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nodejs/doc/api/fs.md
James M Snell 329fc78e49 fs: add initial set of fs.promises APIs 
Initial set of fs.promises APIs with documentation and one
benchmark.

PR-URL: https://github.com/nodejs/node/pull/18297
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
2018-02-05 20:31:04 -08:00

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# File System
<!--introduced_in=v0.10.0-->
> Stability: 2 - Stable
<!--name=fs-->
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:
```js
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`.
For example:
```js
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.
For example:
```js
const fs = require('fs');
try {
fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
```
Note that 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.
```js
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:
```js
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:
*Note*: Omitting the callback function on asynchronous fs functions is
deprecated and may result in an error being thrown in the future.
```txt
$ 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:
```js
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()`):
```js
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:
```js
fs.open(Buffer.from('/open/some/file.txt'), 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
```
*Note:* 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][MSDN-Rel-Path].
### URL object support
<!-- YAML
added: v7.6.0
-->
> Stability: 1 - Experimental
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.
```js
const fs = require('fs');
const fileUrl = new URL('file:///tmp/hello');
fs.readFileSync(fileUrl);
```
*Note*: `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 :
```js
// 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
```
*Note*: `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:
```js
// 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:
```js
// 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:
```js
// 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.
```js
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
Note that 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
<!-- YAML
added: v0.5.8
-->
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'
<!-- YAML
added: v0.5.8
-->
* `eventType` {string} The type of change event that has occurred
* `filename` {string|Buffer} The filename that changed (if relevant/available)
Emitted when something changes in a watched directory or file.
See more details in [`fs.watch()`][].
The `filename` argument may not be provided depending on operating system
support. If `filename` is provided, it will be provided as a `Buffer` if
`fs.watch()` is called with its `encoding` option set to `'buffer'`, otherwise
`filename` will be a UTF-8 string.
```js
// Example when handled through fs.watch listener
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
// Prints: <Buffer ...>
}
});
```
### Event: 'error'
<!-- YAML
added: v0.5.8
-->
* `error` {Error}
Emitted when an error occurs while watching the file.
### watcher.close()
<!-- YAML
added: v0.5.8
-->
Stop watching for changes on the given `fs.FSWatcher`. Once stopped, the
`fs.FSWatcher` object is no longer usable.
## Class: fs.ReadStream
<!-- YAML
added: v0.1.93
-->
A successful call to `fs.createReadStream()` will return a new `fs.ReadStream`
object.
All `fs.ReadStream` objects are [Readable Streams][].
### Event: 'close'
<!-- YAML
added: v0.1.93
-->
Emitted when the `fs.ReadStream`'s underlying file descriptor has been closed.
### Event: 'open'
<!-- YAML
added: v0.1.93
-->
* `fd` {integer} Integer file descriptor used by the ReadStream.
Emitted when the `fs.ReadStream`'s file descriptor has been opened.
### readStream.bytesRead
<!-- YAML
added: 6.4.0
-->
* Value: {number}
The number of bytes that have been read so far.
### readStream.path
<!-- YAML
added: v0.1.93
-->
* Value: {string|Buffer}
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
<!-- YAML
added: v0.1.21
changes:
- version: v8.1.0
pr-url: https://github.com/nodejs/node/pull/13173
description: Added times as numbers.
-->
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.
For example:
```console
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 }
```
### 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.
*Note*: This method is only valid when using [`fs.lstat()`][]
### stats.dev
* Value: {number}
The numeric identifier of the device containing the file.
### stats.ino
* Value: {number}
The file system specific "Inode" number for the file.
### stats.mode
* Value: {number}
A bit-field describing the file type and mode.
### stats.nlink
* Value: {number}
The number of hard-links that exist for the file.
### stats.uid
* Value: {number}
The numeric user identifier of the user that owns the file (POSIX).
### stats.gid
* Value: {number}
The numeric group identifier of the group that owns the file (POSIX).
### stats.rdev
* Value: {number}
A numeric device identifier if the file is considered "special".
### stats.blksize
* Value: {number}
The file system block size for i/o operations.
### stats.blocks
* Value: {number}
The number of blocks allocated for this file.
### stats.atimeMs
* Value: {number}
The timestamp indicating the last time this file was accessed expressed in
milliseconds since the POSIX Epoch.
### stats.mtimeMs
* Value: {number}
The timestamp indicating the last time this file was modified expressed in
milliseconds since the POSIX Epoch.
### stats.ctimeMs
* Value: {number}
The timestamp indicating the last time the file status was changed expressed
in milliseconds since the POSIX Epoch.
### stats.birthtimeMs
* Value: {number}
The timestamp indicating the creation time of this file expressed in
milliseconds since the POSIX Epoch.
### stats.atime
* Value: {Date}
The timestamp indicating the last time this file was accessed.
### stats.mtime
* Value: {Date}
The timestamp indicating the last time this file was modified.
### stats.ctime
* Value: {Date}
The timestamp indicating the last time the file status was changed.
### stats.birthtime
* Value: {Date}
The timestamp indicating the creation time of this file.
### Stat Time Values
*Note*: `atimeMs`, `mtimeMs`, `ctimeMs`, `birthtimeMs` are [numbers][MDN-Number]
that hold the corresponding times in milliseconds. Their precision is platform
specific. `atime`, `mtime`, `ctime`, and `birthtime` are [`Date`][MDN-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 the mknod(2), utimes(2), and read(2) system calls.
* `mtime` "Modified Time" - Time when file data last modified.
Changed by the mknod(2), utimes(2), and write(2) system calls.
* `ctime` "Change Time" - Time when file status was last changed
(inode data modification). Changed by the chmod(2), chown(2),
link(2), mknod(2), rename(2), unlink(2), utimes(2),
read(2), and write(2) system calls.
* `birthtime` "Birth Time" - Time of file creation. Set once when the
file is created. On filesystems where birthtime is not available,
this field may instead hold either the `ctime` or
`1970-01-01T00:00Z` (ie, unix epoch timestamp `0`). Note that this
value may be greater than `atime` or `mtime` in this case. On Darwin
and other FreeBSD variants, also set if the `atime` is explicitly
set to an earlier value than the current `birthtime` using the
utimes(2) system call.
Prior to Node v0.12, the `ctime` held the `birthtime` on Windows
systems. Note that as of v0.12, `ctime` is not "creation time", and
on Unix systems, it never was.
## Class: fs.WriteStream
<!-- YAML
added: v0.1.93
-->
`WriteStream` is a [Writable Stream][].
### Event: 'close'
<!-- YAML
added: v0.1.93
-->
Emitted when the `WriteStream`'s underlying file descriptor has been closed.
### Event: 'open'
<!-- YAML
added: v0.1.93
-->
* `fd` {integer} Integer file descriptor used by the WriteStream.
Emitted when the WriteStream's file is opened.
### writeStream.bytesWritten
<!-- YAML
added: v0.4.7
-->
The number of bytes written so far. Does not include data that is still queued
for writing.
### writeStream.path
<!-- YAML
added: v0.1.93
-->
The path to the file the stream is writing to as specified in the first
argument to `fs.createWriteStream()`. If `path` is passed as a string, then
`writeStream.path` will be a string. If `path` is passed as a `Buffer`, then
`writeStream.path` will be a `Buffer`.
## fs.access(path[, mode], callback)
<!-- YAML
added: v0.11.15
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v6.3.0
pr-url: https://github.com/nodejs/node/pull/6534
description: The constants like `fs.R_OK`, etc which were present directly
on `fs` were moved into `fs.constants` as a soft deprecation.
Thus for Node `< v6.3.0` use `fs` to access those constants, or
do something like `(fs.constants || fs).R_OK` to work with all
versions.
-->
* `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. The following constants define the possible values of
`mode`. It is possible to create a mask consisting of the bitwise OR of two or
more values (e.g. `fs.constants.W_OK | fs.constants.R_OK`).
* `fs.constants.F_OK` - `path` is visible to the calling process. This is useful
for determining if a file exists, but says nothing about `rwx` permissions.
Default if no `mode` is specified.
* `fs.constants.R_OK` - `path` can be read by the calling process.
* `fs.constants.W_OK` - `path` can be written by the calling process.
* `fs.constants.X_OK` - `path` can be executed by the calling process. This has
no effect on Windows (will behave like `fs.constants.F_OK`).
