Elep

# File System

> Stability: 2 - Stable

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`.

```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.

```js
const fs = require('fs');

try {
  fs.unlinkSync('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (err) {
  // handle the error
}
```

There is no guaranteed ordering when using asynchronous methods. So the
following is prone to error because the `fs.stat()` operation may complete
before the `fs.rename()` operation:

```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:

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;
  });
});
```

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

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);
```

`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
```

`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

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.Dirent

When [`fs.readdir()`][] or [`fs.readdirSync()`][] is called with the
`withFileTypes` option set to `true`, the resulting array is filled with
`fs.Dirent` objects, rather than strings or `Buffers`.

### dirent.isBlockDevice()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a block device.

### dirent.isCharacterDevice()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a character device.

### dirent.isDirectory()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a file system
directory.

### dirent.isFIFO()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a first-in-first-out
(FIFO) pipe.

### dirent.isFile()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a regular file.

### dirent.isSocket()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a socket.

### dirent.isSymbolicLink()

* Returns: {boolean}

Returns `true` if the `fs.Dirent` object describes a symbolic link.

### dirent.name

* {string|Buffer}

The file name that this `fs.Dirent` object refers to. The type of this
value is determined by the `options.encoding` passed to [`fs.readdir()`][] or
[`fs.readdirSync()`][].

## Class: fs.FSWatcher

A successful call to [`fs.watch()`][] method will return a new `fs.FSWatcher`
object.

All `fs.FSWatcher` objects are [`EventEmitter`][]'s that will emit a `'change'`
event whenever a specific watched file is modified.

### Event: 'change'

* `eventType` {string} The type of change event that has 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: 'close'

Emitted when the watcher stops watching for changes. The closed
`fs.FSWatcher` object is no longer usable in the event handler.

### Event: 'error'

* `error` {Error}

Emitted when an error occurs while watching the file. The errored
`fs.FSWatcher` object is no longer usable in the event handler.

### watcher.close()

Stop watching for changes on the given `fs.FSWatcher`. Once stopped, the
`fs.FSWatcher` object is no longer usable.

## Class: fs.ReadStream

A successful call to `fs.createReadStream()` will return a new `fs.ReadStream`
object.

All `fs.ReadStream` objects are [Readable Streams][].

### Event: 'close'

Emitted when the `fs.ReadStream`'s underlying file descriptor has been closed.

### Event: 'open'

* `fd` {integer} Integer file descriptor used by the `ReadStream`.

Emitted when the `fs.ReadStream`'s file descriptor has been opened.

### Event: 'ready'

Emitted when the `fs.ReadStream` is ready to be used.

Fires immediately after `'open'`.

### readStream.bytesRead

* {number}

The number of bytes that have been read so far.

### readStream.path

* {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`.

### readStream.pending

* {boolean}

This property is `true` if the underlying file has not been opened yet,
i.e. before the `'ready'` event is emitted.

## Class: fs.Stats

A `fs.Stats` object provides information about a file.

Objects returned from [`fs.stat()`][], [`fs.lstat()`][] and [`fs.fstat()`][] and
their synchronous counterparts are of this type.
If `bigint` in the `options` passed to those methods is true, the numeric values
will be `bigint` instead of `number`.

```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 }
```

`bigint` version:

