message.txt', 'data to append');
console.log('The "data to append" was appended to file!');
} catch (err) {
/* Handle the error */
}
If options
is a string, then it specifies the encoding. Example:
fs.appendFileSync('message.txt', 'data to append', 'utf8');
The path
may be specified as a numeric file descriptor that has been opened
for appending (using fs.open()
or fs.openSync()
). The file descriptor will
not be closed automatically.
let fd;
try {
fd = fs.openSync('message.txt', 'a');
fs.appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
/* Handle the error */
} finally {
if (fd !== undefined)
fs.closeSync(fd);
}
fs.chmod(path, mode, callback)#
Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.
See also: chmod(2)
.
File modes#
The mode
argument used in both the fs.chmod()
and fs.chmodSync()
methods is a numeric bitmask created using a logical OR of the following
constants:
Constant | Octal | Description |
---|---|---|
fs.constants.S_IRUSR | 0o400 | read by owner |
fs.constants.S_IWUSR | 0o200 | write by owner |
fs.constants.S_IXUSR | 0o100 | execute/search by owner |
fs.constants.S_IRGRP | 0o40 | read by group |
fs.constants.S_IWGRP | 0o20 | write by group |
fs.constants.S_IXGRP | 0o10 | execute/search by group |
fs.constants.S_IROTH | 0o4 | read by others |
fs.constants.S_IWOTH | 0o2 | write by others |
fs.constants.S_IXOTH | 0o1 | execute/search by others |
An easier method of constructing the mode
is to use a sequence of three
octal digits (e.g. 765
). The left-most digit (7
in the example), specifies
the permissions for the file owner. The middle digit (6
in the example),
specifies permissions for the group. The right-most digit (5
in the example),
specifies the permissions for others.
Number | Description |
---|---|
7 | read, write, and execute |
6 | read and write |
5 | read and execute |
4 | read only |
3 | write and execute |
2 | write only |
1 | execute only |
0 | no permission |
For example, the octal value 0o765
means:
- The owner may read, write and execute the file.
- The group may read and write the file.
- Others may read and execute the file.
When using raw numbers where file modes are expected, any value larger than
0o777
may result in platform-specific behaviors that are not supported to work
consistently. Therefore constants like S_ISVTX
, S_ISGID
or S_ISUID
are not
exposed in fs.constants
.
Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner or others is not implemented.
fs.chmodSync(path, mode)#
For detailed information, see the documentation of the asynchronous version of
this API: fs.chmod()
.
See also: chmod(2)
.
fs.chown(path, uid, gid, callback)#
Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.
See also: chown(2)
.
fs.chownSync(path, uid, gid)#
Synchronously changes owner and group of a file. Returns undefined
.
This is the synchronous version of fs.chown()
.
See also: chown(2)
.
fs.close(fd, callback)#
fd
<integer>-
callback
<Function>err
<Error>
Asynchronous close(2)
. No arguments other than a possible exception are given
to the completion callback.
fs.closeSync(fd)#
fd
<integer>
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.copyFile(src, dest[, flags], callback)#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationflags
<number> modifiers for copy operation. Default:0
.callback
<Function>
Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. No arguments other than a possible exception are given to the
callback function. Node.js makes no guarantees about the atomicity of the copy
operation. If an error occurs after the destination file has been opened for
writing, Node.js will attempt to remove the destination.
flags
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
- The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
- The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
- The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
Example:
const fs = require('fs');
// destination.txt will be created or overwritten by default.
fs.copyFile('source.txt', 'destination.txt', (err) => {
if (err) throw err;
console.log('source.txt was copied to destination.txt');
});
If the third argument is a number, then it specifies flags
, as shown in the
following example.
const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL, callback);
fs.copyFileSync(src, dest[, flags])#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationflags
<number> modifiers for copy operation. Default:0
.
Synchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. Returns undefined
. Node.js makes no guarantees about the
atomicity of the copy operation. If an error occurs after the destination file
has been opened for writing, Node.js will attempt to remove the destination.
flags
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
- The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
- The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
- The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
Example:
const fs = require('fs');
// destination.txt will be created or overwritten by default.
fs.copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
If the third argument is a number, then it specifies flags
, as shown in the
following example.
const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFileSync('source.txt', 'destination.txt', COPYFILE_EXCL);
fs.createReadStream(path[, options])#
path
<string> | <Buffer> | <URL>- Returns: <fs.ReadStream> See Readable Streams.