The final argument, `callback`, is a callback function that is invoked with
a possible error argument. If any of the accessibility checks fail, the error
argument will be an `Error` object. The following example checks if the file
`/etc/passwd` can be read and written by the current process.
```js
fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => {
console.log(err ? 'no access!' : 'can read/write');
});
```
Using `fs.access()` to check for the accessibility of a file before calling
`fs.open()`, `fs.readFile()` or `fs.writeFile()` is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file is not accessible.
For example:
**write (NOT RECOMMENDED)**
```js
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)**
```js
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)**
```js
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)**
```js
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])
<!-- YAML
added: v0.11.15
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `fs.constants.F_OK`
* Returns: {undefined}
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. The following constants define the
possible values of `mode`. It is possible to create a mask consisting of the
bitwise OR of two or more values (e.g. `fs.constants.W_OK | fs.constants.R_OK`).
* `fs.constants.F_OK` - `path` is visible to the calling process. This is useful
for determining if a file exists, but says nothing about `rwx` permissions.
Default if no `mode` is specified.
* `fs.constants.R_OK` - `path` can be read by the calling process.
* `fs.constants.W_OK` - `path` can be written by the calling process.
* `fs.constants.X_OK` - `path` can be executed by the calling process. This has
no effect on Windows (will behave like `fs.constants.F_OK`).
If any of the accessibility checks fail, an `Error` will be thrown. Otherwise,
the method will return `undefined`.
```js
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(file, data[, options], callback)
<!-- YAML
added: v0.6.7
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7831
description: The passed `options` object will never be modified.
- version: v5.0.0
pr-url: https://github.com/nodejs/node/pull/3163
description: The `file` parameter can be a file descriptor now.
-->
* `file` {string|Buffer|URL|number} filename or file descriptor
* `data` {string|Buffer}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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:
```js
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:
```js
fs.appendFile('message.txt', 'data to append', 'utf8', callback);
```
The `file` 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.
```js
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(file, data[, options])
<!-- YAML
added: v0.6.7
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7831
description: The passed `options` object will never be modified.
- version: v5.0.0
pr-url: https://github.com/nodejs/node/pull/3163
description: The `file` parameter can be a file descriptor now.
-->
* `file` {string|Buffer|URL|number} filename or file descriptor
* `data` {string|Buffer}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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:
```js
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:
```js
fs.appendFileSync('message.txt', 'data to append', 'utf8');
```
The `file` 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.
```js
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)
<!-- YAML
added: v0.1.30
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `mode` {integer}
* `callback` {Function}
* `err` {Error}
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.
## fs.chmodSync(path, mode)
<!-- YAML
added: v0.6.7
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `mode` {integer}
Synchronously changes the permissions of a file. Returns `undefined`.
This is the synchronous version of [`fs.chmod()`][].
See also: chmod(2)
## fs.chown(path, uid, gid, callback)
<!-- YAML
added: v0.1.97
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `uid` {integer}
* `gid` {integer}
* `callback` {Function}
* `err` {Error}
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)
<!-- YAML
added: v0.1.97
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `uid` {integer}
* `gid` {integer}
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)
<!-- YAML
added: v0.0.2
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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)
<!-- YAML
added: v0.1.21
-->
* `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)
<!-- YAML
added: v8.5.0
-->
* `src` {string|Buffer|URL} source filename to copy
* `dest` {string|Buffer|URL} destination filename of the copy operation
* `flags` {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. The only supported flag is `fs.constants.COPYFILE_EXCL`,
which causes the copy operation to fail if `dest` already exists.
Example:
```js
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.
```js
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])
<!-- YAML
added: v8.5.0
-->
* `src` {string|Buffer|URL} source filename to copy
* `dest` {string|Buffer|URL} destination filename of the copy operation
* `flags` {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. The only supported flag is `fs.constants.COPYFILE_EXCL`,
which causes the copy operation to fail if `dest` already exists.
Example:
```js
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.
```js
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])
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7831
description: The passed `options` object will never be modified.
- version: v2.3.0
pr-url: https://github.com/nodejs/node/pull/1845
description: The passed `options` object can be a string now.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `flags` {string}
* `encoding` {string}
* `fd` {integer}
* `mode` {integer}
* `autoClose` {boolean}
* `start` {integer}
* `end` {integer}
* `highWaterMark` {integer}
Returns a new [`ReadStream`][] object. (See [Readable Streams][]).
Be aware that, unlike the default value set for `highWaterMark` on a
readable stream (16 kb), the stream returned by this method has a
default value of 64 kb for the same parameter.
`options` is an object or string with the following defaults:
```js
const defaults = {
flags: 'r',
encoding: null,
fd: null,
mode: 0o666,
autoClose: true,
highWaterMark: 64 * 1024
};
```
`options` can include `start` and `end` values to read a range of bytes from
the file instead of the entire file. Both `start` and `end` are inclusive and
start counting at 0. If `fd` is specified and `start` is omitted or `undefined`,
`fs.createReadStream()` reads sequentially from the current file position.
The `encoding` can be any one of those accepted by [`Buffer`][].
If `fd` is specified, `ReadStream` will ignore the `path` argument and will use
the specified file descriptor. This means that no `'open'` event will be
emitted. Note that `fd` should be blocking; non-blocking `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:
```js
fs.createReadStream('sample.txt', { start: 90, end: 99 });
```
If `options` is a string, then it specifies the encoding.
## fs.createWriteStream(path[, options])
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7831
description: The passed `options` object will never be modified.
- version: v5.5.0
pr-url: https://github.com/nodejs/node/pull/3679
description: The `autoClose` option is supported now.
- version: v2.3.0
pr-url: https://github.com/nodejs/node/pull/1845
description: The passed `options` object can be a string now.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `flags` {string}
* `encoding` {string}
* `fd` {integer}
* `mode` {integer}
* `autoClose` {boolean}
* `start` {integer}
Returns a new [`WriteStream`][] object. (See [Writable Stream][]).
`options` is an object or string with the following defaults:
```js
const defaults = {
flags: 'w',
encoding: 'utf8',
fd: null,
mode: 0o666,
autoClose: true
};
```
`options` may also include a `start` option to allow writing data at
some position past the beginning of the file. Modifying a file rather
than replacing it may require a `flags` mode of `r+` rather than the
default mode `w`. The `encoding` can be any one of those accepted by
[`Buffer`][].
If `autoClose` is set to true (default behavior) on `error` or `end`
the file descriptor will be closed automatically. If `autoClose` is false,
then the file descriptor won't be closed, even if there's an error.
It is 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. Note that `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)
<!-- YAML
added: v0.0.2
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
deprecated: v1.0.0
-->
> Stability: 0 - Deprecated: Use [`fs.stat()`][] or [`fs.access()`][] instead.
* `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:
```js
fs.exists('/etc/passwd', (exists) => {
console.log(exists ? 'it\'s there' : 'no passwd!');
});
```
**Note that the parameter to this callback is not consistent with other
Node.js callbacks.** Normally, the first parameter to a Node.js callback is
an `err` parameter, optionally followed by other parameters. The
`fs.exists()` callback has only one boolean parameter. This is one reason
`fs.access()` is recommended instead of `fs.exists()`.
Using `fs.exists()` to check for the existence of a file before calling
`fs.open()`, `fs.readFile()` or `fs.writeFile()` is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file does not exist.
For example:
**write (NOT RECOMMENDED)**
```js
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)**
```js
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)**
```js
fs.exists('myfile', (exists) => {
if (exists) {
fs.open('myfile', 'r', (err, fd) => {
readMyData(fd);
});
} else {
console.error('myfile does not exist');
}
});
```
**read (RECOMMENDED)**
```js
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)
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
Synchronous version of [`fs.exists()`][].
Returns `true` if the path exists, `false` otherwise.
Note that `fs.exists()` is deprecated, but `fs.existsSync()` is not.
(The `callback` parameter to `fs.exists()` accepts parameters that are
inconsistent with other Node.js callbacks. `fs.existsSync()` does not use
a callback.)
## fs.fchmod(fd, mode, callback)
<!-- YAML
added: v0.4.7
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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)
<!-- YAML
added: v0.4.7
-->
* `fd` {integer}
* `mode` {integer}
Synchronous fchmod(2). Returns `undefined`.
