Node.js v16.9.0 documentation

Promise example#

Promise-based operations return a promise that is fulfilled when the asynchronous operation is complete.

import { unlink } from 'fs/promises';

try {
  await unlink('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (error) {
  console.error('there was an error:', error.message);
}const { unlink } = require('fs/promises');

(async function(path) {
  try {
    await unlink(path);
    console.log(`successfully deleted ${path}`);
  } catch (error) {
    console.error('there was an error:', error.message);
  }
})('/tmp/hello');

Callback example#

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

import { unlink } from 'fs';

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

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

The callback-based versions of the fs module APIs are preferable over the use of the promise APIs when maximal performance (both in terms of execution time and memory allocation are required).

Synchronous example#

The synchronous APIs block the Node.js event loop and further JavaScript execution until the operation is complete. Exceptions are thrown immediately and can be handled using try…catch, or can be allowed to bubble up.

import { unlinkSync } from 'fs';

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

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

Promises API#

The fs/promises API provides asynchronous file system methods that return promises.

The promise APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.

Class: FileHandle#

A <FileHandle> object is an object wrapper for a numeric file descriptor.

Instances of the <FileHandle> object are created by the fsPromises.open() method.

All <FileHandle> objects are <EventEmitter>s.

If a <FileHandle> is not closed using the filehandle.close() method, it will try to automatically close the file descriptor and emit a process warning, helping to prevent memory leaks. Please do not rely on this behavior because it can be unreliable and the file may not be closed. Instead, always explicitly close <FileHandle>s. Node.js may change this behavior in the future.

Event: 'close'#

The 'close' event is emitted when the <FileHandle> has been closed and can no longer be used.

filehandle.appendFile(data[, options])#

• data <string> | <Buffer> | <TypedArray> | <DataView>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
• Returns: <Promise> Fulfills with undefined upon success.

Alias of filehandle.writeFile().

When operating on file handles, the mode cannot be changed from what it was set to with fsPromises.open(). Therefore, this is equivalent to filehandle.writeFile().

filehandle.chmod(mode)#

• mode <integer> the file mode bit mask.
• Returns: <Promise> Fulfills with undefined upon success.

Modifies the permissions on the file. See chmod(2).

filehandle.chown(uid, gid)#

• uid <integer> The file's new owner's user id.
• gid <integer> The file's new group's group id.
• Returns: <Promise> Fulfills with undefined upon success.

Changes the ownership of the file. A wrapper for chown(2).

filehandle.close()#

• Returns: <Promise> Fulfills with undefined upon success.

Closes the file handle after waiting for any pending operation on the handle to complete.

import { open } from 'fs/promises';

let filehandle;
try {
  filehandle = await open('thefile.txt', 'r');
} finally {
  await filehandle?.close();
}

filehandle.datasync()#

• Returns: <Promise> Fulfills with undefined upon success.

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX fdatasync(2) documentation for details.

Unlike filehandle.sync this method does not flush modified metadata.

filehandle.fd#

• <number> The numeric file descriptor managed by the <FileHandle> object.

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

• buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.
• offset <integer> The location in the buffer at which to start filling. Default: 0
• length <integer> The number of bytes to read. Default: buffer.byteLength
• position <integer> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged.
• Returns: <Promise> Fulfills upon success with an object with two properties:
  • bytesRead <integer> The number of bytes read
  • buffer <Buffer> | <TypedArray> | <DataView> A reference to the passed in buffer argument.

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

filehandle.read([options])#

• options <Object>
  • buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read. Default: Buffer.alloc(16384)
  • offset <integer> The location in the buffer at which to start filling. Default: 0
  • length <integer> The number of bytes to read. Default: buffer.byteLength
  • position <integer> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged. Default:: null
• Returns: <Promise> Fulfills upon success with an object with two properties:
  • bytesRead <integer> The number of bytes read
  • buffer <Buffer> | <TypedArray> | <DataView> A reference to the passed in buffer argument.