Unlike the 16 kb default highWaterMark
for a readable stream, the stream
returned by this method has a default highWaterMark
of 64 kb.
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. fd
should be blocking; non-blocking fd
s 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 the application's 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])#
path
<string> | <Buffer> | <URL>- Returns: <fs.WriteStream> See Writable Stream.
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 encoding
can be any one of those accepted by
Buffer
.
If autoClose
is set to true (default behavior) on 'error'
or 'finish'
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 the application's 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. fd
should be blocking; non-blocking fd
s
should be passed to net.Socket
.
If options
is a string, then it specifies the encoding.
fs.exists(path, callback)#
path
<string> | <Buffer> | <URL>-
callback
<Function>exists
<boolean>
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!');
});
The parameters for this callback are 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.
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;
}
throw err;
}
writeMyData(fd);
});
read (NOT RECOMMENDED)
fs.exists('myfile', (exists) => {
if (exists) {
fs.open('myfile', 'r', (err, fd) => {
if (err) throw err;
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;
}
throw err;
}
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)#
Returns true
if the path exists, false
otherwise.
For detailed information, see the documentation of the asynchronous version of
this API: fs.exists()
.
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)#
fd
<integer>mode
<integer>-
callback
<Function>err
<Error>
Asynchronous fchmod(2)
. No arguments other than a possible exception
are given to the completion callback.
fs.fchmodSync(fd, mode)#
Synchronous fchmod(2)
. Returns undefined
.
fs.fchown(fd, uid, gid, callback)#
fd
<integer>uid
<integer>gid
<integer>-
callback
<Function>err
<Error>
Asynchronous fchown(2)
. No arguments other than a possible exception are given
to the completion callback.
fs.fchownSync(fd, uid, gid)#
Synchronous fchown(2)
. Returns undefined
.
fs.fdatasync(fd, callback)#
fd
<integer>-
callback
<Function>err
<Error>
Asynchronous fdatasync(2)
. No arguments other than a possible exception are
given to the completion callback.
fs.fdatasyncSync(fd)#
fd
<integer>
Synchronous fdatasync(2)
. Returns undefined
.
fs.fstat(fd[, options], callback)#
fd
<integer>-
options
<Object> -
callback
<Function>err
<Error>stats
<fs.Stats>
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[, options])#
fd
<integer>-
options
<Object> - Returns: <fs.Stats>
Synchronous fstat(2)
.
fs.fsync(fd, callback)#
fd
<integer>-
callback
<Function>err
<Error>
Asynchronous fsync(2)
. No arguments other than a possible exception are given
to the completion callback.
fs.fsyncSync(fd)#
fd
<integer>
Synchronous fsync(2)
. Returns undefined
.
fs.ftruncate(fd[, len], callback)#
fd
<integer>len
<integer> Default:0
-
callback
<Function>err
<Error>
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', '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 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])#
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.ftruncate()
.
fs.futimes(fd, atime, mtime, callback)#
fd
<integer>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>-
callback
<Function>err
<Error>
Change the file system timestamps of the object referenced by the supplied file
descriptor. See fs.utimes()
.
This function does not work on AIX versions before 7.1, it will return the
error UV_ENOSYS
.
fs.futimesSync(fd, atime, mtime)#
Synchronous version of fs.futimes()
. Returns undefined
.
fs.lchmod(path, mode, callback)#
Asynchronous lchmod(2)
. No arguments other than a possible exception
are given to the completion callback.
Only available on macOS.
fs.lchmodSync(path, mode)#
Synchronous lchmod(2)
. Returns undefined
.
fs.lchown(path, uid, gid, callback)#
Asynchronous lchown(2)
. No arguments other than a possible exception are given
to the completion callback.
fs.lchownSync(path, uid, gid)#
Synchronous lchown(2)
. Returns undefined
.
fs.link(existingPath, newPath, callback)#
existingPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>-
callback
<Function>err
<Error>
Asynchronous link(2)
. No arguments other than a possible exception are given to
the completion callback.
fs.linkSync(existingPath, newPath)#
Synchronous link(2)
. Returns undefined
.
fs.lstat(path[, options], callback)#
path
<string> | <Buffer> | <URL>-
options
<Object> -
callback
<Function>err
<Error>stats
<fs.Stats>
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[, options])#
path
<string> | <Buffer> | <URL>-
options
<Object> - Returns: <fs.Stats>
Synchronous lstat(2)
.
fs.mkdir(path[, mode], callback)#
path
<string> | <Buffer> | <URL>mode
<integer> Not supported on Windows. Default:0o777
.-
callback
<Function>err
<Error>
Asynchronously creates a directory. No arguments other than a possible exception are given to the completion callback.
