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. You can use try/catch to handle exceptions or allow them 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().

Most fs functions let you omit the callback argument. If you do, a default callback is used that rethrows errors. To get a trace to the original call site, set the NODE_DEBUG environment variable:

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();

$ env NODE_DEBUG=fs node script.js
fs.js:66
        throw err;
              ^
Error: EISDIR, read
    at rethrow (fs.js:61:21)
    at maybeCallback (fs.js:79:42)
    at Object.fs.readFile (fs.js:153:18)
    at bad (/path/to/script.js:2:17)
    at Object.<anonymous> (/path/to/script.js:5:1)
    <etc.>

Class: fs.FSWatcher

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

Event: 'change'

• event <String> The type of fs change
• filename <String> The filename that changed (if relevant/available)

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

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: 'open'

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

Emitted when the ReadStream's file is opened.

Event: 'close'

Emitted when the ReadStream's underlying file descriptor has been closed using the fs.close() method.

readStream.path

The path to the file the stream is reading from.

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:

{
  dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  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
}

Please note that atime, mtime, birthtime, and ctime are instances of Date object and to compare the values of these objects you should use appropriate methods. For most general uses getTime() will return the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC and this integer should be sufficient for any comparison, however there are additional methods which can be used for displaying fuzzy information. More details can be found in the MDN JavaScript Reference page.

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: 'open'

• fd <Number> Integer file descriptor used by the WriteStream.

Emitted when the WriteStream's file is opened.

Event: 'close'

Emitted when the WriteStream's underlying file descriptor has been closed using the fs.close() method.

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.

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

Tests a user's permissions for the file specified by path. mode 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.F_OK - File is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
• fs.R_OK - File can be read by the calling process.
• fs.W_OK - File can be written by the calling process.
• fs.X_OK - File can be executed by the calling process. This has no effect on Windows (will behave like fs.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.R_OK | fs.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;
    } else {
      throw err;
    }
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

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

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

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

• file <String> filename
• data <String> | <Buffer>
• options <Object> | <String>
  • encoding <String> | <Null> default = 'utf8'
  • mode <Number> default = 0o666
  • flag <String> default = 'a'
• callback <Function>

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);

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

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

fs.chmod(path, mode, callback)

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

fs.chmodSync(path, mode)

Synchronous chmod(2). Returns undefined.

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

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

fs.chownSync(path, uid, gid)

Synchronous chown(2). Returns undefined.

fs.close(fd, callback)

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

fs.closeSync(fd)

Synchronous close(2). Returns undefined.

fs.createReadStream(path[, options])

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:

{
  flags: 'r',
  encoding: null,
  fd: null,
  mode: 0o666,
  autoClose: true
}

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 your 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])

Returns a new WriteStream object. (See Writable Stream).

options is an object or string with the following defaults:

{
  flags: 'w',
  defaultEncoding: 'utf8',
  fd: null,
  mode: 0o666
}

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 defaultEncoding can be any one of those accepted by Buffer.

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)

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!');
});

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;
    } else {
      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;
    } else {
      throw err;
    }
  } else {
    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)

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

fs.fchmod(fd, mode, callback)

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

fs.fchmodSync(fd, mode)

Synchronous fchmod(2). Returns undefined.

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

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

fs.fchownSync(fd, uid, gid)

Synchronous fchown(2). Returns undefined.

fs.fdatasync(fd, callback)

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

fs.fdatasyncSync(fd)

Synchronous fdatasync(2). Returns undefined.

fs.fstat(fd, callback)

Asynchronous fstat(2). The callback gets two arguments (err, stats) where stats is a 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)

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

fs.fsync(fd, callback)

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

fs.fsyncSync(fd)

Synchronous fsync(2). Returns undefined.

fs.ftruncate(fd, len, callback)

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

fs.ftruncateSync(fd, len)

Synchronous ftruncate(2). Returns undefined.

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

Change the file timestamps of a file referenced by the supplied file descriptor.

fs.futimesSync(fd, atime, mtime)

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

fs.lchmod(path, mode, callback)

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

Only available on Mac OS X.

fs.lchmodSync(path, mode)

Synchronous lchmod(2). Returns undefined.

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

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

fs.lchownSync(path, uid, gid)

Synchronous lchown(2). Returns undefined.

fs.link(srcpath, dstpath, callback)

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

fs.linkSync(srcpath, dstpath)

Synchronous link(2). Returns undefined.

fs.lstat(path, callback)

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)

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

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

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

fs.mkdirSync(path[, mode])

Synchronous mkdir(2). Returns undefined.

fs.mkdtemp(prefix, callback)

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.