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

filehandle.readFile(options)#

• options <Object> | <string>
  • encoding <string> | <null> Default: null
  • signal <AbortSignal> allows aborting an in-progress readFile
• Returns: <Promise> Fulfills upon a successful read with the contents of the file. If no encoding is specified (using options.encoding), the data is returned as a <Buffer> object. Otherwise, the data will be a string.

Asynchronously reads the entire contents of a file.

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

The <FileHandle> has to support reading.

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

filehandle.readv(buffers[, position])#

• buffers <Buffer[]> | <TypedArray[]> | <DataView[]>
• position <integer> The offset from the beginning of the file where the data should be read from. If position is not a number, the data will be read from the current position.
• Returns: <Promise> Fulfills upon success an object containing two properties:
  • bytesRead <integer> the number of bytes read
  • buffers <Buffer[]> | <TypedArray[]> | <DataView[]> property containing a reference to the buffers input.

Read from a file and write to an array of <ArrayBufferView>s

filehandle.stat([options])#

• options <Object>
  • bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.
• Returns: <Promise> Fulfills with an <fs.Stats> for the file.

filehandle.sync()#

• Returns: <Promise> Fufills with undefined upon success.

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX fsync(2) documentation for more detail.

filehandle.truncate(len)#

• len <integer> Default: 0
• Returns: <Promise> Fulfills with undefined upon success.

Truncates the file.

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

The following example retains only the first four bytes of the file:

import { open } from 'fs/promises';

let filehandle = null;
try {
  filehandle = await open('temp.txt', 'r+');
  await filehandle.truncate(4);
} finally {
  await filehandle?.close();
}

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

If len is negative then 0 will be used.

filehandle.utimes(atime, mtime)#

• atime <number> | <string> | <Date>
• mtime <number> | <string> | <Date>
• Returns: <Promise>

Change the file system timestamps of the object referenced by the <FileHandle> then resolves the promise with no arguments upon success.

filehandle.write(buffer[, offset[, length[, position]]])#

• buffer <Buffer> | <TypedArray> | <DataView> | <string> | <Object>
• offset <integer> The start position from within buffer where the data to write begins. Default: 0
• length <integer> The number of bytes from buffer to write. Default: buffer.byteLength
• position <integer> The offset from the beginning of the file where the data from buffer should be written. If position is not a number, the data will be written at the current position. See the POSIX pwrite(2) documentation for more detail.
• Returns: <Promise>

Write buffer to the file.

If buffer is a plain object, it must have an own (not inherited) toString function property.

The promise is resolved with an object containing two properties:

• bytesWritten <integer> the number of bytes written
• buffer <Buffer> | <TypedArray> | <DataView> | <string> | <Object> a reference to the buffer written.

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be resolved (or rejected). For this scenario, use fs.createWriteStream().

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

filehandle.write(string[, position[, encoding]])#

• string <string> | <Object>
• position <integer> The offset from the beginning of the file where the data from string should be written. If position is not a number the data will be written at the current position. See the POSIX pwrite(2) documentation for more detail.
• encoding <string> The expected string encoding. Default: 'utf8'
• Returns: <Promise>

Write string to the file. If string is not a string, or an object with an own toString function property, the promise is rejected with an error.

The promise is resolved with an object containing two properties:

• bytesWritten <integer> the number of bytes written
• buffer <string> | <Object> a reference to the string written.

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be resolved (or rejected). For this scenario, use fs.createWriteStream().

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

filehandle.writeFile(data, options)#

• data <string> | <Buffer> | <TypedArray> | <DataView> | <Object> | <AsyncIterable> | <Iterable> | <Stream>
• options <Object> | <string>
  • encoding <string> | <null> The expected character encoding when data is a string. Default: 'utf8'
• Returns: <Promise>

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string, a buffer, an <AsyncIterable> or <Iterable> object, or an object with an own toString function property. The promise is resolved with no arguments upon success.