## fs.fchown(fd, uid, gid, callback)
<!-- YAML
added: v0.4.7
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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)
<!-- YAML
added: v0.4.7
-->
* `fd` {integer}
* `uid` {integer}
* `gid` {integer}
Synchronous fchown(2). Returns `undefined`.
## fs.fdatasync(fd, callback)
<!-- YAML
added: v0.1.96
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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)
<!-- YAML
added: v0.1.96
-->
* `fd` {integer}
Synchronous fdatasync(2). Returns `undefined`.
## fs.fstat(fd, callback)
<!-- YAML
added: v0.1.95
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `fd` {integer}
* `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)
<!-- YAML
added: v0.1.95
-->
* `fd` {integer}
Synchronous fstat(2). Returns an instance of [`fs.Stats`][].
## fs.fsync(fd, callback)
<!-- YAML
added: v0.1.96
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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)
<!-- YAML
added: v0.1.96
-->
* `fd` {integer}
Synchronous fsync(2). Returns `undefined`.
## fs.ftruncate(fd[, len], callback)
<!-- YAML
added: v0.8.6
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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
```js
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,
```js
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])
<!-- YAML
added: v0.8.6
-->
* `fd` {integer}
* `len` {integer} **Default:** `0`
Synchronous ftruncate(2). Returns `undefined`.
## fs.futimes(fd, atime, mtime, callback)
<!-- YAML
added: v0.4.2
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v4.1.0
pr-url: https://github.com/nodejs/node/pull/2387
description: Numeric strings, `NaN` and `Infinity` are now allowed
time specifiers.
-->
* `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()`][].
*Note*: This function does not work on AIX versions before 7.1, it will return
the error `UV_ENOSYS`.
## fs.futimesSync(fd, atime, mtime)
<!-- YAML
added: v0.4.2
changes:
- version: v4.1.0
pr-url: https://github.com/nodejs/node/pull/2387
description: Numeric strings, `NaN` and `Infinity` are now allowed
time specifiers.
-->
* `fd` {integer}
* `atime` {integer}
* `mtime` {integer}
Synchronous version of [`fs.futimes()`][]. Returns `undefined`.
## fs.lchmod(path, mode, callback)
<!-- YAML
deprecated: v0.4.7
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `mode` {integer}
* `callback` {Function}
* `err` {Error}
Asynchronous lchmod(2). No arguments other than a possible exception
are given to the completion callback.
Only available on macOS.
## fs.lchmodSync(path, mode)
<!-- YAML
deprecated: v0.4.7
-->
* `path` {string|Buffer|URL}
* `mode` {integer}
Synchronous lchmod(2). Returns `undefined`.
## fs.lchown(path, uid, gid, callback)
<!-- YAML
deprecated: v0.4.7
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `uid` {integer}
* `gid` {integer}
* `callback` {Function}
* `err` {Error}
Asynchronous lchown(2). No arguments other than a possible exception are given
to the completion callback.
## fs.lchownSync(path, uid, gid)
<!-- YAML
deprecated: v0.4.7
-->
* `path` {string|Buffer|URL}
* `uid` {integer}
* `gid` {integer}
Synchronous lchown(2). Returns `undefined`.
## fs.link(existingPath, newPath, callback)
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `existingPath` and `newPath` parameters can be WHATWG
`URL` objects using `file:` protocol. Support is currently
still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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)
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `existingPath` and `newPath` parameters can be WHATWG
`URL` objects using `file:` protocol. Support is currently
still *experimental*.
-->
* `existingPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
Synchronous link(2). Returns `undefined`.
## fs.lstat(path, callback)
<!-- YAML
added: v0.1.30
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `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)
<!-- YAML
added: v0.1.30
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
Synchronous lstat(2). Returns an instance of [`fs.Stats`][].
## fs.mkdir(path[, mode], callback)
<!-- YAML
added: v0.1.8
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `0o777`
* `callback` {Function}
* `err` {Error}
Asynchronously creates a directory. No arguments other than a possible exception
are given to the completion callback. `mode` defaults to `0o777`.
See also: mkdir(2)
## fs.mkdirSync(path[, mode])
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `0o777`
Synchronously creates a directory. Returns `undefined`.
This is the synchronous version of [`fs.mkdir()`][].
See also: mkdir(2)
## fs.mkdtemp(prefix[, options], callback)
<!-- YAML
added: v5.10.0
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v6.2.1
pr-url: https://github.com/nodejs/node/pull/6828
description: The `callback` parameter is optional now.
-->
* `prefix` {string}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* `callback` {Function}
* `err` {Error}
* `folder` {string}
Creates a unique temporary directory.
Generates six random characters to be appended behind a required
`prefix` to create a unique temporary directory.
The created folder path is passed as a string to the callback's second
parameter.
The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use.
Example:
```js
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
});
```
*Note*: The `fs.mkdtemp()` method will append the six randomly selected
characters directly to the `prefix` string. For instance, given a directory
`/tmp`, if the intention is to create a temporary directory *within* `/tmp`,
the `prefix` *must* end with a trailing platform-specific path separator
(`require('path').sep`).
```js
// 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`.
// Note that 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])
<!-- YAML
added: v5.10.0
-->
* `prefix` {string}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
The synchronous version of [`fs.mkdtemp()`][]. Returns the created
folder path.
The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use.
## fs.open(path, flags[, mode], callback)
<!-- YAML
added: v0.0.2
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `flags` {string|number}
* `mode` {integer} **Default:** `0o666`
* `callback` {Function}
* `err` {Error}
* `fd` {integer}
Asynchronous file open. See open(2). `flags` can be:
* `'r'` - Open file for reading.
An exception occurs if the file does not exist.
* `'r+'` - Open file for reading and writing.
An exception occurs if the file does not exist.
* `'rs+'` - Open file for reading and writing in synchronous mode. Instructs
the operating system to bypass the local file system cache.
This is primarily useful for opening files on NFS mounts as it allows 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.
Note that this doesn't turn `fs.open()` into a synchronous blocking call.
If synchronous operation is desired `fs.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 `path` exists.
* `'w+'` - Open file for reading and writing.
The file is created (if it does not exist) or truncated (if it exists).
* `'wx+'` - Like `'w+'` but fails if `path` exists.
* `'a'` - Open file for appending.
The file is created if it does not exist.
* `'ax'` - Like `'a'` but fails if `path` exists.
* `'a+'` - Open file for reading and appending.
The file is created if it does not exist.
* `'ax+'` - Like `'a+'` but fails if `path` exists.
`mode` sets the file mode (permission and sticky bits), but only if the file was
created. It defaults to `0o666` (readable and writable).
The callback gets two arguments `(err, fd)`.
The exclusive flag `'x'` (`O_EXCL` flag in open(2)) ensures that `path` is newly
created. On POSIX systems, `path` is considered to exist even if it is a symlink
to a non-existent file. The exclusive flag may or may not work with network file
systems.
`flags` can also be a number as documented by open(2); commonly used constants
are available from `fs.constants`. On Windows, flags are translated to
their equivalent ones where applicable, e.g. `O_WRONLY` to `FILE_GENERIC_WRITE`,
or `O_EXCL|O_CREAT` to `CREATE_NEW`, as accepted by CreateFileW.
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
*Note*: The behavior of `fs.open()` is platform-specific for some flags. As
such, opening a directory on macOS and Linux with the `'a+'` flag - see example
below - will return an error. In contrast, on Windows and FreeBSD, a file
descriptor will be returned.
```js
// 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>
});
```
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][MSDN-Using-Streams].
Functions based on `fs.open()` exhibit this behavior as well. eg.
`fs.writeFile()`, `fs.readFile()`, etc.
*Note:* On Windows, opening an existing hidden file using the `w` flag (either
through `fs.open()` or `fs.writeFile()`) will fail with `EPERM`. Existing hidden
files can be opened for writing with the `r+` flag. A call to `fs.ftruncate()`
can be used to reset the file contents.
## fs.openSync(path, flags[, mode])
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `flags` {string|number}
* `mode` {integer} **Default:** `0o666`
Synchronous version of [`fs.open()`][]. Returns an integer representing the file
descriptor.
## fs.read(fd, buffer, offset, length, position, callback)
<!-- YAML
added: v0.0.2
changes:
- version: v7.4.0
pr-url: https://github.com/nodejs/node/pull/10382
description: The `buffer` parameter can now be a `Uint8Array`.