Example:

fs.mkdtemp('/tmp/foo-', (err, folder) => {
  console.log(folder);
    // Prints: /tmp/foo-itXde2
});

fs.mkdtempSync(template)

The synchronous version of [fs.mkdtemp()][]. Returns the created folder path.

fs.open(path, flags[, mode], callback)

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 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 you to skip the potentially stale local cache. It has a very real impact on I/O performance so don't use this flag unless you need it.

  Note that this doesn't turn fs.open() into a synchronous blocking call. If that's what you want then you should be using fs.openSync()

• 'rs+' - Open file for reading and writing, telling the OS to open it synchronously. See notes for 'rs' about using this with caution.

• '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 0666, 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 require('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.

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

Synchronous version of fs.open(). Returns an integer representing the file descriptor.

fs.read(fd, buffer, offset, length, position, callback)

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 integer specifying where to begin reading from in the file. If position is null, data will be read from the current file position.

The callback is given the three arguments, (err, bytesRead, buffer).

fs.readdir(path, callback)

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 '..'.

fs.readdirSync(path)

Synchronous readdir(3). Returns an array of filenames excluding '.' and '..'.

fs.readFile(file[, options], callback)

• file <String> filename
• options <Object> | <String>
  • encoding <String> | <Null> default = null
  • flag <String> default = 'r'
• callback <Function>

Asynchronously reads the entire contents of a file. Example:

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

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

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

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

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

fs.readFileSync(file[, options])

Synchronous version of fs.readFile. Returns the contents of the file.

If the encoding option is specified then this function returns a string. Otherwise it returns a buffer.

fs.readlink(path, callback)

Asynchronous readlink(2). The callback gets two arguments (err, linkString).

fs.readlinkSync(path)

Synchronous readlink(2). Returns the symbolic link's string value.

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

Asynchronous realpath(2). The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths. cache is an object literal of mapped paths that can be used to force a specific path resolution or avoid additional fs.stat calls for known real paths.

Example:

var cache = {'/etc':'/private/etc'};
fs.realpath('/etc/passwd', cache, (err, resolvedPath) => {
  if (err) throw err;
  console.log(resolvedPath);
});

fs.readSync(fd, buffer, offset, length, position)

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

fs.realpathSync(path[, cache])

Synchronous realpath(2). Returns the resolved path. cache is an object literal of mapped paths that can be used to force a specific path resolution or avoid additional fs.stat calls for known real paths.

fs.rename(oldPath, newPath, callback)

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

fs.renameSync(oldPath, newPath)

Synchronous rename(2). Returns undefined.

fs.rmdir(path, callback)

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

fs.rmdirSync(path)

Synchronous rmdir(2). Returns undefined.

fs.stat(path, callback)

Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is a fs.Stats object. See the fs.Stats section for more information.

fs.statSync(path)

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

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

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');

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

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

Synchronous symlink(2). Returns undefined.

fs.truncate(path, len, callback)

Asynchronous truncate(2). No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called.

fs.truncateSync(path, len)

Synchronous truncate(2). Returns undefined.

fs.unlink(path, callback)

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

fs.unlinkSync(path)

Synchronous unlink(2). Returns undefined.

fs.unwatchFile(filename[, listener])

Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed and you have effectively stopped watching 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)

Change file timestamps of the file referenced by the supplied path.

Note: the arguments atime and mtime of the following related functions does follow the below rules:

• If the value is a numberable string like '123456789', the value would get converted to corresponding number.
• If the value is NaN or Infinity, the value would get converted to Date.now().

fs.utimesSync(path, atime, mtime)

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

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

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. The options if provided should be an object. The supported boolean members are persistent and recursive. persistent indicates whether the process should continue to run as long as files are being watched. recursive 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).

The default is { persistent: true, recursive: false }.

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

Caveats

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

The recursive option is only supported on OS X 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 OS X, 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.

You can still use fs.watchFile, which uses stat polling, but it is slower and less reliable.

Inodes

On Linux and OS X 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.

Filename Argument

Providing filename argument in the callback is only supported on Linux and Windows. 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', (event, filename) => {
  console.log(`event is: ${event}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

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

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.

If you want to be notified when the file was modified, not just accessed, you need 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.

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

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, written, buffer) where written specifies how many bytes were written from buffer.

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, data[, position[, encoding]], callback)

Write data to the file specified by fd. If data is not a Buffer instance then the value will be coerced to a string.

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> filename
• data <String> | <Buffer>
• options <Object> | <String>
  • encoding <String> | <Null> default = 'utf8'
  • mode <Number> default = 0o666
  • flag <String> default = 'w'
• callback <Function>

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('It\'s saved!');
});

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

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

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.

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

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

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

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

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