See also: mkdir(2)
.
fs.mkdirSync(path[, mode])#
Synchronously creates a directory. Returns undefined
.
This is the synchronous version of fs.mkdir()
.
See also: mkdir(2)
.
fs.mkdtemp(prefix[, options], callback)#
prefix
<string>-
encoding
<string> Default:'utf8'
-
callback
<Function>
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.
fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => {
if (err) throw err;
console.log(folder);
// Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});
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 = os.tmpdir();
// This method is *INCORRECT*:
fs.mkdtemp(tmpDir, (err, folder) => {
if (err) throw err;
console.log(folder);
// Will print something similar to `/tmpabc123`.
// A new temporary directory is created at the file system root
// rather than *within* the /tmp directory.
});
// This method is *CORRECT*:
const { sep } = require('path');
fs.mkdtemp(`${tmpDir}${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])#
Returns the created folder path.
For detailed information, see the documentation of the asynchronous version of
this API: fs.mkdtemp()
.
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)#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
.mode
<integer> Default:0o666
(readable and writable)-
callback
<Function>
Asynchronous file open. See open(2)
.
mode
sets the file mode (permission and sticky bits), but only if the file was
created. On Windows, only the write permission can be manipulated; see
fs.chmod()
.
The callback gets two arguments (err, fd)
.
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented
by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains
a colon, Node.js will open a file system stream, as described by
this MSDN page.
Functions based on fs.open()
exhibit this behavior as well:
fs.writeFile()
, fs.readFile()
, etc.
fs.openSync(path, flags[, mode])#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
.mode
<integer> Default:0o666
- Returns: <number>
Returns an integer representing the file descriptor.
For detailed information, see the documentation of the asynchronous version of
this API: fs.open()
.
fs.read(fd, buffer, offset, length, position, callback)#
fd
<integer>buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>-
callback
<Function>
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 argument specifying where to begin reading from in the file.
If position
is null
, data will be read from the current file position,
and the file position will be updated.
If position
is an integer, the file position will remain unchanged.
The callback is given the three arguments, (err, bytesRead, buffer)
.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise
for an Object
with bytesRead
and buffer
properties.
fs.readdir(path[, options], callback)#
path
<string> | <Buffer> | <URL>-
encoding
<string> Default:'utf8'
-
callback
<Function>err
<Error>files
<string[]> | <Buffer[]>
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])#
path
<string> | <Buffer> | <URL>-
encoding
<string> Default:'utf8'
- Returns: <string[]> An array of filenames excluding
'.'
and'..'
.
Synchronous readdir(3)
.
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 returned. If the encoding
is set to 'buffer'
,
the filenames returned will be passed as Buffer
objects.
fs.readFile(path[, options], callback)#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptor-
encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.
-
callback
<Function>
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);
When the path is a directory, the behavior of fs.readFile()
and
fs.readFileSync()
is platform-specific. On macOS, Linux, and Windows, an
error will be returned. On FreeBSD, a representation of the directory's contents
will be returned.
// macOS, Linux, and Windows
fs.readFile('<directory>', (err, data) => {
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
});
// FreeBSD
fs.readFile('<directory>', (err, data) => {
// => null, <data>
});
Any specified file descriptor has to support reading.
If a file descriptor is specified as the path
, it will not be closed
automatically.
The fs.readFile()
function buffers the entire file. To minimize memory costs,
when possible prefer streaming via fs.createReadStream()
.
fs.readFileSync(path[, options])#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptor-
encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.
- Returns: <string> | <Buffer>
Returns the contents of the path
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.readFile()
.
If the encoding
option is specified then this function returns a
string. Otherwise it returns a buffer.
Similar to fs.readFile()
, when the path is a directory, the behavior of
fs.readFileSync()
is platform-specific.
// macOS, Linux, and Windows
fs.readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
// FreeBSD
fs.readFileSync('<directory>'); // => <data>
fs.readlink(path[, options], callback)#
path
<string> | <Buffer> | <URL>-
encoding
<string> Default:'utf8'
-
callback
<Function>
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])#
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 returned. 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)#
fd
<integer>buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>- Returns: <number>
Returns the number of bytesRead
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.read()
.
fs.realpath(path[, options], callback)#
path
<string> | <Buffer> | <URL>-
encoding
<string> Default:'utf8'
-
callback
<Function>
Asynchronously computes the canonical pathname by resolving .