```console
Stats {
  dev: 2114n,
  ino: 48064969n,
  mode: 33188n,
  nlink: 1n,
  uid: 85n,
  gid: 100n,
  rdev: 0n,
  size: 527n,
  blksize: 4096n,
  blocks: 8n,
  atimeMs: 1318289051000n,
  mtimeMs: 1318289051000n,
  ctimeMs: 1318289051000n,
  birthtimeMs: 1318289051000n,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
```

### stats.isBlockDevice()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a block device.

### stats.isCharacterDevice()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a character device.

### stats.isDirectory()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a file system directory.

### stats.isFIFO()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a first-in-first-out (FIFO)
pipe.

### stats.isFile()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a regular file.

### stats.isSocket()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a socket.

### stats.isSymbolicLink()

* Returns: {boolean}

Returns `true` if the `fs.Stats` object describes a symbolic link.

This method is only valid when using [`fs.lstat()`][].

### stats.dev

* {number|bigint}

The numeric identifier of the device containing the file.

### stats.ino

* {number|bigint}

The file system specific "Inode" number for the file.

### stats.mode

* {number|bigint}

A bit-field describing the file type and mode.

### stats.nlink

* {number|bigint}

The number of hard-links that exist for the file.

### stats.uid

* {number|bigint}

The numeric user identifier of the user that owns the file (POSIX).

### stats.gid

* {number|bigint}

The numeric group identifier of the group that owns the file (POSIX).

### stats.rdev

* {number|bigint}

A numeric device identifier if the file is considered "special".

### stats.size

* {number|bigint}

The size of the file in bytes.

### stats.blksize

* {number|bigint}

The file system block size for i/o operations.

### stats.blocks

* {number|bigint}

The number of blocks allocated for this file.

### stats.atimeMs

* {number|bigint}

The timestamp indicating the last time this file was accessed expressed in
milliseconds since the POSIX Epoch.

### stats.mtimeMs

* {number|bigint}

The timestamp indicating the last time this file was modified expressed in
milliseconds since the POSIX Epoch.

### stats.ctimeMs

* {number|bigint}

The timestamp indicating the last time the file status was changed expressed
in milliseconds since the POSIX Epoch.

### stats.birthtimeMs

* {number|bigint}

The timestamp indicating the creation time of this file expressed in
milliseconds since the POSIX Epoch.

### stats.atime

* {Date}

The timestamp indicating the last time this file was accessed.

### stats.mtime

* {Date}

The timestamp indicating the last time this file was modified.

### stats.ctime

* {Date}

The timestamp indicating the last time the file status was changed.

### stats.birthtime

* {Date}

The timestamp indicating the creation time of this file.

### Stat Time Values

The `atimeMs`, `mtimeMs`, `ctimeMs`, `birthtimeMs` properties 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`). 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.js 0.12, the `ctime` held the `birthtime` on Windows systems. As
of 0.12, `ctime` is not "creation time", and on Unix systems, it never was.

## Class: fs.WriteStream

`WriteStream` is a [Writable Stream][].

### Event: 'close'

Emitted when the `WriteStream`'s underlying file descriptor has been closed.

### Event: 'open'

* `fd` {integer} Integer file descriptor used by the `WriteStream`.

Emitted when the `WriteStream`'s file is opened.

### Event: 'ready'

Emitted when the `fs.WriteStream` is ready to be used.

Fires immediately after `'open'`.

### writeStream.bytesWritten

The number of bytes written so far. Does not include data that is still queued
for writing.

### writeStream.path

The path to the file the stream is writing to as specified in the first
argument to [`fs.createWriteStream()`][]. If `path` is passed as a string, then
`writeStream.path` will be a string. If `path` is passed as a `Buffer`, then
`writeStream.path` will be a `Buffer`.

### writeStream.pending

* {boolean}

This property is `true` if the underlying file has not been opened yet,
i.e. before the `'ready'` event is emitted.

## fs.access(path[, mode], callback)

* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `fs.constants.F_OK`
* `callback` {Function}
  * `err` {Error}

Tests a user's permissions for the file or directory specified by `path`.
The `mode` argument is an optional integer that specifies the accessibility
checks to be performed. Check [File Access Constants][] for possible values
of `mode`. It is possible to create a mask consisting of the bitwise OR of
two or more values (e.g. `fs.constants.W_OK | fs.constants.R_OK`).

The final argument, `callback`, is a callback function that is invoked with
a possible error argument. If any of the accessibility checks fail, the error
argument will be an `Error` object. The following examples check if
`package.json` exists, and if it is readable or writable.

```js
const file = 'package.json';

// Check if the file exists in the current directory.
fs.access(file, fs.constants.F_OK, (err) => {
  console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});

// Check if the file is readable.
fs.access(file, fs.constants.R_OK, (err) => {
  console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});

// Check if the file is writable.
fs.access(file, fs.constants.W_OK, (err) => {
  console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});

// Check if the file exists in the current directory, and if it is writable.
fs.access(file, fs.constants.F_OK | fs.constants.W_OK, (err) => {
  if (err) {
    console.error(
      `${file} ${err.code === 'ENOENT' ? 'does not exist' : 'is read-only'}`);
  } else {
    console.log(`${file} exists, and it is writable`);
  }
});
```

Using `fs.access()` to check for the accessibility of a file before calling
`fs.open()`, `fs.readFile()` or `fs.writeFile()` is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file is not accessible.

**write (NOT RECOMMENDED)**

```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.

On Windows, access-control policies (ACLs) on a directory may limit access to
a file or directory. The `fs.access()` function, however, does not check the
ACL and therefore may report that a path is accessible even if the ACL restricts
the user from reading or writing to it.

## fs.accessSync(path[, mode])

* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `fs.constants.F_OK`

Synchronously tests a user's permissions for the file or directory specified
by `path`. The `mode` argument is an optional integer that specifies the
accessibility checks to be performed. Check [File Access Constants][] for
possible values of `mode`. It is possible to create a mask consisting of
the bitwise OR of two or more values
(e.g. `fs.constants.W_OK | fs.constants.R_OK`).

If any of the accessibility checks fail, an `Error` will be thrown. Otherwise,
the method will return `undefined`.

```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(path, data[, options], callback)

* `path` {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} See [support of file system `flags`][]. **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`][].

```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:

```js
fs.appendFile('message.txt', 'data to append', 'utf8', callback);
```

The `path` may be specified as a numeric file descriptor that has been opened
for appending (using `fs.open()` or `fs.openSync()`). The file descriptor will
not be closed automatically.

```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(path, data[, options])

Synchronously append data to a file, creating the file if it does not yet
exist. `data` can be a string or a [`Buffer`][].

