File System

File I/O is provided by simple wrappers around standard POSIX functions. To use this module do require('fs'). All the methods have asynchronous and synchronous forms.

The asynchronous form always takes a completion callback as its last argument. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation was completed successfully, then the first argument will be null or undefined.

When using the synchronous form any exceptions are immediately thrown. Exceptions may be handled using try/catch, or they may be allowed to bubble up.

Here is an example of the asynchronous version:

const fs = require('fs');

fs.unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

Here is the synchronous version:

const fs = require('fs');

fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');

With the asynchronous methods there is no guaranteed ordering. So the following is prone to error:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
  if (err) throw err;
  console.log(`stats: ${JSON.stringify(stats)}`);
});

It could be that fs.stat is executed before fs.rename. The correct way to do this is to chain the callbacks.

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  fs.stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});

In busy processes, the programmer is strongly encouraged to use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete--halting all connections.

The relative path to a filename can be used. Remember, however, that this path will be relative to process.cwd().

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.

$ 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.>

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.

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.

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.

WHATWG URL object support

For most fs module functions, the path or filename argument may be passed as a WHATWG URL object. Only URL objects using the file: protocol are supported.

const fs = require('fs');
const { URL } = require('url');
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 :

// 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:

// On other platforms:

// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute

// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
fs.readFileSync(new URL('file:///tmp/hello'));

A file: URL having encoded slash characters will result in a throw on all platforms:

// On Windows
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */

// On POSIX
fs.readFileSync(new URL('file:///p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */

On Windows, file: URLs having encoded backslash will result in a throw:

// On Windows
fs.readFileSync(new URL('file:///C:/path/%5C'));
fs.readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */

Buffer API

fs functions support passing and receiving paths as both strings and Buffers. The latter is intended to make it possible to work with filesystems that allow for non-UTF-8 filenames. For most typical uses, working with paths as Buffers will be unnecessary, as the string API converts to and from UTF-8 automatically.

Note: On certain file systems (such as NTFS and HFS+) filenames will always be encoded as UTF-8. On such file systems, passing non-UTF-8 encoded Buffers to fs functions will not work as expected.

Class: fs.FSWatcher

Objects returned from fs.watch() are of this type.

The listener callback provided to fs.watch() receives the returned FSWatcher's change events.

The object itself emits these events:

Event: 'change'

• eventType <string> The type of fs change
• filename <string> | <Buffer> The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file. See more details in fs.watch().

The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a Buffer if fs.watch() is called with its encoding option set to 'buffer', otherwise filename will be a string.

// Example when handled through fs.watch listener
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
  if (filename) {
    console.log(filename);
    // Prints: <Buffer ...>
  }
});

Event: 'error'

• error <Error>

Emitted when an error occurs.

watcher.close()

Stop watching for changes on the given fs.FSWatcher.

Class: fs.ReadStream

ReadStream is a Readable Stream.

Event: 'close'

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

Event: 'open'

• fd <integer> Integer file descriptor used by the ReadStream.

Emitted when the ReadStream's file is opened.

readStream.bytesRead

The number of bytes read so far.

readStream.path

The path to the file the stream is reading from as specified in the first argument to fs.createReadStream(). If path is passed as a string, then readStream.path will be a string. If path is passed as a Buffer, then readStream.path will be a Buffer.

Class: fs.Stats

Objects returned from fs.stat(), fs.lstat() and fs.fstat() and their synchronous counterparts are of this type.

• stats.isFile()
• stats.isDirectory()
• stats.isBlockDevice()
• stats.isCharacterDevice()
• stats.isSymbolicLink() (only valid with fs.lstat())
• stats.isFIFO()
• stats.isSocket()

For a regular file util.inspect(stats) would return a string very similar to this:

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 }

Note: atimeMs, mtimeMs, ctimeMs, birthtimeMs are numbers that hold the corresponding times in milliseconds. Their precision is platform specific. atime, mtime, ctime, and birthtime are Date object alternate representations of the various times. The Date and number values are not connected. Assigning a new number value, or mutating the Date value, will not be reflected in the corresponding alternate representation.