, ..
and
symbolic links.
A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.
This function behaves like realpath(3)
, with some exceptions:
-
No case conversion is performed on case-insensitive file systems.
-
The maximum number of symbolic links is platform-independent and generally (much) higher than what the native
realpath(3)
implementation supports.
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.
If path
resolves to a socket or a pipe, the function will return a system
dependent name for that object.
fs.realpath.native(path[, options], callback)#
path
<string> | <Buffer> | <URL>-
encoding
<string> Default:'utf8'
-
callback
<Function>
Asynchronous realpath(3)
.
The callback
gets two arguments (err, resolvedPath)
.
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.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fs.realpathSync(path[, options])#
Returns the resolved pathname.
For detailed information, see the documentation of the asynchronous version of
this API: fs.realpath()
.
fs.realpathSync.native(path[, options])#
Synchronous realpath(3)
.
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 returned. If the encoding
is set to 'buffer'
,
the path returned will be passed as a Buffer
object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fs.rename(oldPath, newPath, callback)#
oldPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>-
callback
<Function>err
<Error>
Asynchronously rename file at oldPath
to the pathname provided
as newPath
. In the case that newPath
already exists, it will
be overwritten. No arguments other than a possible exception are
given to the completion callback.
See also: rename(2)
.
fs.rename('oldFile.txt', 'newFile.txt', (err) => {
if (err) throw err;
console.log('Rename complete!');
});
fs.renameSync(oldPath, newPath)#
Synchronous rename(2)
. Returns undefined
.
fs.rmdir(path, callback)#
path
<string> | <Buffer> | <URL>-
callback
<Function>err
<Error>
Asynchronous rmdir(2)
. No arguments other than a possible exception are given
to the completion callback.
Using fs.rmdir()
on a file (not a directory) results in an ENOENT
error on
Windows and an ENOTDIR
error on POSIX.
fs.rmdirSync(path)#
Synchronous rmdir(2)
. Returns undefined
.
Using fs.rmdirSync()
on a file (not a directory) results in an ENOENT
error
on Windows and an ENOTDIR
error on POSIX.
fs.stat(path[, options], callback)#
path
<string> | <Buffer> | <URL>-
options
<Object> -
callback
<Function>err
<Error>stats
<fs.Stats>
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[, options])#
path
<string> | <Buffer> | <URL>-
options
<Object> - Returns: <fs.Stats>
Synchronous stat(2)
.
fs.symlink(target, path[, type], callback)#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> Default:'file'
-
callback
<Function>err
<Error>
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'
and is only available on
Windows (ignored on other platforms). 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', callback);
It creates a symbolic link named "new-port" that points to "foo".
fs.symlinkSync(target, path[, type])#
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.symlink()
.
fs.truncate(path[, len], callback)#
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.
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
fs.truncateSync(path[, len])#
Synchronous truncate(2)
. Returns undefined
. A file descriptor can also be
passed as the first argument. In this case, fs.ftruncateSync()
is called.
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
fs.unlink(path, callback)#
path
<string> | <Buffer> | <URL>-
callback
<Function>err
<Error>
Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.
// Assuming that 'path/file.txt' is a regular file.
fs.unlink('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was deleted');
});
fs.unlink()
will not work on a directory, empty or otherwise. To remove a
directory, use fs.rmdir()
.
See also: unlink(2)
.
fs.unlinkSync(path)#
Synchronous unlink(2)
. Returns undefined
.
fs.unwatchFile(filename[, listener])#
filename
<string> | <Buffer> | <URL>listener
<Function> Optional, a listener previously attached usingfs.watchFile()
Stop watching for changes on filename
. If listener
is specified, only that
particular listener is removed. Otherwise, all listeners are removed,
effectively stopping watching of filename
.
Calling fs.unwatchFile()
with a filename that is not being watched is a
no-op, not an error.
Using 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)#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>-
callback
<Function>err
<Error>
Change the file system timestamps of the object referenced by path
.