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

The <FileHandle> has to support writing.

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

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

filehandle.writev(buffers[, position])#

• buffers <Buffer[]> | <TypedArray[]> | <DataView[]>
• position <integer> The offset from the beginning of the file where the data from buffers should be written. If position is not a number, the data will be written at the current position.
• Returns: <Promise>

Write an array of <ArrayBufferView>s to the file.

The promise is resolved with an object containing a two properties:

• bytesWritten <integer> the number of bytes written
• buffers <Buffer[]> | <TypedArray[]> | <DataView[]> a reference to the buffers input.

It is unsafe to call writev() multiple times on the same file without waiting for the promise to be resolved (or rejected).

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.

fsPromises.access(path[, mode])#

• path <string> | <Buffer> | <URL>
• mode <integer> Default: fs.constants.F_OK
• Returns: <Promise> Fulfills with undefined upon success.

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

If the accessibility check is successful, the promise is resolved with no value. If any of the accessibility checks fail, the promise is rejected with an <Error> object. The following example checks if the file /etc/passwd can be read and written by the current process.

import { access } from 'fs/promises';
import { constants } from 'fs';

try {
  await access('/etc/passwd', constants.R_OK | constants.W_OK);
  console.log('can access');
} catch {
  console.error('cannot access');
}

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

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

• path <string> | <Buffer> | <URL> | <FileHandle> filename or <FileHandle>
• data <string> | <Buffer>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • mode <integer> Default: 0o666
  • flag <string> See support of file system flags. Default: 'a'.
• Returns: <Promise> Fulfills with undefined upon success.

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.

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

The mode option only affects the newly created file. See fs.open() for more details.

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

fsPromises.chmod(path, mode)#

• path <string> | <Buffer> | <URL>
• mode <string> | <integer>
• Returns: <Promise> Fulfills with undefined upon success.

Changes the permissions of a file.

fsPromises.chown(path, uid, gid)#

• path <string> | <Buffer> | <URL>
• uid <integer>
• gid <integer>
• Returns: <Promise> Fulfills with undefined upon success.

Changes the ownership of a file.

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

• src <string> | <Buffer> | <URL> source filename to copy
• dest <string> | <Buffer> | <URL> destination filename of the copy operation
• mode <integer> Optional modifiers that specify the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE) Default: 0.
  • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
  • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
• Returns: <Promise> Fulfills with undefined upon success.

Asynchronously copies src to dest. By default, dest is overwritten if it already exists.

No guarantees are made about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, an attempt will be made to remove the destination.

import { constants } from 'fs';
import { copyFile } from 'fs/promises';

try {
  await copyFile('source.txt', 'destination.txt');
  console.log('source.txt was copied to destination.txt');
} catch {
  console.log('The file could not be copied');
}

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
try {
  await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
  console.log('source.txt was copied to destination.txt');
} catch {
  console.log('The file could not be copied');
}

fsPromises.cp(src, dest[, options])#

• src <string> | <URL> source path to copy.
• dest <string> | <URL> destination path to copy to.
• options <Object>
  • dereference <boolean> dereference symlinks. Default: false.
  • errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.
  • filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. Can also return a Promise that resolves to true or false Default: undefined.
  • force <boolean> overwrite existing file or directory. _The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.
  • preserveTimestamps <boolean> When true timestamps from src will be preserved. Default: false.
  • recursive <boolean> copy directories recursively Default: false
• Returns: <Promise> Fulfills with undefined upon success.

Asynchronously copies the entire directory structure from src to dest, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.

fsPromises.lchmod(path, mode)#

• path <string> | <Buffer> | <URL>
• mode <integer>
• Returns: <Promise> Fulfills with undefined upon success.

Changes the permissions on a symbolic link.