```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:

```js
fs.appendFileSync('message.txt', 'data to append', 'utf8');
```

The `path` may be specified as a numeric file descriptor that has been opened
for appending (using `fs.open()` or `fs.openSync()`). The file descriptor will
not be closed automatically.

```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)

* `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: unlink(2).

## fs.unlinkSync(path)

* `path` {string|Buffer|URL}

Synchronous unlink(2). Returns `undefined`.

## fs.unwatchFile(filename[, listener])

* `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.

Using [`fs.watch()`][] is more efficient than `fs.watchFile()` and
`fs.unwatchFile()`. `fs.watch()` should be used instead of `fs.watchFile()`
and `fs.unwatchFile()` when possible.

## fs.utimes(path, atime, mtime, callback)

* `path` {string|Buffer|URL}
* `atime` {number|string|Date}
* `mtime` {number|string|Date}
* `callback` {Function}
  * `err` {Error}

Change the file system timestamps of the object referenced by `path`.

The `atime` and `mtime` arguments follow these rules:
- Values can be either numbers representing Unix epoch time, `Date`s, or a
  numeric string like `'123456789.0'`.
- If the value can not be converted to a number, or is `NaN`, `Infinity` or
  `-Infinity`, an `Error` will be thrown.

## fs.utimesSync(path, atime, mtime)

* `path` {string|Buffer|URL}
* `atime` {integer}
* `mtime` {integer}

Returns `undefined`.

For detailed information, see the documentation of the asynchronous version of
this API: [`fs.utimes()`][].

## fs.watch(filename[, options][, listener])

* `filename` {string|Buffer|URL}
* `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}
* Returns: {fs.FSWatcher}

Watch for changes on `filename`, where `filename` is either a file or a
directory.

The second argument is optional. If `options` is provided as a string, it
specifies the `encoding`. Otherwise `options` should be passed as an object.

The listener callback gets two arguments `(eventType, filename)`. `eventType`
is either `'rename'` or `'change'`, and `filename` is the name of the file
which triggered the event.

On most platforms, `'rename'` is emitted whenever a filename appears or
disappears in the directory.

The listener callback is attached to the `'change'` event fired by
[`fs.FSWatcher`][], but it is not the same thing as the `'change'` value of
`eventType`.

### Caveats

The `fs.watch` API is not 100% consistent across platforms, and is
unavailable in some situations.

The recursive option is only supported on macOS and Windows.

#### Availability

This feature depends on the underlying operating system providing a way
to be notified of filesystem changes.

* On Linux systems, this uses [`inotify(7)`].
* On BSD systems, this uses [`kqueue(2)`].
* On macOS, this uses [`kqueue(2)`] for files 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

On Linux and macOS systems, `fs.watch()` resolves the path to an [inode][] and
watches the inode. If the watched path is deleted and recreated, it is assigned
a new inode. The watch will emit an event for the delete but will continue
watching the *original* inode. Events for the new inode will not be emitted.
This is expected behavior.

AIX files retain the same inode for the lifetime of a file. Saving and closing a
watched file on AIX will result in two notifications (one for adding new
content, and one for truncation).

#### Filename Argument

Providing `filename` argument in the callback is only supported on Linux,
macOS, Windows, and AIX. Even on supported platforms, `filename` is not always
guaranteed to be provided. Therefore, don't assume that `filename` argument is
always provided in the callback, and have some fallback logic if it is `null`.

```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)

* `filename` {string|Buffer|URL}
* `options` {Object}
  * `bigint` {boolean} **Default:** `false`
  * `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 `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`. If the `bigint` option is `true`,
the numeric values in these objects are specified as `BigInt`s.

To be notified when the file was modified, not just accessed, it is necessary
to compare `curr.mtime` and `prev.mtime`.

When an `fs.watchFile` operation results in an `ENOENT` error, it
will invoke the listener once, with all the fields zeroed (or, for dates, the
Unix Epoch). In Windows, `blksize` and `blocks` fields will be `undefined`,
instead of zero. If the file is created later on, the listener will be called
again, with the latest stat objects. This is a change in functionality since
v0.10.

Using [`fs.watch()`][] is more efficient than `fs.watchFile` and
`fs.unwatchFile`. `fs.watch` should be used instead of `fs.watchFile` and
`fs.unwatchFile` when possible.

When a file being watched by `fs.watchFile()` disappears and reappears,
then the `previousStat` reported in the second callback event (the file's
reappearance) will be the same as the `previousStat` of the first callback
event (its disappearance).

This happens when:
- the file is deleted, followed by a restore
- the file is renamed twice - the second time back to its original name

## fs.write(fd, buffer[, offset[, length[, position]]], callback)

* `fd` {integer}
* `buffer` {Buffer|TypedArray|DataView}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
* `callback` {Function}
  * `err` {Error}
  * `bytesWritten` {integer}
  * `buffer` {Buffer|TypedArray|DataView}

Write `buffer` to the file specified by `fd`.

`offset` determines the part of the buffer to be written, and `length` is
an integer specifying the number of bytes to write.