The atime
and mtime
arguments follow these rules:
- Values can be either numbers representing Unix epoch time,
Date
s, or a numeric string like'123456789.0'
. - If the value can not be converted to a number, or is
NaN
,Infinity
or-Infinity
, anError
will be thrown.
fs.utimesSync(path, atime, mtime)#
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.utimes()
.
fs.watch(filename[, options][, listener])#
filename
<string> | <Buffer> | <URL>-
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. This 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> | <undefined> Default:undefined
- Returns: <fs.FSWatcher>
Watch for changes on filename
, where filename
is either a file or a
directory.
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.
On most platforms, 'rename'
is emitted whenever a filename appears or
disappears in the directory.
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 macOS 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(7)
. - On BSD systems, this uses
kqueue(2)
. - On macOS, this uses
kqueue(2)
for files andFSEvents
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.
It is still possible to use fs.watchFile()
, which uses stat polling, but
this method is slower and less reliable.
Inodes#
On Linux and macOS 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.
AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).
Filename Argument#
Providing filename
argument in the callback is only supported on Linux,
macOS, Windows, and AIX. 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)#
filename
<string> | <Buffer> | <URL>-
options
<Object> -
listener
<Function>current
<fs.Stats>previous
<fs.Stats>
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 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
.
To be notified when the file was modified, not just accessed, it is necessary
to compare curr.mtime
and prev.mtime
.
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.
Using 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.
When a file being watched by fs.watchFile()
disappears and reappears,
then the previousStat
reported in the second callback event (the file's
reappearance) will be the same as the previousStat
of the first callback
event (its disappearance).
This happens when:
- the file is deleted, followed by a restore
- the file is renamed twice - the second time back to its original name
fs.write(fd, buffer[, offset[, length[, position]]], callback)#
fd
<integer>buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>-
callback
<Function>err
<Error>bytesWritten
<integer>buffer
<Buffer> | <Uint8Array>
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, bytesWritten, buffer)
where
bytesWritten
specifies how many bytes were written from buffer
.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise
for an Object
with bytesWritten
and buffer
properties.
It is unsafe to use fs.write()
multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream()
is 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)#
fd
<integer>string
<string>position
<integer>encoding
<string>-
callback
<Function>
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. Bytes
written is not necessarily the same as string characters written. See
Buffer.byteLength
.
It is unsafe to use fs.write()
multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream()
is 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)#
file
<string> | <Buffer> | <URL> | <integer> filename or file descriptordata
<string> | <Buffer> | <Uint8Array>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.
-
callback
<Function>err
<Error>
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.
Example:
fs.writeFile('message.txt', 'Hello Node.js', (err) => {
if (err) throw err;
console.log('The file has been 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.
It is unsafe to use fs.writeFile()
multiple times on the same file without
waiting for the callback. For this scenario, fs.createWriteStream()
is
recommended.
If a file descriptor is specified as the file
, it will not be closed
automatically.
fs.writeFileSync(file, data[, options])#
file
<string> | <Buffer> | <URL> | <integer> filename or file descriptordata
<string> | <Buffer> | <Uint8Array>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.writeFile()
.
fs.writeSync(fd, buffer[, offset[, length[, position]]])#
fd
<integer>buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>- Returns: <number>
fs.writeSync(fd, string[, position[, encoding]])#
Returns the number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write()
.
fs Promises API#
The fs.promises
API provides an alternative set of asynchronous file system
methods that return Promise
objects rather than using callbacks. The
API is accessible via require('fs').promises
.
class: FileHandle#
A FileHandle
object is a wrapper for a numeric file descriptor.
Instances of FileHandle
are distinct from numeric file descriptors
in that, if the FileHandle
is not explicitly closed using the
filehandle.close()
method, they will automatically close the file descriptor
and will emit a process warning, thereby helping to prevent memory leaks.
Instances of the FileHandle
object are created internally by the
fsPromises.open()
method.
Unlike the callback-based API (fs.fstat()
, fs.fchown()
, fs.fchmod()
, and
so on), a numeric file descriptor is not used by the promise-based API. Instead,
the promise-based API uses the FileHandle
class in order to help avoid
accidental leaking of unclosed file descriptors after a Promise
is resolved or
rejected.
filehandle.appendFile(data, options)#
data
<string> | <Buffer>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
- Returns: <Promise>
Asynchronously append data to this file, creating the file if it does not yet
exist. data
can be a string or a Buffer
. The Promise
will be
resolved with no arguments upon success.
If options
is a string, then it specifies the encoding.