- version: v6.0.0
pr-url: https://github.com/nodejs/node/pull/4518
description: The `length` parameter can now be `0`.
-->
* `fd` {integer}
* `buffer` {Buffer|Uint8Array}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
* `callback` {Function}
* `err` {Error}
* `bytesRead` {integer}
* `buffer` {Buffer}
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)
<!-- YAML
added: v0.1.8
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v6.0.0
pr-url: https://github.com/nodejs/node/pull/5616
description: The `options` parameter was added.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `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])
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
Synchronous readdir(3). Returns an array of filenames excluding `'.'` and
`'..'`.
The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use for
the filenames passed to the callback. If the `encoding` is set to `'buffer'`,
the filenames returned will be passed as `Buffer` objects.
## fs.readFile(path[, options], callback)
<!-- YAML
added: v0.1.29
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v5.1.0
pr-url: https://github.com/nodejs/node/pull/3740
description: The `callback` will always be called with `null` as the `error`
parameter in case of success.
- version: v5.0.0
pr-url: https://github.com/nodejs/node/pull/3163
description: The `path` parameter can be a file descriptor now.
-->
* `path` {string|Buffer|URL|integer} filename or file descriptor
* `options` {Object|string}
* `encoding` {string|null} **Default:** `null`
* `flag` {string} **Default:** `'r'`
* `callback` {Function}
* `err` {Error}
* `data` {string|Buffer}
Asynchronously reads the entire contents of a file. Example:
```js
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:
```js
fs.readFile('/etc/passwd', 'utf8', callback);
```
*Note*: 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.
```js
// 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.
*Note*: If a file descriptor is specified as the `path`, it will not be closed
automatically.
*Note*: `fs.readFile()` buffers the entire file.
To minimize memory costs, when possible prefer streaming via
`fs.createReadStream()`.
## fs.readFileSync(path[, options])
<!-- YAML
added: v0.1.8
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v5.0.0
pr-url: https://github.com/nodejs/node/pull/3163
description: The `path` parameter can be a file descriptor now.
-->
* `path` {string|Buffer|URL|integer} filename or file descriptor
* `options` {Object|string}
* `encoding` {string|null} **Default:** `null`
* `flag` {string} **Default:** `'r'`
Synchronous version of [`fs.readFile()`][]. Returns the contents of the `path`.
If the `encoding` option is specified then this function returns a
string. Otherwise it returns a buffer.
*Note*: Similar to [`fs.readFile()`][], when the path is a directory, the
behavior of `fs.readFileSync()` is platform-specific.
```js
// macOS, Linux, and Windows
fs.readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
// FreeBSD
fs.readFileSync('<directory>'); // => null, <data>
```
## fs.readlink(path[, options], callback)
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* `callback` {Function}
* `err` {Error}
* `linkString` {string|Buffer}
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])
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
Synchronous readlink(2). Returns the symbolic link's string value.
The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use for
the link path passed to the callback. If the `encoding` is set to `'buffer'`,
the link path returned will be passed as a `Buffer` object.
## fs.readSync(fd, buffer, offset, length, position)
<!-- YAML
added: v0.1.21
changes:
- version: v6.0.0
pr-url: https://github.com/nodejs/node/pull/4518
description: The `length` parameter can now be `0`.
-->
* `fd` {integer}
* `buffer` {Buffer|Uint8Array}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
Synchronous version of [`fs.read()`][]. Returns the number of `bytesRead`.
## fs.realpath(path[, options], callback)
<!-- YAML
added: v0.1.31
changes:
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/13028
description: Pipe/Socket resolve support was added.
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v6.4.0
pr-url: https://github.com/nodejs/node/pull/7899
description: Calling `realpath` now works again for various edge cases
on Windows.
- version: v6.0.0
pr-url: https://github.com/nodejs/node/pull/3594
description: The `cache` parameter was removed.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* `callback` {Function}
* `err` {Error}
* `resolvedPath` {string|Buffer}
Asynchronously computes the canonical pathname by resolving `.`, `..` and
symbolic links.
Note that "canonical" does not mean "unique": hard links and bind mounts can
expose a file system entity through many pathnames.
This function behaves like realpath(3), with some exceptions:
1. No case conversion is performed on case-insensitive file systems.
2. 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.
*Note*: 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)
<!-- YAML
added: v9.2.0
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* `callback` {Function}
* `err` {Error}
* `resolvedPath` {string|Buffer}
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.
*Note*: 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])
<!-- YAML
added: v0.1.31
changes:
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/13028
description: Pipe/Socket resolve support was added.
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
- version: v6.4.0
pr-url: https://github.com/nodejs/node/pull/7899
description: Calling `realpathSync` now works again for various edge cases
on Windows.
- version: v6.0.0
pr-url: https://github.com/nodejs/node/pull/3594
description: The `cache` parameter was removed.
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
Synchronously computes the canonical pathname by resolving `.`, `..` and
symbolic links.
Note that "canonical" does not mean "unique": hard links and bind mounts can
expose a file system entity through many pathnames.
This function behaves like realpath(3), with some exceptions:
1. No case conversion is performed on case-insensitive file systems.
2. The maximum number of symbolic links is platform-independent and generally
(much) higher than what the native realpath(3) implementation supports.
The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use for
the returned value. If the `encoding` is set to `'buffer'`, the path returned
will be passed as a `Buffer` object.
*Note*: If `path` resolves to a socket or a pipe, the function will return a
system dependent name for that object.
## fs.realpathSync.native(path[, options])
<!-- YAML
added: v9.2.0
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
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 passed to the callback. If the `encoding` is set to `'buffer'`,
the path returned will be passed as a `Buffer` object.
*Note*: 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)
<!-- YAML
added: v0.0.2
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `oldPath` and `newPath` parameters can be WHATWG `URL`
objects using `file:` protocol. Support is currently still
*experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `oldPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
* `callback` {Function}
* `err` {Error}
Asynchronous rename(2). No arguments other than a possible exception are given
to the completion callback.
## fs.renameSync(oldPath, newPath)
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `oldPath` and `newPath` parameters can be WHATWG `URL`
objects using `file:` protocol. Support is currently still
*experimental*.
-->
* `oldPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
Synchronous rename(2). Returns `undefined`.
## fs.rmdir(path, callback)
<!-- YAML
added: v0.0.2
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameters can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `callback` {Function}
* `err` {Error}
Asynchronous rmdir(2). No arguments other than a possible exception are given
to the completion callback.
*Note*: 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)
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameters can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
Synchronous rmdir(2). Returns `undefined`.
*Note*: 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, callback)
<!-- YAML
added: v0.0.2
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `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)
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
Synchronous stat(2). Returns an instance of [`fs.Stats`][].
## fs.symlink(target, path[, type], callback)
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `target` and `path` parameters can be WHATWG `URL` objects
using `file:` protocol. Support is currently still
*experimental*.
-->
* `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'` (default is `'file'`) and is only available on
Windows (ignored on other platforms). Note that Windows junction points require
the destination path to be absolute. When using `'junction'`, the `target`
argument will automatically be normalized to absolute path.
Here is an example below:
```js
fs.symlink('./foo', './new-port', callback);
```
It creates a symbolic link named "new-port" that points to "foo".
## fs.symlinkSync(target, path[, type])
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `target` and `path` parameters can be WHATWG `URL` objects
using `file:` protocol. Support is currently still
*experimental*.
-->
* `target` {string|Buffer|URL}
* `path` {string|Buffer|URL}
* `type` {string} **Default:** `'file'`
Synchronous symlink(2). Returns `undefined`.
## fs.truncate(path[, len], callback)
<!-- YAML
added: v0.8.6
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `len` {integer} **Default:** `0`
* `callback` {Function}
* `err` {Error}
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.
*Note*: Passing a file descriptor is deprecated and may result in an error
being thrown in the future.
## fs.truncateSync(path[, len])
<!-- YAML
added: v0.8.6
-->
* `path` {string|Buffer|URL}
* `len` {integer} **Default:** `0`
Synchronous truncate(2). Returns `undefined`. A file descriptor can also be
passed as the first argument. In this case, `fs.ftruncateSync()` is called.
*Note*: Passing a file descriptor is deprecated and may result in an error
being thrown in the future.
## fs.unlink(path, callback)
<!-- YAML
added: v0.0.2
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `path` {string|Buffer|URL}
* `callback` {Function}
* `err` {Error}
Asynchronous unlink(2). No arguments other than a possible exception are given
to the completion callback.