Stat Time Values

The times in the stat object have the following semantics:

• atime "Access Time" - Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.
• mtime "Modified Time" - Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.
• ctime "Change Time" - Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.
• birthtime "Birth Time" - Time of file creation. Set once when the file is created. On filesystems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, unix epoch timestamp 0). Note that this value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call.

Prior to Node v0.12, the ctime held the birthtime on Windows systems. Note that as of v0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.WriteStream

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.

writeStream.bytesWritten

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

writeStream.path

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

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

• path <string> | <Buffer> | <URL>
• mode <integer> Default: fs.constants.F_OK
• callback <Function>
  • err <Error>

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. The following constants define the possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values.

• fs.constants.F_OK - path is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
• fs.constants.R_OK - path can be read by the calling process.
• fs.constants.W_OK - path can be written by the calling process.
• fs.constants.X_OK - path can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

The final argument, callback, is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be populated. The following example checks if the file /etc/passwd can be read and written by the current process.

fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => {
  console.log(err ? 'no access!' : 'can read/write');
});

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

For example:

write (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (!err) {
    console.error('myfile already exists');
    return;
  }

  fs.open('myfile', 'wx', (err, fd) => {
    if (err) throw err;
    writeMyData(fd);
  });
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  fs.open('myfile', 'r', (err, fd) => {
    if (err) throw err;
    readMyData(fd);
  });
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the accessibility of a file only if the file won’t be used directly, for example when its accessibility is a signal from another process.

fs.accessSync(path[, mode])

• path <string> | <Buffer> | <URL>
• mode <integer> Default: fs.constants.F_OK

Synchronous version of fs.access(). This throws if any accessibility checks fail, and does nothing otherwise.

fs.appendFile(file, data[, options], callback)

• file <string> | <Buffer> | <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:

fs.appendFile('message.txt', 'data to append', (err) => {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
});

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

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

Any specified file descriptor has to have been opened for appending.

Note: If a file descriptor is specified as the file, it will not be closed automatically.

fs.appendFileSync(file, data[, options])

• file <string> | <Buffer> | <number> filename or file descriptor
• data <string> | <Buffer>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • mode <integer> Default: 0o666
  • flag <string> Default: 'a'

The synchronous version of fs.appendFile(). Returns undefined.

fs.chmod(path, mode, callback)

• path <string> | <Buffer> | <URL>
• mode <integer>
• callback <Function>
  • err <Error>

Asynchronous chmod(2). No arguments other than a possible exception are given to the completion callback.

fs.chmodSync(path, mode)

• path <string> | <Buffer> | <URL>
• mode <integer>

Synchronous chmod(2). Returns undefined.

fs.chown(path, uid, gid, callback)

• path <string> | <Buffer> | <URL>
• uid <integer>
• gid <integer>
• callback <Function>
  • err <Error>

Asynchronous chown(2). No arguments other than a possible exception are given to the completion callback.

fs.chownSync(path, uid, gid)

• path <string> | <Buffer> | <URL>
• uid <integer>
• gid <integer>

Synchronous chown(2). Returns undefined.

fs.close(fd, callback)

• fd <integer>
• callback <Function>
  • err <Error>

Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.

fs.closeSync(fd)

• fd <integer>

Synchronous close(2). Returns undefined.

fs.constants

Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.

fs.copyFile(src, dest[, flags], callback)

• src <string> | <Buffer> | <URL> source filename to 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:

const fs = require('fs');

// destination.txt will be created or overwritten by default.
fs.copyFile('source.txt', 'destination.txt', (err) => {
  if (err) throw err;
  console.log('source.txt was copied to destination.txt');
});

If the third argument is a number, then it specifies flags, as shown in the following example.

const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL, callback);

fs.copyFileSync(src, dest[, flags])

• src <string> | <Buffer> | <URL> source filename to 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:

const fs = require('fs');

// destination.txt will be created or overwritten by default.
fs.copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');

If the third argument is a number, then it specifies flags, as shown in the following example.

const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFileSync('source.txt', 'destination.txt', COPYFILE_EXCL);

fs.createReadStream(path[, options])

• path <string> | <Buffer> | <URL>
• 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 Stream).