The FileHandle
must have been opened for appending.
filehandle.chmod(mode)#
Modifies the permissions on the file. The Promise
is resolved with no
arguments upon success.
filehandle.chown(uid, gid)#
Changes the ownership of the file then resolves the Promise
with no arguments
upon success.
filehandle.close()#
- Returns: <Promise> A
Promise
that will be resolved once the underlying file descriptor is closed, or will be rejected if an error occurs while closing.
Closes the file descriptor.
const fsPromises = require('fs').promises;
async function openAndClose() {
let filehandle;
try {
filehandle = await fsPromises.open('thefile.txt', 'r');
} finally {
if (filehandle !== undefined)
await filehandle.close();
}
}
filehandle.datasync()#
- Returns: <Promise>
Asynchronous fdatasync(2)
. The Promise
is resolved with no arguments upon
success.
filehandle.fd#
- <number> The numeric file descriptor managed by the
FileHandle
object.
filehandle.read(buffer, offset, length, position)#
buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>- Returns: <Promise>
Read data from the file.
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 argument specifying where to begin reading from in the file.
If position
is null
, data will be read from the current file position,
and the file position will be updated.
If position
is an integer, the file position will remain unchanged.
Following successful read, the Promise
is resolved with an object with a
bytesRead
property specifying the number of bytes read, and a buffer
property that is a reference to the passed in buffer
argument.
filehandle.readFile(options)#
-
encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.
- Returns: <Promise>
Asynchronously reads the entire contents of a file.
The Promise
is resolved with the contents of the file. If no encoding is
specified (using options.encoding
), the data is returned as a Buffer
object. Otherwise, the data will be a string.
If options
is a string, then it specifies the encoding.
When the path
is a directory, the behavior of fsPromises.readFile()
is
platform-specific. On macOS, Linux, and Windows, the promise will be rejected
with an error. On FreeBSD, a representation of the directory's contents will be
returned.
The FileHandle
has to support reading.
filehandle.stat([options])#
Retrieves the fs.Stats
for the file.
filehandle.sync()#
- Returns: <Promise>
Asynchronous fsync(2)
. The Promise
is resolved with no arguments upon
success.
filehandle.truncate(len)#
Truncates the file then resolves the Promise
with no arguments upon success.
If the file 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:
const fs = require('fs');
const fsPromises = fs.promises;
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
async function doTruncate() {
let filehandle = null;
try {
filehandle = await fsPromises.open('temp.txt', 'r+');
await filehandle.truncate(4);
} finally {
if (filehandle) {
// close the file if it is opened.
await filehandle.close();
}
}
console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node
}
doTruncate().catch(console.error);
If the file previously was shorter than len
bytes, it is extended, and the
extended part is filled with null bytes ('\0'
). For example,
const fs = require('fs');
const fsPromises = fs.promises;
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
async function doTruncate() {
let filehandle = null;
try {
filehandle = await fsPromises.open('temp.txt', 'r+');
await filehandle.truncate(10);
} finally {
if (filehandle) {
// close the file if it is opened.
await filehandle.close();
}
}
console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints Node.js\0\0\0
}
doTruncate().catch(console.error);
The last three bytes are null bytes ('\0'
), to compensate the over-truncation.
filehandle.utimes(atime, mtime)#
Change the file system timestamps of the object referenced by the FileHandle
then resolves the Promise
with no arguments upon success.
This function does not work on AIX versions before 7.1, it will resolve the
Promise
with an error using code UV_ENOSYS
.
filehandle.write(buffer, offset, length, position)#
buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>- Returns: <Promise>
Write buffer
to the file.
The Promise
is resolved with an object containing a bytesWritten
property
identifying the number of bytes written, and a buffer
property containing
a reference to the buffer
written.
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)
.
It is unsafe to use filehandle.write()
multiple times on the same file
without waiting for the Promise
to be resolved (or rejected). For this
scenario, fs.createWriteStream
is strongly recommended.
On Linux, positional writes do not 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.
filehandle.writeFile(data, options)#
data
<string> | <Buffer> | <Uint8Array>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.
- Returns: <Promise>
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string or a buffer. The Promise
will be resolved with no
arguments upon success.
The encoding
option is ignored if data
is a buffer.
If options
is a string, then it specifies the encoding.
The FileHandle
has to support writing.
It is unsafe to use filehandle.writeFile()
multiple times on the same file
without waiting for the Promise
to be resolved (or rejected).
fsPromises.access(path[, mode])#
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. Check File Access Constants for possible values
of mode
. It is possible to create a mask consisting of the bitwise OR of
two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK
).