This method is only implemented on macOS.

fsPromises.lchown(path, uid, gid)#

• path <string> | <Buffer> | <URL>
• uid <integer>
• gid <integer>
• Returns: <Promise> Fulfills with undefined upon success.

Changes the ownership on a symbolic link.

fsPromises.lutimes(path, atime, mtime)#

• path <string> | <Buffer> | <URL>
• atime <number> | <string> | <Date>
• mtime <number> | <string> | <Date>
• Returns: <Promise> Fulfills with undefined upon success.

Changes the access and modification times of a file in the same way as fsPromises.utimes(), with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.

fsPromises.link(existingPath, newPath)#

• existingPath <string> | <Buffer> | <URL>
• newPath <string> | <Buffer> | <URL>
• Returns: <Promise> Fulfills with undefined upon success.

Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail.

fsPromises.lstat(path[, options])#

• path <string> | <Buffer> | <URL>
• options <Object>
  • bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.
• Returns: <Promise> Fulfills with the <fs.Stats> object for the given symbolic link path.

Equivalent to fsPromises.stat() unless path refers to a symbolic link, in which case the link itself is stat-ed, not the file that it refers to. Refer to the POSIX lstat(2) document for more detail.

fsPromises.mkdir(path[, options])#

• path <string> | <Buffer> | <URL>
• options <Object> | <integer>
  • recursive <boolean> Default: false
  • mode <string> | <integer> Not supported on Windows. Default: 0o777.
• Returns: <Promise> Upon success, fulfills with undefined if recursive is false, or the first directory path created if recursive is true.

Asynchronously creates a directory.

The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fsPromises.mkdir() when path is a directory that exists results in a rejection only when recursive is false.

fsPromises.mkdtemp(prefix[, options])#

• prefix <string>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
• Returns: <Promise> Fulfills with a string containing the filesystem path of the newly created temporary directory.

Creates a unique temporary directory. A unique directory name is generated by appending six random characters to the end of the provided prefix. Due to platform inconsistencies, avoid trailing X characters in prefix. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing X characters in prefix with random characters.

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

import { mkdtemp } from 'fs/promises';

try {
  await mkdtemp(path.join(os.tmpdir(), 'foo-'));
} catch (err) {
  console.error(err);
}

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

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

• path <string> | <Buffer> | <URL>
• flags <string> | <number> See support of file system flags. Default: 'r'.
• mode <string> | <integer> Sets the file mode (permission and sticky bits) if the file is created. Default: 0o666 (readable and writable)
• Returns: <Promise> Fulfills with a <FileHandle> object.

Opens a <FileHandle>.

Refer to the POSIX open(2) documentation for more detail.

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

fsPromises.opendir(path[, options])#

• path <string> | <Buffer> | <URL>
• options <Object>
  • encoding <string> | <null> Default: 'utf8'
  • bufferSize <number> Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: 32
• Returns: <Promise> Fulfills with an <fs.Dir>.

Asynchronously open a directory for iterative scanning. See the POSIX opendir(3) documentation for more detail.

Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

Example using async iteration:

import { opendir } from 'fs/promises';

try {
  const dir = await opendir('./');
  for await (const dirent of dir)
    console.log(dirent.name);
} catch (err) {
  console.error(err);
}

When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.

fsPromises.readdir(path[, options])#

• path <string> | <Buffer> | <URL>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
  • withFileTypes <boolean> Default: false
• Returns: <Promise> Fulfills with an array of the names of the files in the directory excluding '.' and '..'.

Reads the contents of a directory.

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

If options.withFileTypes is set to true, the resolved array will contain <fs.Dirent> objects.

import { readdir } from 'fs/promises';

try {
  const files = await readdir(path);
  for (const file of files)
    console.log(file);
} catch (err) {
  console.error(err);
}

fsPromises.readFile(path[, options])#

• path <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle
• options <Object> | <string>
  • encoding <string> | <null> Default: null
  • flag <string> See support of file system flags. Default: 'r'.
  • signal <AbortSignal> allows aborting an in-progress readFile
• Returns: <Promise> Fulfills with the contents of the file.