`position` refers to the offset from the beginning of the file where this data
should be written. If `typeof position !== 'number'`, the data will be written
at the current position. See pwrite(2).

The callback will be given three arguments `(err, bytesWritten, buffer)` where
`bytesWritten` specifies how many _bytes_ were written from `buffer`.

If this method is invoked as its [`util.promisify()`][]ed version, it returns
a `Promise` for an `Object` with `bytesWritten` and `buffer` properties.

It is unsafe to use `fs.write()` multiple times on the same file without waiting
for the callback. For this scenario, [`fs.createWriteStream()`][] is
recommended.

On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

## fs.write(fd, string[, position[, encoding]], callback)

* `fd` {integer}
* `string` {string}
* `position` {integer}
* `encoding` {string} **Default:** `'utf8'`
* `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. Bytes
written is not necessarily the same as string characters written. See
[`Buffer.byteLength`][].

It is unsafe to use `fs.write()` multiple times on the same file without waiting
for the callback. For this scenario, [`fs.createWriteStream()`][] is
recommended.

On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

On Windows, if the file descriptor is connected to the console (e.g. `fd == 1`
or `stdout`) a string containing non-ASCII characters will not be rendered
properly by default, regardless of the encoding used.
It is possible to configure the console to render UTF-8 properly by changing the
active codepage with the `chcp 65001` command. See the [chcp][] docs for more
details.

## fs.writeFile(file, data[, options], callback)

* `file` {string|Buffer|URL|integer} filename or file descriptor
* `data` {string|Buffer|TypedArray|DataView}
* `options` {Object|string}
  * `encoding` {string|null} **Default:** `'utf8'`
  * `mode` {integer} **Default:** `0o666`
  * `flag` {string} See [support of file system `flags`][]. **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.

```js
const data = new Uint8Array(Buffer.from('Hello Node.js'));
fs.writeFile('message.txt', data, (err) => {
  if (err) throw err;
  console.log('The file has been saved!');
});
```

If `options` is a string, then it specifies the encoding:

```js
fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
```

It is unsafe to use `fs.writeFile()` multiple times on the same file without
waiting for the callback. For this scenario, [`fs.createWriteStream()`][] is
recommended.

### File Descriptors
1. Any specified file descriptor has to support writing.
2. If a file descriptor is specified as the `file`, it will not be closed
automatically.
3. The writing will begin at the beginning of the file. For example, if the
file already had `'Hello World'` and the newly written content is `'Aloha'`,
then the contents of the file would be `'Aloha World'`, rather than just
`'Aloha'`.

## fs.writeFileSync(file, data[, options])

Returns `undefined`.

For detailed information, see the documentation of the asynchronous version of
this API: [`fs.writeFile()`][].

## fs.writeSync(fd, buffer[, offset[, length[, position]]])

* `fd` {integer}
* `buffer` {Buffer|TypedArray|DataView}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
* Returns: {number} The number of bytes written.

For detailed information, see the documentation of the asynchronous version of
this API: [`fs.write(fd, buffer...)`][].

## fs.writeSync(fd, string[, position[, encoding]])

* `fd` {integer}
* `string` {string}
* `position` {integer}
* `encoding` {string}
* Returns: {number} The number of bytes written.

For detailed information, see the documentation of the asynchronous version of
this API: [`fs.write(fd, string...)`][].

## fs Promises API

> Stability: 2 - Stable

The `fs.promises` API provides an alternative set of asynchronous file system
methods that return `Promise` objects rather than using callbacks. The
API is accessible via `require('fs').promises`.

### class: FileHandle

A `FileHandle` object is a wrapper for a numeric file descriptor.
Instances of `FileHandle` are distinct from numeric file descriptors
in that, if the `FileHandle` is not explicitly closed using the
`filehandle.close()` method, they will automatically close the file descriptor
and will emit a process warning, thereby helping to prevent memory leaks.

Instances of the `FileHandle` object are created internally by the
`fsPromises.open()` method.

Unlike the callback-based API (`fs.fstat()`, `fs.fchown()`, `fs.fchmod()`, and
so on), a numeric file descriptor is not used by the promise-based API. Instead,
the promise-based API uses the `FileHandle` class in order to help avoid
accidental leaking of unclosed file descriptors after a `Promise` is resolved or
rejected.

#### filehandle.appendFile(data, options)

* `data` {string|Buffer}
* `options` {Object|string}
  * `encoding` {string|null} **Default:** `'utf8'`
  * `mode` {integer} **Default:** `0o666`
  * `flag` {string} See [support of file system `flags`][]. **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)

* `mode` {integer}
* Returns: {Promise}

Modifies the permissions on the file. The `Promise` is resolved with no
arguments upon success.

#### filehandle.chown(uid, gid)

* `uid` {integer}
* `gid` {integer}
* Returns: {Promise}

Changes the ownership of the file then resolves the `Promise` with no arguments
upon success.

#### filehandle.close()

* Returns: {Promise} A `Promise` that will be resolved once the underlying
  file descriptor is closed, or will be rejected if an error occurs while
  closing.