If the accessibility check is successful, the Promise
is resolved with no
value. If any of the accessibility checks fail, the Promise
is rejected
with an Error
object. The following example checks if the file
/etc/passwd
can be read and written by the current process.
const fs = require('fs');
const fsPromises = fs.promises;
fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
.then(() => console.log('can access'))
.catch(() => console.error('cannot access'));
Using fsPromises.access()
to check for the accessibility of a file before
calling fsPromises.open()
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.
fsPromises.appendFile(path, data[, options])#
path
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
data
<string> | <Buffer>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'a'
.
- Returns: <Promise>
Asynchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a Buffer
. The Promise
will be
resolved with no arguments upon success.
If options
is a string, then it specifies the encoding.
The path
may be specified as a FileHandle
that has been opened
for appending (using fsPromises.open()
).
fsPromises.chmod(path, mode)#
Changes the permissions of a file then resolves the Promise
with no
arguments upon succces.
fsPromises.chown(path, uid, gid)#
Changes the ownership of a file then resolves the Promise
with no arguments
upon success.
fsPromises.copyFile(src, dest[, flags])#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationflags
<number> modifiers for copy operation. Default:0
.- Returns: <Promise>
Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. The Promise
will be resolved with no arguments upon success.
Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.
flags
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
- The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
- The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
- The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
Example:
const fsPromises = require('fs').promises;
// destination.txt will be created or overwritten by default.
fsPromises.copyFile('source.txt', 'destination.txt')
.then(() => console.log('source.txt was copied to destination.txt'))
.catch(() => console.log('The file could not be copied'));
If the third argument is a number, then it specifies flags
, as shown in the
following example.
const fs = require('fs');
const fsPromises = fs.promises;
const { COPYFILE_EXCL } = fs.constants;
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fsPromises.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL)
.then(() => console.log('source.txt was copied to destination.txt'))
.catch(() => console.log('The file could not be copied'));
fsPromises.lchmod(path, mode)#
Changes the permissions on a symbolic link then resolves the Promise
with
no arguments upon success. This method is only implemented on macOS.
fsPromises.lchown(path, uid, gid)#
Changes the ownership on a symbolic link then resolves the Promise
with
no arguments upon success.
fsPromises.link(existingPath, newPath)#
Asynchronous link(2)
. The Promise
is resolved with no arguments upon success.
fsPromises.lstat(path[, options])#
Asynchronous lstat(2)
. The Promise
is resolved with the fs.Stats
object
for the given symbolic link path
.
fsPromises.mkdir(path[, mode])#
Asynchronously creates a directory then resolves the Promise
with no
arguments upon success.
fsPromises.mkdtemp(prefix[, options])#
Creates a unique temporary directory and resolves the Promise
with the created
folder path. A unique directory name is generated by appending six random
characters to the end of the provided prefix
.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
.catch(console.error);
The fsPromises.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
).
fsPromises.open(path, flags[, mode])#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
.mode
<integer> Default:0o666
(readable and writable)- Returns: <Promise>
Asynchronous file open that returns a Promise
that, when resolved, yields a
FileHandle
object. See open(2)
.
mode
sets the file mode (permission and sticky bits), but only if the file was
created.
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented
by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains
a colon, Node.js will open a file system stream, as described by
this MSDN page.
fsPromises.readdir(path[, options])#
Reads the contents of a directory then resolves the Promise
with 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. If the encoding
is set to 'buffer'
, the filenames returned
will be passed as Buffer
objects.
fsPromises.readFile(path[, options])#
path
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
-
encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.
- Returns: <Promise>
Asynchronously reads the entire contents of a file.
The Promise
is resolved with the contents of the file. If no encoding is
specified (using options.encoding
), the data is returned as a Buffer
object. Otherwise, the data will be a string.
If options
is a string, then it specifies the encoding.
When the path
is a directory, the behavior of fsPromises.readFile()
is
platform-specific. On macOS, Linux, and Windows, the promise will be rejected
with an error. On FreeBSD, a representation of the directory's contents will be
returned.
Any specified FileHandle
has to support reading.
fsPromises.readlink(path[, options])#
Asynchronous readlink(2)
. The Promise
is resolved with the linkString
upon
success.