## fs.unlinkSync(path)
<!-- YAML
added: v0.1.21
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
-->
* `path` {string|Buffer|URL}
Synchronous unlink(2). Returns `undefined`.
## fs.unwatchFile(filename[, listener])
<!-- YAML
added: v0.1.31
-->
* `filename` {string|Buffer|URL}
* `listener` {Function} Optional, a listener previously attached using
`fs.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.
*Note*: [`fs.watch()`][] is more efficient than `fs.watchFile()` and
`fs.unwatchFile()`. `fs.watch()` should be used instead of `fs.watchFile()`
and `fs.unwatchFile()` when possible.
## fs.utimes(path, atime, mtime, callback)
<!-- YAML
added: v0.4.2
changes:
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/11919
description: "`NaN`, `Infinity`, and `-Infinity` are no longer valid time
specifiers."
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v4.1.0
pr-url: https://github.com/nodejs/node/pull/2387
description: Numeric strings, `NaN` and `Infinity` are now allowed
time specifiers.
-->
* `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`, a `Error` will be thrown.
## fs.utimesSync(path, atime, mtime)
<!-- YAML
added: v0.4.2
changes:
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/11919
description: "`NaN`, `Infinity`, and `-Infinity` are no longer valid time
specifiers."
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `path` parameter can be a WHATWG `URL` object using `file:`
protocol. Support is currently still *experimental*.
- version: v4.1.0
pr-url: https://github.com/nodejs/node/pull/2387
description: Numeric strings, `NaN` and `Infinity` are now allowed
time specifiers.
-->
* `path` {string|Buffer|URL}
* `atime` {integer}
* `mtime` {integer}
Synchronous version of [`fs.utimes()`][]. Returns `undefined`.
## fs.watch(filename[, options][, listener])
<!-- YAML
added: v0.5.10
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `filename` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7831
description: The passed `options` object will never be modified.
-->
* `filename` {string|Buffer|URL}
* `options` {string|Object}
* `persistent` {boolean} Indicates whether the process should continue to run
as long as files are being watched. **Default:** `true`
* `recursive` {boolean} Indicates whether all subdirectories should be
watched, or only the current directory. 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`
* `eventType` {string}
* `filename` {string|Buffer}
Watch for changes on `filename`, where `filename` is either a file or a
directory. The returned object is a [`fs.FSWatcher`][].
The second argument is optional. If `options` is provided as a string, it
specifies the `encoding`. Otherwise `options` should be passed as an object.
The listener callback gets two arguments `(eventType, filename)`. `eventType` is either
`'rename'` or `'change'`, and `filename` is the name of the file which triggered
the event.
Note that on most platforms, `'rename'` is emitted whenever a filename appears
or disappears in the directory.
Also note the listener callback is attached to the `'change'` event fired by
[`fs.FSWatcher`][], but it is not the same thing as the `'change'` value of
`eventType`.
### Caveats
<!--type=misc-->
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
<!--type=misc-->
This feature depends on the underlying operating system providing a way
to be notified of filesystem changes.
* On Linux systems, this uses [`inotify`]
* On BSD systems, this uses [`kqueue`]
* On macOS, this uses [`kqueue`] for files and [`FSEvents`] for directories.
* On SunOS systems (including Solaris and SmartOS), this uses [`event ports`].
* On Windows systems, this feature depends on [`ReadDirectoryChangesW`].
* On Aix systems, this feature depends on [`AHAFS`], which must be enabled.
If the underlying functionality is not available for some reason, then
`fs.watch` will not be able to function. For example, watching files or
directories can be unreliable, and in some cases impossible, on network file
systems (NFS, SMB, etc), or host file systems when using virtualization software
such as Vagrant, Docker, etc.
It is still possible to use `fs.watchFile()`, which uses stat polling, but
this method is slower and less reliable.
#### Inodes
<!--type=misc-->
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
<!--type=misc-->
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.
```js
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)
<!-- YAML
added: v0.1.31
changes:
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `filename` parameter can be a WHATWG `URL` object using
`file:` protocol. Support is currently still *experimental*.
-->
* `filename` {string|Buffer|URL}
* `options` {Object}
* `persistent` {boolean} **Default:** `true`
* `interval` {integer} **Default:** `5007`
* `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 default is
`{ persistent: true, interval: 5007 }`.
The `listener` gets two arguments the current stat object and the previous
stat object:
```js
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`.
*Note*: When an `fs.watchFile` operation results in an `ENOENT` error, it
will invoke the listener once, with all the fields zeroed (or, for dates, the
Unix Epoch). In Windows, `blksize` and `blocks` fields will be `undefined`,
instead of zero. If the file is created later on, the listener will be called
again, with the latest stat objects. This is a change in functionality since
v0.10.
*Note*: [`fs.watch()`][] is more efficient than `fs.watchFile` and
`fs.unwatchFile`. `fs.watch` should be used instead of `fs.watchFile` and
`fs.unwatchFile` when possible.
*Note:* 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)
<!-- YAML
added: v0.0.2
changes:
- version: v7.4.0
pr-url: https://github.com/nodejs/node/pull/10382
description: The `buffer` parameter can now be a `Uint8Array`.
- version: v7.2.0
pr-url: https://github.com/nodejs/node/pull/7856
description: The `offset` and `length` parameters are optional now.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `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.
Note that it is unsafe to use `fs.write` multiple times on the same file
without waiting for the callback. For this scenario,
`fs.createWriteStream` is strongly recommended.
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
## fs.write(fd, string[, position[, encoding]], callback)
<!-- YAML
added: v0.11.5
changes:
- version: v7.2.0
pr-url: https://github.com/nodejs/node/pull/7856
description: The `position` parameter is optional now.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
-->
* `fd` {integer}
* `string` {string}
* `position` {integer}
* `encoding` {string}
* `callback` {Function}
* `err` {Error}
* `written` {integer}
* `string` {string}
Write `string` to the file specified by `fd`. If `string` is not a string, then
the value will be coerced to one.
`position` refers to the offset from the beginning of the file where this data
should be written. If `typeof position !== 'number'` the data will be written at
the current position. See pwrite(2).
`encoding` is the expected string encoding.
The callback will receive the arguments `(err, written, string)` where `written`
specifies how many _bytes_ the passed string required to be written. Note that
bytes written is not the same as string characters. See [`Buffer.byteLength`][].
Unlike when writing `buffer`, the entire string must be written. No substring
may be specified. This is because the byte offset of the resulting data may not
be the same as the string offset.
Note that it is unsafe to use `fs.write` multiple times on the same file
without waiting for the callback. For this scenario,
`fs.createWriteStream` is strongly recommended.
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
## fs.writeFile(file, data[, options], callback)
<!-- YAML
added: v0.1.29
changes:
- version: v7.4.0
pr-url: https://github.com/nodejs/node/pull/10382
description: The `data` parameter can now be a `Uint8Array`.
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/7897
description: The `callback` parameter is no longer optional. Not passing
it will emit a deprecation warning.
- version: v5.0.0
pr-url: https://github.com/nodejs/node/pull/3163
description: The `file` parameter can be a file descriptor now.
-->
* `file` {string|Buffer|URL|integer} filename or file descriptor
* `data` {string|Buffer|Uint8Array}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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. It defaults
to `'utf8'`.
Example:
```js
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:
```js
fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
```
Any specified file descriptor has to support writing.
Note that it is unsafe to use `fs.writeFile` multiple times on the same file
without waiting for the callback. For this scenario,
`fs.createWriteStream` is strongly recommended.
*Note*: If a file descriptor is specified as the `file`, it will not be closed
automatically.
## fs.writeFileSync(file, data[, options])
<!-- YAML
added: v0.1.29
changes:
- version: v7.4.0
pr-url: https://github.com/nodejs/node/pull/10382
description: The `data` parameter can now be a `Uint8Array`.
- version: v5.0.0
pr-url: https://github.com/nodejs/node/pull/3163
description: The `file` parameter can be a file descriptor now.
-->
* `file` {string|Buffer|URL|integer} filename or file descriptor
* `data` {string|Buffer|Uint8Array}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **Default:** `'w'`
The synchronous version of [`fs.writeFile()`][]. Returns `undefined`.