Closes the file descriptor.

```js
const fsPromises = require('fs').promises;
async function openAndClose() {
  let filehandle;
  try {
    filehandle = await fsPromises.open('thefile.txt', 'r');
  } finally {
    if (filehandle !== undefined)
      await filehandle.close();
  }
}
```

#### filehandle.datasync()

* Returns: {Promise}

Asynchronous fdatasync(2). The `Promise` is resolved with no arguments upon
success.

#### filehandle.fd

* {number} The numeric file descriptor managed by the `FileHandle` object.

#### filehandle.read(buffer, offset, length, position)

* `buffer` {Buffer|Uint8Array}
* `offset` {integer}
* `length` {integer}
* `position` {integer}
* Returns: {Promise}

Read data from the file.

`buffer` is the buffer that the data will be written to.

`offset` is the offset in the buffer to start writing at.

`length` is an integer specifying the number of bytes to read.

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)

* `options` {Object|string}
  * `encoding` {string|null} **Default:** `null`
  * `flag` {string} See [support of file system `flags`][]. **Default:** `'r'`.
* Returns: {Promise}

Asynchronously reads the entire contents of a file.

The `Promise` is resolved with the contents of the file. If no encoding is
specified (using `options.encoding`), the data is returned as a `Buffer`
object. Otherwise, the data will be a string.

If `options` is a string, then it specifies the encoding.

When the `path` is a directory, the behavior of `fsPromises.readFile()` is
platform-specific. On macOS, Linux, and Windows, the promise will be rejected
with an error. On FreeBSD, a representation of the directory's contents will be
returned.

The `FileHandle` has to support reading.

If one or more `filehandle.read()` calls are made on a file handle and then a
`filehandle.readFile()` call is made, the data will be read from the current
position till the end of the file. It doesn't always read from the beginning
of the file.

#### filehandle.stat([options])

* `options` {Object}
  * `bigint` {boolean} Whether the numeric values in the returned
    [`fs.Stats`][] object should be `bigint`. **Default:** `false`.
* Returns: {Promise}

Retrieves the [`fs.Stats`][] for the file.

#### filehandle.sync()

* Returns: {Promise}

Asynchronous fsync(2). The `Promise` is resolved with no arguments upon
success.

#### filehandle.truncate(len)

* `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
const fs = require('fs');
const fsPromises = fs.promises;

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  let filehandle = null;
  try {
    filehandle = await fsPromises.open('temp.txt', 'r+');
    await filehandle.truncate(4);
  } finally {
    if (filehandle) {
      // close the file if it is opened.
      await filehandle.close();
    }
  }
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints: Node
}

doTruncate().catch(console.error);
```

If the file previously was shorter than `len` bytes, it is extended, and the
extended part is filled with null bytes (`'\0'`):

```js
const fs = require('fs');
const fsPromises = fs.promises;

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  let filehandle = null;
  try {
    filehandle = await fsPromises.open('temp.txt', 'r+');
    await filehandle.truncate(10);
  } finally {
    if (filehandle) {
      // close the file if it is opened.
      await filehandle.close();
    }
  }
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints Node.js\0\0\0
}

doTruncate().catch(console.error);
```

The last three bytes are null bytes (`'\0'`), to compensate the over-truncation.

#### filehandle.utimes(atime, mtime)

* `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)

* `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.write(string[, position[, encoding]])

* `string` {string}
* `position` {integer}
* `encoding` {string} **Default:** `'utf8'`
* Returns: {Promise}

Write `string` to the file. If `string` is not a string, then
the value will be coerced to one.

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 `string` written.

`position` refers to the offset from the beginning of the file where this data
should be written. If the type of `position` is not a `number` the data
will be written at the current position. See pwrite(2).

`encoding` is the expected string encoding.

On Linux, positional writes do not work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

#### filehandle.writeFile(data, options)

* `data` {string|Buffer|Uint8Array}
* `options` {Object|string}
  * `encoding` {string|null} **Default:** `'utf8'`
  * `mode` {integer} **Default:** `0o666`
  * `flag` {string} See [support of file system `flags`][]. **Default:** `'w'`.
* Returns: {Promise}

Asynchronously writes data to a file, replacing the file if it already exists.
`data` can be a string or a buffer. The `Promise` will be resolved with no
arguments upon success.

The `encoding` option is ignored if `data` is a buffer.

If `options` is a string, then it specifies the encoding.

The `FileHandle` has to support writing.

It is unsafe to use `filehandle.writeFile()` multiple times on the same file
without waiting for the `Promise` to be resolved (or rejected).

If one or more `filehandle.write()` calls are made on a file handle and then a
`filehandle.writeFile()` call is made, the data will be written from the
current position till the end of the file. It doesn't always write from the
beginning of the file.

### fsPromises.access(path[, mode])

* `path` {string|Buffer|URL}
* `mode` {integer} **Default:** `fs.constants.F_OK`
* Returns: {Promise}

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
const fs = require('fs');
const fsPromises = fs.promises;

fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
  .then(() => console.log('can access'))
  .catch(() => console.error('cannot access'));
```

Using `fsPromises.access()` to check for the accessibility of a file before
calling `fsPromises.open()` is not recommended. Doing so introduces a race
condition, since other processes may change the file's state between the two
calls. Instead, user code should open/read/write the file directly and handle
the error raised if the file is not accessible.