Asynchronously reads the entire contents of a file.

If no encoding is specified (using options.encoding), the data is returned as a <Buffer> object. Otherwise, the data will be a string.

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

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

It is possible to abort an ongoing readFile using an <AbortSignal>. If a request is aborted the promise returned is rejected with an AbortError:

import { readFile } from 'fs/promises';

try {
  const controller = new AbortController();
  const { signal } = controller;
  const promise = readFile(fileName, { signal });

  // Abort the request before the promise settles.
  controller.abort();

  await promise;
} catch (err) {
  // When a request is aborted - err is an AbortError
  console.error(err);
}

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.readFile performs.

Any specified <FileHandle> has to support reading.

fsPromises.readlink(path[, options])#

• path <string> | <Buffer> | <URL>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
• Returns: <Promise> Fulfills with the linkString upon success.

Reads the contents of the symbolic link referred to by path. See the POSIX readlink(2) documentation for more detail. The promise is resolved with the linkString upon success.

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

fsPromises.realpath(path[, options])#

• path <string> | <Buffer> | <URL>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
• Returns: <Promise> Fulfills with the resolved path upon success.

Determines the actual location of path using the same semantics as the fs.realpath.native() function.

Only paths that can be converted to UTF8 strings are supported.

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

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fsPromises.rename(oldPath, newPath)#

• oldPath <string> | <Buffer> | <URL>
• newPath <string> | <Buffer> | <URL>
• Returns: <Promise> Fulfills with undefined upon success.

Renames oldPath to newPath.

fsPromises.rmdir(path[, options])#

• path <string> | <Buffer> | <URL>
• options <Object>
  • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
  • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
  • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
• Returns: <Promise> Fulfills with undefined upon success.

Removes the directory identified by path.

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

To get a behavior similar to the rm -rf Unix command, use fsPromises.rm() with options { recursive: true, force: true }.

fsPromises.rm(path[, options])#

• path <string> | <Buffer> | <URL>
• options <Object>
  • force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.
  • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
  • recursive <boolean> If true, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: false.
  • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
• Returns: <Promise> Fulfills with undefined upon success.

Removes files and directories (modeled on the standard POSIX rm utility).

fsPromises.stat(path[, options])#

• path <string> | <Buffer> | <URL>
• options <Object>
  • bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.
• Returns: <Promise> Fulfills with the <fs.Stats> object for the given path.

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

• target <string> | <Buffer> | <URL>
• path <string> | <Buffer> | <URL>
• type <string> Default: 'file'
• Returns: <Promise> Fulfills with undefined upon success.

Creates a symbolic link.

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

fsPromises.truncate(path[, len])#

• path <string> | <Buffer> | <URL>
• len <integer> Default: 0
• Returns: <Promise> Fulfills with undefined upon success.

Truncates (shortens or extends the length) of the content at path to len bytes.

fsPromises.unlink(path)#

• path <string> | <Buffer> | <URL>
• Returns: <Promise> Fulfills with undefined upon success.

If path refers to a symbolic link, then the link is removed without affecting the file or directory to which that link refers. If the path refers to a file path that is not a symbolic link, the file is deleted. See the POSIX unlink(2) documentation for more detail.

fsPromises.utimes(path, atime, mtime)#

• path <string> | <Buffer> | <URL>
• atime <number> | <string> | <Date>
• mtime <number> | <string> | <Date>
• Returns: <Promise> Fulfills with undefined upon success.

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, an Error will be thrown.

fsPromises.watch(filename[, options])#

• 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'.
  • signal <AbortSignal> An <AbortSignal> used to signal when the watcher should stop.
• Returns: <AsyncIterator> of objects with the properties:
  • eventType <string> The type of change
  • filename <string> | <Buffer> The name of the file changed.