## fs.writeSync(fd, buffer[, offset[, length[, position]]])
<!-- YAML
added: v0.1.21
changes:
- version: v7.4.0
pr-url: https://github.com/nodejs/node/pull/10382
description: The `buffer` parameter can now be a `Uint8Array`.
- version: v7.2.0
pr-url: https://github.com/nodejs/node/pull/7856
description: The `offset` and `length` parameters are optional now.
-->
* `fd` {integer}
* `buffer` {Buffer|Uint8Array}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
## fs.writeSync(fd, string[, position[, encoding]])
<!-- YAML
added: v0.11.5
changes:
- version: v7.2.0
pr-url: https://github.com/nodejs/node/pull/7856
description: The `position` parameter is optional now.
-->
* `fd` {integer}
* `string` {string}
* `position` {integer}
* `encoding` {string}
Synchronous versions of [`fs.write()`][]. Returns the number of bytes written.
## fs Promises API
> Stability: 1 - Experimental
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 `fs.promises`.
### class: FileHandle
<!-- YAML
added: REPLACEME
-->
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
`fs.promises.open()` method.
Unlike callback-based such as `fs.fstat()`, `fs.fchown()`, `fs.fchmod()`,
`fs.ftruncate()`, `fs.read()`, and `fs.write()`, operations -- all of which
use a simple numeric file descriptor, all `fs.promises.*` variations use the
`FileHandle` class in order to help protect against accidental leaking of
unclosed file descriptors after a `Promise` is resolved or rejected.
#### filehandle.fd
<!-- YAML
added: REPLACEME
-->
Value: {number} The numeric file descriptor managed by the `FileHandle` object.
#### filehandle.appendFile(data, options)
<!-- YAML
added: REPLACEME
-->
* `data` {string|Buffer}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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)
<!-- YAML
added: REPLACEME
-->
* `mode` {integer}
* Returns: {Promise}
Modifies the permissions on the file. The `Promise` is resolved with no
arguments upon success.
#### filehandle.chown(uid, gid)
<!-- YAML
added: REPLACEME
-->
* `uid` {integer}
* `gid` {integer}
* Returns: {Promise}
Changes the ownership of the file then resolves the `Promise` with no arguments
upon success.
#### filehandle.close()
<!-- YAML
added: REPLACEME
-->
* 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.
```js
async function openAndClose() {
let filehandle;
try {
filehandle = await fs.promises.open('thefile.txt', 'r');
} finally {
if (filehandle !== undefined)
await filehandle.close();
}
}
```
#### filehandle.datasync()
<!-- YAML
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-->
* Returns: {Promise}
Asynchronous fdatasync(2). The `Promise` is resolved with no arguments upon
success.
#### filehandle.read(buffer, offset, length, position)
<!-- YAML
added: REPLACEME
-->
* `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)
<!-- YAML
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-->
* `options` {Object|string}
* `encoding` {string|null} **Default:** `null`
* `flag` {string} **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 `fs.promises.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()
<!-- YAML
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-->
* Returns: {Promise}
Retrieves the [`fs.Stats`][] for the file.
#### filehandle.sync()
<!-- YAML
added: REPLACEME
-->
* Returns: {Promise}
Asynchronous fsync(2). The `Promise` is resolved with no arguments upon
success.
#### filehandle.truncate(len = 0)
<!-- YAML
added: REPLACEME
-->
* `len` {integer} **Default:** `0`
* Returns: {Promise}
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:
```js
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
async function doTruncate() {
const fd = await fs.promises.open('temp.txt', 'r+');
await fs.promises.ftruncate(fd, 4);
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,
```js
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
async function doTruncate() {
const fd = await fs.promises.open('temp.txt', 'r+');
await fs.promises.ftruncate(fd, 10);
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)
<!-- YAML
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-->
* `atime` {number|string|Date}
* `mtime` {number|string|Date}`
* Returns: {Promise}
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)
<!-- YAML
added: REPLACEME
-->
* `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)
<!-- YAML
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-->
* `data` {string|Buffer|Uint8Array}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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. It defaults
to `'utf8'`.
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).
### fs.promises.access(path[, mode])
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `fs.constants.F_OK`
* Returns: {Promise}
Tests a user's permissions for the file or directory specified by `path`.
The `mode` argument is an optional integer that specifies the accessibility
checks to be performed. The following constants define the possible values of
`mode`. It is possible to create a mask consisting of the bitwise OR of two or
more values (e.g. `fs.constants.W_OK | fs.constants.R_OK`).
* `fs.constants.F_OK` - `path` is visible to the calling process. This is useful
for determining if a file exists, but says nothing about `rwx` permissions.
Default if no `mode` is specified.
* `fs.constants.R_OK` - `path` can be read by the calling process.
* `fs.constants.W_OK` - `path` can be written by the calling process.
* `fs.constants.X_OK` - `path` can be executed by the calling process. This has
no effect on Windows (will behave like `fs.constants.F_OK`).
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.
```js
fs.promises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
.then(() => console.log('can access'))
.catch(() => console.error('cannot access'));
```
Using `fs.promises.access()` to check for the accessibility of a file before
calling `fs.promises.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.
### fs.promises.appendFile(file, data[, options])
<!-- YAML
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-->
* `file` {string|Buffer|[FileHandle][]} filename or `FileHandle`
* `data` {string|Buffer}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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 `file` may be specified as a `FileHandle` that has been opened
for appending (using `fs.promises.open()`).
### fs.promises.chmod(path, mode)
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* `mode` {integer}
* Returns: {Promise}
Changes the permissions of a file then resolves the `Promise` with no
arguments upon succces.
### fs.promises.chown(path, uid, gid)
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* `uid` {integer}
* `gid` {integer}
* Returns: {Promise}
Changes the ownership of a file then resolves the `Promise` with no arguments
upon success.
### fs.promises.copyFile(src, dest[, flags])
<!-- YAML
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-->
* `src` {string|Buffer|URL} source filename to copy
* `dest` {string|Buffer|URL} destination filename of the copy operation
* `flags` {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. The only supported flag is `fs.constants.COPYFILE_EXCL`,
which causes the copy operation to fail if `dest` already exists.
Example:
```js
const fs = require('fs');
// destination.txt will be created or overwritten by default.
fs.promises.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.
```js
const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.promises.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'));
```
### fs.promises.fchmod(filehandle, mode)
<!-- YAML
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-->
* `filehandle` {[FileHandle][]}
* `mode` {integer}
* Returns: {Promise}
Asynchronous fchmod(2). The `Promise` is resolved with no arguments upon
success.
### fs.promises.fchown(filehandle, uid, gid)
<!-- YAML
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-->
* `filehandle` {[FileHandle][]}
* `uid` {integer}
* `gid` {integer}
* Returns: {Promise}
Changes the ownership of the file represented by `filehandle` then resolves
the `Promise` with no arguments upon success.
### fs.promises.fdatasync(filehandle)
<!-- YAML
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-->
* `filehandle` {[FileHandle][]}
* Returns: {Promise}
Asynchronous fdatasync(2). The `Promise` is resolved with no arguments upon
success.
### fs.promises.fstat(filehandle)
<!-- YAML
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-->
* `filehandle` {[FileHandle][]}
* Returns: {Promise}
Retrieves the [`fs.Stats`][] for the given `filehandle`.
### fs.promises.fsync(filehandle)
<!-- YAML
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-->
* `filehandle` {[FileHandle][]}
* Returns: {Promise}
Asynchronous fsync(2). The `Promise` is resolved with no arguments upon
success.
### fs.promises.ftruncate(filehandle[, len])
<!-- YAML
added: REPLACEME
-->
* `filehandle` {[FileHandle][]}
* `len` {integer} **Default:** `0`
* Returns: {Promise}
Truncates the file represented by `filehandle` then resolves the `Promise`
with no arguments upon success.
If the file referred to by the `FileHandle` 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:
```js
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
async function doTruncate() {
const fd = await fs.promises.open('temp.txt', 'r+');
await fs.promises.ftruncate(fd, 4);
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,
```js
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
async function doTruncate() {
const fd = await fs.promises.open('temp.txt', 'r+');
await fs.promises.ftruncate(fd, 10);
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.
### fs.promises.futimes(filehandle, atime, mtime)
<!-- YAML
added: REPLACEME
-->
* `filehandle` {[FileHandle][]}
* `atime` {number|string|Date}
* `mtime` {number|string|Date}`
* Returns: {Promise}
Change the file system timestamps of the object referenced by the supplied
`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`.