### fsPromises.appendFile(path, data[, options])

* `path` {string|Buffer|URL|FileHandle} filename or `FileHandle`
* `data` {string|Buffer}
* `options` {Object|string}
  * `encoding` {string|null} **Default:** `'utf8'`
  * `mode` {integer} **Default:** `0o666`
  * `flag` {string} See [support of file system `flags`][]. **Default:** `'a'`.
* Returns: {Promise}

Asynchronously append data to a file, creating the file if it does not yet
exist. `data` can be a string or a [`Buffer`][]. The `Promise` will be
resolved with no arguments upon success.

If `options` is a string, then it specifies the encoding.

The `path` may be specified as a `FileHandle` that has been opened
for appending (using `fsPromises.open()`).

### fsPromises.chmod(path, mode)

* `path` {string|Buffer|URL}
* `mode` {integer}
* Returns: {Promise}

Changes the permissions of a file then resolves the `Promise` with no
arguments upon succces.

### fsPromises.chown(path, uid, gid)

* `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.

### fsPromises.copyFile(src, dest[, flags])

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.

```js
const fsPromises = require('fs').promises;

// destination.txt will be created or overwritten by default.
fsPromises.copyFile('source.txt', 'destination.txt')
  .then(() => console.log('source.txt was copied to destination.txt'))
  .catch(() => console.log('The file could not be copied'));
```

If the third argument is a number, then it specifies `flags`:

```js
const fs = require('fs');
const fsPromises = fs.promises;
const { COPYFILE_EXCL } = fs.constants;

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fsPromises.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL)
  .then(() => console.log('source.txt was copied to destination.txt'))
  .catch(() => console.log('The file could not be copied'));
```

### fsPromises.lchmod(path, mode)

* `path` {string|Buffer|URL}
* `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.

### fsPromises.lchown(path, uid, gid)

* `path` {string|Buffer|URL}
* `uid` {integer}
* `gid` {integer}
* Returns: {Promise}

Changes the ownership on a symbolic link then resolves the `Promise` with
no arguments upon success.

### fsPromises.link(existingPath, newPath)

* `existingPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
* Returns: {Promise}

Asynchronous link(2). The `Promise` is resolved with no arguments upon success.

### fsPromises.lstat(path[, options])

Asynchronous lstat(2). The `Promise` is resolved with the [`fs.Stats`][] object
for the given symbolic link `path`.

### fsPromises.mkdir(path[, options])

* `path` {string|Buffer|URL}
* `options` {Object|integer}
  * `recursive` {boolean} **Default:** `false`
  * `mode` {integer} Not supported on Windows. **Default:** `0o777`.
* Returns: {Promise}

Asynchronously creates a directory then resolves the `Promise` with no
arguments upon success.

### fsPromises.mkdtemp(prefix[, options])

* `prefix` {string}
* `options` {string|Object}
  * `encoding` {string} **Default:** `'utf8'`
* Returns: {Promise}

Creates a unique temporary directory and resolves the `Promise` with the created
folder path. A unique directory name is generated by appending six random
characters to the end of the provided `prefix`.

The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use.

```js
fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
  .catch(console.error);
```

The `fsPromises.mkdtemp()` method will append the six randomly selected
characters directly to the `prefix` string. For instance, given a directory
`/tmp`, if the intention is to create a temporary directory *within* `/tmp`, the
`prefix` must end with a trailing platform-specific path separator
(`require('path').sep`).

### fsPromises.open(path, flags[, mode])

Asynchronous file open that returns a `Promise` that, when resolved, yields a
`FileHandle` object. See open(2).

`mode` sets the file mode (permission and sticky bits), but only if the file was
created.

### fsPromises.readdir(path[, options])

* `path` {string|Buffer|URL}
* `options` {string|Object}
  * `encoding` {string} **Default:** `'utf8'`
  * `withFileTypes` {boolean} **Default:** `false`
* Returns: {Promise}

Reads the contents of a directory then resolves the `Promise` with an array
of the names of the files in the directory excluding `'.'` and `'..'`.

The optional `options` argument can be a string specifying an encoding, or an
object with an `encoding` property specifying the character encoding to use for
the filenames. If the `encoding` is set to `'buffer'`, the filenames returned
will be passed as `Buffer` objects.

If `options.withFileTypes` is set to `true`, the resolved array will contain
[`fs.Dirent`][] objects.

### fsPromises.readFile(path[, options])

* `path` {string|Buffer|URL|FileHandle} filename or `FileHandle`
* `options` {Object|string}
  * `encoding` {string|null} **Default:** `null`
  * `flag` {string} See [support of file system `flags`][]. **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.