Be aware that, unlike the default value set for highWaterMark on a readable stream (16 kb), the stream returned by this method has a default value of 64 kb for the same parameter.

options is an object or string with the following defaults:

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 fds should be passed to net.Socket.

If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on error or end the file descriptor will be closed automatically.

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

An example to read the last 10 bytes of a file which is 100 bytes long:

fs.createReadStream('sample.txt', { start: 90, end: 99 });

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

fs.createWriteStream(path[, options])

• path <string> | <Buffer> | <URL>
• 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:

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 fds should be passed to net.Socket.

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

fs.exists(path, callback)

• path <string> | <Buffer> | <URL>
• callback <Function>
  • exists <Boolean>

Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false. Example:

fs.exists('/etc/passwd', (exists) => {
  console.log(exists ? 'it\'s there' : 'no passwd!');
});

Note that the parameter to this callback is not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().

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

For example:

write (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    console.error('myfile already exists');
  } else {
    fs.open('myfile', 'wx', (err, fd) => {
      if (err) throw err;
      writeMyData(fd);
    });
  }
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    fs.open('myfile', 'r', (err, fd) => {
      readMyData(fd);
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    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)

• path <string> | <Buffer> | <URL>

Synchronous version of fs.exists(). Returns true if the file exists, false otherwise.

Note that fs.exists() is deprecated, but fs.existsSync() is not. (The callback parameter to fs.exists() accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync() does not use a callback.)

fs.fchmod(fd, mode, callback)

• 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)

• fd <integer>
• mode <integer>

Synchronous fchmod(2). Returns undefined.

fs.fchown(fd, uid, gid, callback)

• fd <integer>
• uid <integer>
• gid <integer>
• callback <Function>
  • err <Error>

Asynchronous fchown(2). No arguments other than a possible exception are given to the completion callback.

fs.fchownSync(fd, uid, gid)

• fd <integer>
• uid <integer>
• gid <integer>

Synchronous fchown(2). Returns undefined.

fs.fdatasync(fd, callback)

• fd <integer>
• callback <Function>
  • err <Error>

Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.

fs.fdatasyncSync(fd)

• fd <integer>

Synchronous fdatasync(2). Returns undefined.

fs.fstat(fd, callback)

• 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)

• fd <integer>

Synchronous fstat(2). Returns an instance of fs.Stats.

fs.fsync(fd, callback)

• fd <integer>
• callback <Function>
  • err <Error>

Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.

fs.fsyncSync(fd)

• fd <integer>

Synchronous fsync(2). Returns undefined.

fs.ftruncate(fd[, len], callback)

• fd <integer>
• len <integer> Default: 0
• callback <Function>
  • err <Error>

Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.

If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file

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

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to first four bytes
fs.ftruncate(fd, 4, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt', 'utf8'));
});
// Prints: Node

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

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

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to 10 bytes, whereas the actual size is 7 bytes
fs.ftruncate(fd, 10, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt'));
});
// Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
// ('Node.js\0\0\0' in UTF8)

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

fs.ftruncateSync(fd[, len])

• fd <integer>
• len <integer> Default: 0

Synchronous ftruncate(2). Returns undefined.

fs.futimes(fd, atime, mtime, callback)

• fd <integer>
• atime <number> | <string> | <Date>
• mtime <number> | <string> | <Date>
• callback <Function>
  • err <Error>

Change the file system timestamps of the object referenced by the supplied file descriptor. See fs.utimes().