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 returned. If the encoding
is set to 'buffer'
, the link path
returned will be passed as a Buffer
object.
fsPromises.realpath(path[, options])#
Determines the actual location of path
using the same semantics as the
fs.realpath.native()
function then resolves the Promise
with 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 path. If the encoding
is set to 'buffer'
, the path returned will be
passed as a Buffer
object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fsPromises.rename(oldPath, newPath)#
Renames oldPath
to newPath
and resolves the Promise
with no arguments
upon success.
fsPromises.rmdir(path)#
Removes the directory identified by path
then resolves the Promise
with
no arguments upon success.
Using fsPromises.rmdir()
on a file (not a directory) results in the
Promise
being rejected with an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
fsPromises.stat(path[, options])#
The Promise
is resolved with the fs.Stats
object for the given path
.
fsPromises.symlink(target, path[, type])#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> Default:'file'
- Returns: <Promise>
Creates a symbolic link then resolves the Promise
with no arguments upon
success.
The type
argument is only used on Windows platforms and can be one of 'dir'
,
'file'
, or 'junction'
. Windows junction points require the destination path
to be absolute. When using 'junction'
, the target
argument will
automatically be normalized to absolute path.
fsPromises.truncate(path[, len])#
Truncates the path
then resolves the Promise
with no arguments upon
success. The path
must be a string or Buffer
.
fsPromises.unlink(path)#
Asynchronous unlink(2)
. The Promise
is resolved with no arguments upon
success.
fsPromises.utimes(path, atime, mtime)#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>- Returns: <Promise>
Change the file system timestamps of the object referenced by path
then
resolves the Promise
with no arguments upon success.
The atime
and mtime
arguments follow these rules:
- Values can be either numbers representing Unix epoch time,
Date
s, or a numeric string like'123456789.0'
. - If the value can not be converted to a number, or is
NaN
,Infinity
or-Infinity
, anError
will be thrown.
fsPromises.writeFile(file, data[, options])#
file
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
data
<string> | <Buffer> | <Uint8Array>-
encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.
- Returns: <Promise>
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string or a buffer. The Promise
will be resolved with no
arguments upon success.
The encoding
option is ignored if data
is a buffer.
If options
is a string, then it specifies the encoding.
Any specified FileHandle
has to support writing.
It is unsafe to use fsPromises.writeFile()
multiple times on the same file
without waiting for the Promise
to be resolved (or rejected).
FS Constants#
The following constants are exported by fs.constants
.
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.
This is useful for determining if a file exists, but says nothing
about rwx permissions. Default if no mode is specified. |
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. This has no effect on Windows
(will behave like fs.constants.F_OK ). |
File Copy Constants#
The following constants are meant for use with fs.copyFile()
.
Constant | Description |
---|---|
COPYFILE_EXCL |
If present, the copy operation will fail with an error if the destination path already exists. |
COPYFILE_FICLONE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used. |
COPYFILE_FICLONE_FORCE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error. |
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 synchronized I/O with write operations waiting for file integrity. |
O_DSYNC |
Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity. |
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. |
File System Flags#
The following flags are available wherever the flag
option takes a
string:
-
'a'
- Open file for appending. The file is created if it does not exist. -
'ax'
- Like'a'
but fails if the path exists. -
'a+'
- Open file for reading and appending. The file is created if it does not exist. -
'ax+'
- Like'a+'
but fails if the path exists. -
'as'
- Open file for appending in synchronous mode. The file is created if it does not exist. -
'as+'
- Open file for reading and appending in synchronous mode. The file is created if it does not exist. -
'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 skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.
This doesn't turn
fs.open()
orfsPromises.open()
into a synchronous blocking call. If synchronous operation is desired, something likefs.openSync()
should be used. -
'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 the 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 the path exists.
flag
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
.
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.
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.
Modifying a file rather than replacing it may require a flags mode of 'r+'
rather than the default mode 'w'
.
The behavior of some flags are platform-specific. As such, opening a directory
on macOS and Linux with the 'a+'
flag - see example below - will return an
error. In contrast, on Windows and FreeBSD, a file descriptor or a FileHandle
will be returned.
// macOS 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>
});
On Windows, opening an existing hidden file using the 'w'
flag (either
through fs.open()
or fs.writeFile()
or fsPromises.open()
) will fail with
EPERM
. Existing hidden files can be opened for writing with the 'r+'
flag.
A call to fs.ftruncate()
or fsPromises.ftruncate()
can be used to reset
the file contents.