Any specified `FileHandle` has to support reading.

### fsPromises.readlink(path[, options])

* `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.

### fsPromises.realpath(path[, options])

* `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.

### fsPromises.rename(oldPath, newPath)

* `oldPath` {string|Buffer|URL}
* `newPath` {string|Buffer|URL}
* Returns: {Promise}

Renames `oldPath` to `newPath` and resolves the `Promise` with no arguments
upon success.

### fsPromises.rmdir(path)

* `path` {string|Buffer|URL}
* Returns: {Promise}

Removes the directory identified by `path` then resolves the `Promise` with
no arguments upon success.

Using `fsPromises.rmdir()` on a file (not a directory) results in the
`Promise` being rejected with an `ENOENT` error on Windows and an `ENOTDIR`
error on POSIX.

### fsPromises.stat(path[, options])

The `Promise` is resolved with the [`fs.Stats`][] object for the given `path`.

### fsPromises.symlink(target, path[, type])

* `target` {string|Buffer|URL}
* `path` {string|Buffer|URL}
* `type` {string} **Default:** `'file'`
* Returns: {Promise}

Creates a symbolic link then resolves the `Promise` with no arguments upon
success.

The `type` argument is only used on Windows platforms and can be one of `'dir'`,
`'file'`, or `'junction'`. Windows junction points require the destination path
to be absolute. When using `'junction'`, the `target` argument will
automatically be normalized to absolute path.

### fsPromises.truncate(path[, len])

* `path` {string|Buffer|URL}
* `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`.

### fsPromises.unlink(path)

* `path` {string|Buffer|URL}
* Returns: {Promise}

Asynchronous unlink(2). The `Promise` is resolved with no arguments upon
success.

### fsPromises.utimes(path, atime, mtime)

* `path` {string|Buffer|URL}
* `atime` {number|string|Date}
* `mtime` {number|string|Date}
* Returns: {Promise}

Change the file system timestamps of the object referenced by `path` then
resolves the `Promise` with no arguments upon success.

### fsPromises.writeFile(file, data[, options])

* `file` {string|Buffer|URL|FileHandle} filename or `FileHandle`
* `data` {string|Buffer|Uint8Array}
* `options` {Object|string}
  * `encoding` {string|null} **Default:** `'utf8'`
  * `mode` {integer} **Default:** `0o666`
  * `flag` {string} See [support of file system `flags`][]. **Default:** `'w'`.
* Returns: {Promise}

Asynchronously writes data to a file, replacing the file if it already exists.
`data` can be a string or a buffer. The `Promise` will be resolved with no
arguments upon success.

The `encoding` option is ignored if `data` is a buffer.

If `options` is a string, then it specifies the encoding.

Any specified `FileHandle` has to support writing.

It is unsafe to use `fsPromises.writeFile()` multiple times on the same file
without waiting for the `Promise` to be resolved (or rejected).

## FS Constants

The following constants are exported by `fs.constants`.

Not every constant will be available on every operating system.

### File Access Constants

The following constants are meant for use with [`fs.access()`][].

<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.
     This is useful for determining if a file exists, but says nothing
     about <code>rwx</code> permissions. Default if no mode is specified.</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. This has no effect on Windows
    (will behave like <code>fs.constants.F_OK</code>).</td>
  </tr>
</table>

### File Copy Constants

The following constants are meant for use with [`fs.copyFile()`][].

<table>
  <tr>
    <th>Constant</th>
    <th>Description</th>
  </tr>
  <tr>
    <td><code>COPYFILE_EXCL</code></td>
    <td>If present, the copy operation will fail with an error if the
    destination path already exists.</td>
  </tr>
  <tr>
    <td><code>COPYFILE_FICLONE</code></td>
    <td>If present, the copy operation will attempt to create a
    copy-on-write reflink. If the underlying platform does not support
    copy-on-write, then a fallback copy mechanism is used.</td>
  </tr>
  <tr>
    <td><code>COPYFILE_FICLONE_FORCE</code></td>
    <td>If present, the copy operation will attempt to create a
    copy-on-write reflink. If the underlying platform does not support
    copy-on-write, then the operation will fail with an error.</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 <code>atime</code> 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>

## File System Flags

The following flags are available wherever the `flag` option takes a
string:

* `'a'` - Open file for appending.
  The file is created if it does not exist.

* `'ax'` - Like `'a'` but fails if the path exists.

* `'a+'` - Open file for reading and appending.
  The file is created if it does not exist.

* `'ax+'` - Like `'a+'` but fails if the path exists.

* `'as'` - Open file for appending in synchronous mode.
  The file is created if it does not exist.

* `'as+'` - Open file for reading and appending in synchronous mode.
  The file is created if it does not exist.

* `'r'` - Open file for reading.
  An exception occurs if the file does not exist.

* `'r+'` - Open file for reading and writing.
  An exception occurs if the file does not exist.

* `'rs+'` - Open file for reading and writing in synchronous mode. Instructs
  the operating system to bypass the local file system cache.