### fs.promises.lchmod(path, mode)
<!-- YAML
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-->
* `path` {string|Buffer}
* `mode` {integer}
* Returns: {Promise}
Changes the permissions on a symbolic link then resolves the `Promise` with
no arguments upon success. This method is only implemented on macOS.
### fs.promises.lchown(path, uid, gid)
<!-- YAML
deprecated: REPLACEME
-->
* `path` {string|Buffer}
* `uid` {integer}
* `gid` {integer}
* Returns: {Promise}
Changes the ownership on a symbolic link then resolves the `Promise` with
no arguments upon success. This method is only implemented on macOS.
### fs.promises.link(existingPath, newPath)
<!-- YAML
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-->
* `existingPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
* Returns: {Promise}
Asynchronous link(2). The `Promise` is resolved with no arguments upon success.
### fs.promises.lstat(path)
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* Returns: {Promise}
Asynchronous lstat(2). The `Promise` is resolved with the [`fs.Stats`][] object
for the given symbolic link `path`.
### fs.promises.mkdir(path[, mode])
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `0o777`
* Returns: {Promise}
Asynchronously creates a directory then resolves the `Promise` with no
arguments upon success.
### fs.promises.mkdtemp(prefix[, options])
<!-- YAML
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-->
* `prefix` {string}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* Returns: {Promise}
Creates a unique temporary directory then 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.
Example:
```js
fs.promises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
.catch(console.error);
```
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`).
### fs.promises.open(path, flags[, mode])
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* `flags` {string|number}
* `mode` {integer} **Default:** `0o666`
* Return: {Promise}
Asynchronous file open that returns a `Promise` that, when resolved, yields a
`FileHandle` object. See open(2).
The `flags` argument can be:
* `'r'` - Open file for reading.
An exception occurs if the file does not exist.
* `'r+'` - Open file for reading and writing.
An exception occurs if the file does not exist.
* `'rs+'` - Open file for reading and writing in synchronous mode. Instructs
the operating system to bypass the local file system cache.
This is primarily useful for opening files on NFS mounts as it allows 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.
Note that this does not turn `fs.promises.open()` into a synchronous blocking
call.
* `'w'` - Open file for writing.
The file is created (if it does not exist) or truncated (if it exists).
* `'wx'` - Like `'w'` but fails if `path` exists.
* `'w+'` - Open file for reading and writing.
The file is created (if it does not exist) or truncated (if it exists).
* `'wx+'` - Like `'w+'` but fails if `path` exists.
* `'a'` - Open file for appending.
The file is created if it does not exist.
* `'ax'` - Like `'a'` but fails if `path` exists.
* `'a+'` - Open file for reading and appending.
The file is created if it does not exist.
* `'ax+'` - Like `'a+'` but fails if `path` exists.
`mode` sets the file mode (permission and sticky bits), but only if the file was
created. It defaults to `0o666` (readable and writable).
The exclusive flag `'x'` (`O_EXCL` flag in open(2)) ensures that `path` is newly
created. On POSIX systems, `path` is considered to exist even if it is a symlink
to a non-existent file. The exclusive flag may or may not work with network file
systems.
`flags` can also be a number as documented by open(2); commonly used constants
are available from `fs.constants`. On Windows, flags are translated to
their equivalent ones where applicable, e.g. `O_WRONLY` to `FILE_GENERIC_WRITE`,
or `O_EXCL|O_CREAT` to `CREATE_NEW`, as accepted by CreateFileW.
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
The behavior of `fs.promises.open()` is platform-specific for some
flags. As such, opening a directory on macOS and Linux with the `'a+'` flag will
return an error. In contrast, on Windows and FreeBSD, a `FileHandle` will be
returned.
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][MSDN-Using-Streams].
*Note:* On Windows, opening an existing hidden file using the `w` flag (e.g.
using `fs.promises.open()`) will fail with `EPERM`. Existing hidden
files can be opened for writing with the `r+` flag. A call to
`fs.promises.ftruncate()` can be used to reset the file contents.
### fs.promises.read(filehandle, buffer, offset, length, position)
<!-- YAML
added: REPLACEME
-->
* `filehandle` {[FileHandle][]}
* `buffer` {Buffer|Uint8Array}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
* Returns: {Promise}
Read data from the file specified by `filehandle`.
`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.
### fs.promises.readdir(path[, options])
<!-- YAML
added: REPLACEME
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* Returns: {Promise}
Reads the contents of a directory then resolves the `Promise` with an array
of the names of the files in the directory excludiing `'.'` 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.
### fs.promises.readFile(path[, options])
<!-- YAML
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-->
* `path` {string|Buffer|URL|[FileHandle][]} filename or `FileHandle`
* `options` {Object|string}
* `encoding` {string|null} **Default:** `null`
* `flag` {string} **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 `fs.promises.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.
### fs.promises.readlink(path[, options])
<!-- YAML
added: REPLACEME
-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* Returns: {Promise}
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.
### fs.promises.realpath(path[, options])
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* `options` {string|Object}
* `encoding` {string} **Default:** `'utf8'`
* Returns: {Promise}
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.
### fs.promises.rename(oldPath, newPath)
<!-- YAML
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-->
* `oldPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
* Returns: {Promise}
Renames `oldPath` to `newPath` and resolves the `Promise` with no arguments
upon success.
### fs.promises.rmdir(path)
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* Returns: {Promise}
Removes the directory identified by `path` then resolves the `Promise` with
no arguments upon success.
Using `fs.promises.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.
### fs.promises.stat(path)
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* Returns: {Promise}
The `Promise` is resolved with the [`fs.Stats`][] object for the given `path`.
### fs.promises.symlink(target, path[, type])
<!-- YAML
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-->
* `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'` (default is `'file'`). Note that Windows junction
points require the destination path to be absolute. When using `'junction'`,
the `target` argument will automatically be normalized to absolute path.
### fs.promises.truncate(path[, len])
<!-- YAML
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-->
* `path` {string|Buffer}
* `len` {integer} **Default:** `0`
* Returns: {Promise}
Truncates the `path` then resolves the `Promise` with no arguments upon
success. The `path` *must* be a string or `Buffer`.
### fs.promises.unlink(path)
<!-- YAML
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-->
* `path` {string|Buffer|URL}
* Returns: {Promise}
Asynchronous unlink(2). The `Promise` is resolved with no arguments upon
success.
### fs.promises.utimes(path, atime, mtime)
<!-- YAML
added: REPLACEME
-->
* `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`, an `Error` will be thrown.
### fs.promises.write(filehandle, buffer[, offset[, length[, position]]])
<!-- YAML
added: REPLACEME
-->
* `filehandle` {[FileHandle][]}
* `buffer` {Buffer|Uint8Array}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
* Returns: {Promise}
Write `buffer` to the file specified by `filehandle`.
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 `fs.promises.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.
### fs.promises.writeFile(file, data[, options])
<!-- YAML
added: REPLACEME
-->
* `file` {string|Buffer|[FileHandle][]} filename or `FileHandle`
* `data` {string|Buffer|Uint8Array}
* `options` {Object|string}
* `encoding` {string|null} **Default:** `'utf8'`
* `mode` {integer} **Default:** `0o666`
* `flag` {string} **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. It defaults
to `'utf8'`.
If `options` is a string, then it specifies the encoding.
Any specified `FileHandle` has to support writing.
It is unsafe to use `fs.promises.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`.
*Note*: Not every constant will be available on every operating system.
### File Access Constants
The following constants are meant for use with [`fs.access()`][].
<table>
<tr>
<th>Constant</th>
<th>Description</th>
</tr>
<tr>
<td><code>F_OK</code></td>
<td>Flag indicating that the file is visible to the calling process.</td>
</tr>
<tr>
<td><code>R_OK</code></td>
<td>Flag indicating that the file can be read by the calling process.</td>
</tr>
<tr>
<td><code>W_OK</code></td>
<td>Flag indicating that the file can be written by the calling
process.</td>
</tr>
<tr>
<td><code>X_OK</code></td>
<td>Flag indicating that the file can be executed by the calling
process.</td>
</tr>
</table>
### File Open Constants
The following constants are meant for use with `fs.open()`.