Note: This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS.

fs.futimesSync(fd, atime, mtime)

• fd <integer>
• atime <integer>
• mtime <integer>

Synchronous version of fs.futimes(). Returns undefined.

fs.lchmod(path, mode, callback)

• path <string> | <Buffer>
• 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)

• path <string> | <Buffer>
• mode <integer>

Synchronous lchmod(2). Returns undefined.

fs.lchown(path, uid, gid, callback)

• path <string> | <Buffer>
• 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)

• path <string> | <Buffer>
• uid <integer>
• gid <integer>

Synchronous lchown(2). Returns undefined.

fs.link(existingPath, newPath, callback)

• existingPath <string> | <Buffer> | <URL>
• newPath <string> | <Buffer> | <URL>
• callback <Function>
  • err <Error>

Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.

fs.linkSync(existingPath, newPath)

• existingPath <string> | <Buffer> | <URL>
• newPath <string> | <Buffer> | <URL>

Synchronous link(2). Returns undefined.

fs.lstat(path, callback)

• 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)

• path <string> | <Buffer> | <URL>

Synchronous lstat(2). Returns an instance of fs.Stats.

fs.mkdir(path[, mode], callback)

• path <string> | <Buffer> | <URL>
• mode <integer> Default: 0o777
• callback <Function>
  • err <Error>

Asynchronous mkdir(2). No arguments other than a possible exception are given to the completion callback. mode defaults to 0o777.

fs.mkdirSync(path[, mode])

• path <string> | <Buffer> | <URL>
• mode <integer> Default: 0o777

Synchronous mkdir(2). Returns undefined.

fs.mkdtemp(prefix[, options], callback)

• 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:

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).

// 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])

• 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)

• 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.

// 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.

Functions based on fs.open() exhibit this behavior as well. eg. fs.writeFile(), fs.readFile(), etc.

fs.openSync(path, flags[, mode])

• 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)

• 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)

• 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])

• 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)

• 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:

fs.readFile('/etc/passwd', (err, data) => {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the contents of the file.

If no encoding is specified, then the raw buffer is returned.

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

fs.readFile('/etc/passwd', 'utf8', callback);

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.

// 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() reads the entire file in a single threadpool request. To minimize threadpool task length variation, prefer the partitioned APIs fs.read() and fs.createReadStream() when reading files as part of fulfilling a client request.

fs.readFileSync(path[, options])

• 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.

// 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)

• 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])

• 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)

• fd <integer>
• buffer <string> | <Buffer> | <Uint8Array>
• offset <integer>
• length <integer>
• position <integer>

Synchronous version of fs.read(). Returns the number of bytesRead.

fs.realpath(path[, options], callback)

• 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)

• 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])

• 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])

• 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)

• 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)

• oldPath <string> | <Buffer> | <URL>
• newPath <string> | <Buffer> | <URL>

Synchronous rename(2). Returns undefined.

fs.rmdir(path, callback)

• path <string> | <Buffer> | <URL>
• callback <Function>
  • err <Error>

Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.

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)

• 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)

• 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)

• path <string> | <Buffer> | <URL>

Synchronous stat(2). Returns an instance of fs.Stats.

fs.symlink(target, path[, type], callback)

• target <string> | <Buffer> | <URL>
• path <string> | <Buffer> | <URL>
• type <string> Default: 'file'
• callback <Function>
  • err <Error>

Asynchronous symlink(2). No arguments other than a possible exception are given to the completion callback. The type argument can be set to 'dir', 'file', or 'junction' (default is 'file') and is only available on Windows (ignored on other platforms). Note that Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Here is an example below:

fs.symlink('./foo', './new-port', callback);

It creates a symbolic link named "new-port" that points to "foo".

fs.symlinkSync(target, path[, type])

• target <string> | <Buffer> | <URL>
• path <string> | <Buffer> | <URL>
• type <string> Default: 'file'

Synchronous symlink(2). Returns undefined.

fs.truncate(path[, len], callback)

• path <string> | <Buffer>
• 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])

• path <string> | <Buffer>
• 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)

• 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)

• path <string> | <Buffer> | <URL>

Synchronous unlink(2). Returns undefined.

fs.unwatchFile(filename[, listener])

• filename <string> | <Buffer>
• listener <Function> | <undefined> Default: undefined
  • eventType <string>
  • filename <string> | <Buffer>

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)

• 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, Dates, 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)

• path <string> | <Buffer> | <URL>
• atime <integer>
• mtime <integer>

Synchronous version of fs.utimes(). Returns undefined.