This is primarily useful for opening files on NFS mounts as it allows
  skipping the potentially stale local cache. It has a very real impact on
  I/O performance so using this flag is not recommended unless it is needed.

This doesn't turn `fs.open()` or `fsPromises.open()` into a synchronous
  blocking call. If synchronous operation is desired, something like
  `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 the path exists.

* `'w+'` - Open file for reading and writing.
The file is created (if it does not exist) or truncated (if it exists).

* `'wx+'` - Like `'w+'` but fails if the path exists.

`flag` can also be a number as documented by open(2); commonly used constants
are available from `fs.constants`. On Windows, flags are translated to
their equivalent ones where applicable, e.g. `O_WRONLY` to `FILE_GENERIC_WRITE`,
or `O_EXCL|O_CREAT` to `CREATE_NEW`, as accepted by `CreateFileW`.

The exclusive flag `'x'` (`O_EXCL` flag in open(2)) ensures that path is newly
created. On POSIX systems, path is considered to exist even if it is a symlink
to a non-existent file. The exclusive flag may or may not work with network
file systems.

On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

Modifying a file rather than replacing it may require a flags mode of `'r+'`
rather than the default mode `'w'`.

The behavior of some flags are platform-specific. As such, opening a directory
on macOS and Linux with the `'a+'` flag - see example below - will return an
error. In contrast, on Windows and FreeBSD, a file descriptor or a `FileHandle`
will be returned.

```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>
});
```

On Windows, opening an existing hidden file using the `'w'` flag (either
through `fs.open()` or `fs.writeFile()` or `fsPromises.open()`) will fail with
`EPERM`. Existing hidden files can be opened for writing with the `'r+'` flag.

A call to `fs.ftruncate()` or `filehandle.truncate()` can be used to reset
the file contents.

[`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
[`EventEmitter`]: events.html
[`FSEvents`]: https://developer.apple.com/documentation/coreservices/file_system_events
[`ReadDirectoryChangesW`]: https://docs.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-readdirectorychangesw
[`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
[`event ports`]: http://illumos.org/man/port_create
[`fs.Dirent`]: #fs_class_fs_dirent
[`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.copyFile()`]: #fs_fs_copyfile_src_dest_flags_callback
[`fs.createWriteStream()`]: #fs_fs_createwritestream_path_options
[`fs.exists()`]: fs.html#fs_fs_exists_path_callback
[`fs.fstat()`]: #fs_fs_fstat_fd_options_callback
[`fs.ftruncate()`]: #fs_fs_ftruncate_fd_len_callback
[`fs.futimes()`]: #fs_fs_futimes_fd_atime_mtime_callback
[`fs.lstat()`]: #fs_fs_lstat_path_options_callback
[`fs.mkdir()`]: #fs_fs_mkdir_path_options_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.readdir()`]: #fs_fs_readdir_path_options_callback
[`fs.readdirSync()`]: #fs_fs_readdirsync_path_options
[`fs.realpath()`]: #fs_fs_realpath_path_options_callback
[`fs.rmdir()`]: #fs_fs_rmdir_path_callback
[`fs.stat()`]: #fs_fs_stat_path_options_callback
[`fs.symlink()`]: #fs_fs_symlink_target_path_type_callback
[`fs.utimes()`]: #fs_fs_utimes_path_atime_mtime_callback
[`fs.watch()`]: #fs_fs_watch_filename_options_listener
[`fs.write(fd, buffer...)`]: #fs_fs_write_fd_buffer_offset_length_position_callback
[`fs.write(fd, string...)`]: #fs_fs_write_fd_string_position_encoding_callback
[`fs.writeFile()`]: #fs_fs_writefile_file_data_options_callback
[`inotify(7)`]: http://man7.org/linux/man-pages/man7/inotify.7.html
[`kqueue(2)`]: https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
[`net.Socket`]: net.html#net_class_net_socket
[`stat()`]: fs.html#fs_fs_stat_path_options_callback
[`util.promisify()`]: util.html#util_util_promisify_original
[Caveats]: #fs_caveats
[Common System Errors]: errors.html#errors_common_system_errors
[FS Constants]: #fs_fs_constants_1
[File Access Constants]: #fs_file_access_constants
[MDN-Date]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date
[MDN-Number]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type
[MSDN-Rel-Path]: https://docs.microsoft.com/en-us/windows/desktop/FileIO/naming-a-file#fully-qualified-vs-relative-paths
[MSDN-Using-Streams]: https://docs.microsoft.com/en-us/windows/desktop/FileIO/using-streams
[Naming Files, Paths, and Namespaces]: https://docs.microsoft.com/en-us/windows/desktop/FileIO/naming-a-file
[Readable Streams]: stream.html#stream_class_stream_readable
[Writable Stream]: stream.html#stream_class_stream_writable
[inode]: https://en.wikipedia.org/wiki/Inode
[support of file system `flags`]: #fs_file_system_flags
[chcp]: https://ss64.com/nt/chcp.html