<table>
<tr>
<th>Constant</th>
<th>Description</th>
</tr>
<tr>
<td><code>O_RDONLY</code></td>
<td>Flag indicating to open a file for read-only access.</td>
</tr>
<tr>
<td><code>O_WRONLY</code></td>
<td>Flag indicating to open a file for write-only access.</td>
</tr>
<tr>
<td><code>O_RDWR</code></td>
<td>Flag indicating to open a file for read-write access.</td>
</tr>
<tr>
<td><code>O_CREAT</code></td>
<td>Flag indicating to create the file if it does not already exist.</td>
</tr>
<tr>
<td><code>O_EXCL</code></td>
<td>Flag indicating that opening a file should fail if the
<code>O_CREAT</code> flag is set and the file already exists.</td>
</tr>
<tr>
<td><code>O_NOCTTY</code></td>
<td>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).</td>
</tr>
<tr>
<td><code>O_TRUNC</code></td>
<td>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.</td>
</tr>
<tr>
<td><code>O_APPEND</code></td>
<td>Flag indicating that data will be appended to the end of the file.</td>
</tr>
<tr>
<td><code>O_DIRECTORY</code></td>
<td>Flag indicating that the open should fail if the path is not a
directory.</td>
</tr>
<tr>
<td><code>O_NOATIME</code></td>
<td>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.</td>
</tr>
<tr>
<td><code>O_NOFOLLOW</code></td>
<td>Flag indicating that the open should fail if the path is a symbolic
link.</td>
</tr>
<tr>
<td><code>O_SYNC</code></td>
<td>Flag indicating that the file is opened for synchronized I/O with write
operations waiting for file integrity.</td>
</tr>
<tr>
<td><code>O_DSYNC</code></td>
<td>Flag indicating that the file is opened for synchronized I/O with write
operations waiting for data integrity.</td>
</tr>
<tr>
<td><code>O_SYMLINK</code></td>
<td>Flag indicating to open the symbolic link itself rather than the
resource it is pointing to.</td>
</tr>
<tr>
<td><code>O_DIRECT</code></td>
<td>When set, an attempt will be made to minimize caching effects of file
I/O.</td>
</tr>
<tr>
<td><code>O_NONBLOCK</code></td>
<td>Flag indicating to open the file in nonblocking mode when possible.</td>
</tr>
</table>
### File Type Constants
The following constants are meant for use with the [`fs.Stats`][] object's
`mode` property for determining a file's type.
<table>
<tr>
<th>Constant</th>
<th>Description</th>
</tr>
<tr>
<td><code>S_IFMT</code></td>
<td>Bit mask used to extract the file type code.</td>
</tr>
<tr>
<td><code>S_IFREG</code></td>
<td>File type constant for a regular file.</td>
</tr>
<tr>
<td><code>S_IFDIR</code></td>
<td>File type constant for a directory.</td>
</tr>
<tr>
<td><code>S_IFCHR</code></td>
<td>File type constant for a character-oriented device file.</td>
</tr>
<tr>
<td><code>S_IFBLK</code></td>
<td>File type constant for a block-oriented device file.</td>
</tr>
<tr>
<td><code>S_IFIFO</code></td>
<td>File type constant for a FIFO/pipe.</td>
</tr>
<tr>
<td><code>S_IFLNK</code></td>
<td>File type constant for a symbolic link.</td>
</tr>
<tr>
<td><code>S_IFSOCK</code></td>
<td>File type constant for a socket.</td>
</tr>
</table>
### 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.
<table>
<tr>
<th>Constant</th>
<th>Description</th>
</tr>
<tr>
<td><code>S_IRWXU</code></td>
<td>File mode indicating readable, writable, and executable by owner.</td>
</tr>
<tr>
<td><code>S_IRUSR</code></td>
<td>File mode indicating readable by owner.</td>
</tr>
<tr>
<td><code>S_IWUSR</code></td>
<td>File mode indicating writable by owner.</td>
</tr>
<tr>
<td><code>S_IXUSR</code></td>
<td>File mode indicating executable by owner.</td>
</tr>
<tr>
<td><code>S_IRWXG</code></td>
<td>File mode indicating readable, writable, and executable by group.</td>
</tr>
<tr>
<td><code>S_IRGRP</code></td>
<td>File mode indicating readable by group.</td>
</tr>
<tr>
<td><code>S_IWGRP</code></td>
<td>File mode indicating writable by group.</td>
</tr>
<tr>
<td><code>S_IXGRP</code></td>
<td>File mode indicating executable by group.</td>
</tr>
<tr>
<td><code>S_IRWXO</code></td>
<td>File mode indicating readable, writable, and executable by others.</td>
</tr>
<tr>
<td><code>S_IROTH</code></td>
<td>File mode indicating readable by others.</td>
</tr>
<tr>
<td><code>S_IWOTH</code></td>
<td>File mode indicating writable by others.</td>
</tr>
<tr>
<td><code>S_IXOTH</code></td>
<td>File mode indicating executable by others.</td>
</tr>
</table>
[`AHAFS`]: https://www.ibm.com/developerworks/aix/library/au-aix_event_infrastructure/
[`Buffer.byteLength`]: buffer.html#buffer_class_method_buffer_bytelength_string_encoding
[`Buffer`]: buffer.html#buffer_buffer
[`FSEvents`]: https://developer.apple.com/library/mac/documentation/Darwin/Conceptual/FSEvents_ProgGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40005289-CH1-SW1
[`ReadDirectoryChangesW`]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365465%28v=vs.85%29.aspx
[`ReadStream`]: #fs_class_fs_readstream
[`URL`]: url.html#url_the_whatwg_url_api
[`UV_THREADPOOL_SIZE`]: cli.html#cli_uv_threadpool_size_size
[`WriteStream`]: #fs_class_fs_writestream
[`EventEmitter`]: events.html
[`event ports`]: http://illumos.org/man/port_create
[`fs.FSWatcher`]: #fs_class_fs_fswatcher
[`fs.Stats`]: #fs_class_fs_stats
[`fs.access()`]: #fs_fs_access_path_mode_callback
[`fs.chmod()`]: #fs_fs_chmod_path_mode_callback
[`fs.chown()`]: #fs_fs_chown_path_uid_gid_callback
[`fs.exists()`]: fs.html#fs_fs_exists_path_callback
[`fs.fstat()`]: #fs_fs_fstat_fd_callback
[`fs.futimes()`]: #fs_fs_futimes_fd_atime_mtime_callback
[`fs.lstat()`]: #fs_fs_lstat_path_callback
[`fs.mkdir()`]: #fs_fs_mkdir_path_mode_callback
[`fs.mkdtemp()`]: #fs_fs_mkdtemp_prefix_options_callback
[`fs.open()`]: #fs_fs_open_path_flags_mode_callback
[`fs.read()`]: #fs_fs_read_fd_buffer_offset_length_position_callback
[`fs.readFile()`]: #fs_fs_readfile_path_options_callback
[`fs.readFileSync()`]: #fs_fs_readfilesync_path_options
[`fs.stat()`]: #fs_fs_stat_path_callback
[`fs.utimes()`]: #fs_fs_utimes_path_atime_mtime_callback
[`fs.watch()`]: #fs_fs_watch_filename_options_listener
[`fs.write()`]: #fs_fs_write_fd_buffer_offset_length_position_callback
[`fs.writeFile()`]: #fs_fs_writefile_file_data_options_callback
[`inotify`]: http://man7.org/linux/man-pages/man7/inotify.7.html
[`kqueue`]: https://www.freebsd.org/cgi/man.cgi?kqueue
[`net.Socket`]: net.html#net_class_net_socket
[`stat()`]: fs.html#fs_fs_stat_path_callback
[`util.promisify()`]: util.html#util_util_promisify_original
[Caveats]: #fs_caveats
[Common System Errors]: errors.html#errors_common_system_errors
[FileHandle]: #fs_class_filehandle
[FS Constants]: #fs_fs_constants_1
[MDN-Date]: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Date
[MDN-Number]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type
[MSDN-Rel-Path]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx#fully_qualified_vs._relative_paths
[Readable Streams]: stream.html#stream_class_stream_readable
[Writable Stream]: stream.html#stream_class_stream_writable
[inode]: https://en.wikipedia.org/wiki/Inode
[Naming Files, Paths, and Namespaces]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
[MSDN-Using-Streams]: https://msdn.microsoft.com/en-us/library/windows/desktop/bb540537.aspx
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