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>

Watch for changes on filename, where filename is either a file or a directory. The returned object is a fs.FSWatcher.

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

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

Note that on most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

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

Caveats

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

The recursive option is only supported on 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
• 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

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

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

Filename Argument

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

fs.watch('somedir', (eventType, filename) => {
  console.log(`event type is: ${eventType}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

fs.watchFile(filename[, options], listener)

• filename <string> | <Buffer> | <URL>
• options <Object>
  • 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:

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)

• 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)

• 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)

• file <string> | <Buffer> | <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:

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

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

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

Any specified file descriptor has to support writing.

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])

• file <string> | <Buffer> | <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]]])

• fd <integer>
• buffer <Buffer> | <Uint8Array>
• offset <integer>
• length <integer>
• position <integer>

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

• fd <integer>
• string <string>
• position <integer>
• encoding <string>

Synchronous versions of fs.write(). Returns the number of bytes written.

FS Constants

The following constants are exported by fs.constants.

Note: Not every constant will be available on every operating system.

File Access Constants

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

Constant	Description
F_OK	Flag indicating that the file is visible to the calling process.
R_OK	Flag indicating that the file can be read by the calling process.
W_OK	Flag indicating that the file can be written by the calling process.
X_OK	Flag indicating that the file can be executed by the calling process.

File Open Constants

The following constants are meant for use with fs.open().

Constant	Description
O_RDONLY	Flag indicating to open a file for read-only access.
O_WRONLY	Flag indicating to open a file for write-only access.
O_RDWR	Flag indicating to open a file for read-write access.
O_CREAT	Flag indicating to create the file if it does not already exist.
O_EXCL	Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists.
O_NOCTTY	Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one).
O_TRUNC	Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero.
O_APPEND	Flag indicating that data will be appended to the end of the file.
O_DIRECTORY	Flag indicating that the open should fail if the path is not a directory.
O_NOATIME	Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only.
O_NOFOLLOW	Flag indicating that the open should fail if the path is a symbolic link.
O_SYNC	Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity.
O_DSYNC	Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity.
O_SYMLINK	Flag indicating to open the symbolic link itself rather than the resource it is pointing to.
O_DIRECT	When set, an attempt will be made to minimize caching effects of file I/O.
O_NONBLOCK	Flag indicating to open the file in nonblocking mode when possible.

File Type Constants

The following constants are meant for use with the fs.Stats object's mode property for determining a file's type.

Constant	Description
S_IFMT	Bit mask used to extract the file type code.
S_IFREG	File type constant for a regular file.
S_IFDIR	File type constant for a directory.
S_IFCHR	File type constant for a character-oriented device file.
S_IFBLK	File type constant for a block-oriented device file.
S_IFIFO	File type constant for a FIFO/pipe.
S_IFLNK	File type constant for a symbolic link.
S_IFSOCK	File type constant for a socket.

File Mode Constants

The following constants are meant for use with the fs.Stats object's mode property for determining the access permissions for a file.

Constant	Description
S_IRWXU	File mode indicating readable, writable, and executable by owner.
S_IRUSR	File mode indicating readable by owner.
S_IWUSR	File mode indicating writable by owner.
S_IXUSR	File mode indicating executable by owner.
S_IRWXG	File mode indicating readable, writable, and executable by group.
S_IRGRP	File mode indicating readable by group.
S_IWGRP	File mode indicating writable by group.
S_IXGRP	File mode indicating executable by group.
S_IRWXO	File mode indicating readable, writable, and executable by others.
S_IROTH	File mode indicating readable by others.
S_IWOTH	File mode indicating writable by others.
S_IXOTH	File mode indicating executable by others.