Returns an async iterator that watches for changes on filename, where filename is either a file or a directory.

const { watch } = require('fs/promises');

const ac = new AbortController();
const { signal } = ac;
setTimeout(() => ac.abort(), 10000);

(async () => {
  try {
    const watcher = watch(__filename, { signal });
    for await (const event of watcher)
      console.log(event);
  } catch (err) {
    if (err.name === 'AbortError')
      return;
    throw err;
  }
})();

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

All the caveats for fs.watch() also apply to fsPromises.watch().

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

• file <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle
• data <string> | <Buffer> | <TypedArray> | <DataView> | <Object> | <AsyncIterable> | <Iterable> | <Stream>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • mode <integer> Default: 0o666
  • flag <string> See support of file system flags. Default: 'w'.
  • signal <AbortSignal> allows aborting an in-progress writeFile
• Returns: <Promise> Fulfills with undefined upon success.

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string, a <Buffer>, or, an object with an own (not inherited) toString function property.

The encoding option is ignored if data is a buffer.

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

The mode option only affects the newly created file. See fs.open() for more details.

Any specified <FileHandle> has to support writing.

It is unsafe to use fsPromises.writeFile() multiple times on the same file without waiting for the promise to be settled.

Similarly to fsPromises.readFile - fsPromises.writeFile is a convenience method that performs multiple write calls internally to write the buffer passed to it. For performance sensitive code consider using fs.createWriteStream().

It is possible to use an <AbortSignal> to cancel an fsPromises.writeFile(). Cancelation is "best effort", and some amount of data is likely still to be written.

import { writeFile } from 'fs/promises';
import { Buffer } from 'buffer';

try {
  const controller = new AbortController();
  const { signal } = controller;
  const data = new Uint8Array(Buffer.from('Hello Node.js'));
  const promise = writeFile('message.txt', data, { signal });

  // Abort the request before the promise settles.
  controller.abort();

  await promise;
} catch (err) {
  // When a request is aborted - err is an AbortError
  console.error(err);
}

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.writeFile performs.

Callback API#

The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.

The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.

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

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

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

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

import { access, constants } from 'fs';

const file = 'package.json';

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

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

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

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

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

write (NOT RECOMMENDED)

import { access, open, close } from 'fs';

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

  open('myfile', 'wx', (err, fd) => {
    if (err) throw err;

    try {
      writeMyData(fd);
    } finally {
      close(fd, (err) => {
        if (err) throw err;
      });
    }
  });
});

write (RECOMMENDED)

import { open, close } from 'fs';

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

    throw err;
  }

  try {
    writeMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

read (NOT RECOMMENDED)

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

    throw err;
  }

  open('myfile', 'r', (err, fd) => {
    if (err) throw err;

    try {
      readMyData(fd);
    } finally {
      close(fd, (err) => {
        if (err) throw err;
      });
    }
  });
});

read (RECOMMENDED)

import { open, close } from 'fs';

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

    throw err;
  }

  try {
    readMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

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

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

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

fs.appendFile(path, data[, options], callback)#

• path <string> | <Buffer> | <URL> | <number> filename or file descriptor
• data <string> | <Buffer>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • mode <integer> Default: 0o666
  • flag <string> See support of file system flags. Default: 'a'.
• callback <Function>
  • err <Error>

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.

The mode option only affects the newly created file. See fs.open() for more details.

import { appendFile } from '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:

import { appendFile } from 'fs';

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

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

import { open, close, appendFile } from 'fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('message.txt', 'a', (err, fd) => {
  if (err) throw err;

  try {
    appendFile(fd, 'data to append', 'utf8', (err) => {
      closeFd(fd);
      if (err) throw err;
    });
  } catch (err) {
    closeFd(fd);
    throw err;
  }
});

fs.chmod(path, mode, callback)#

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

Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.

See the POSIX chmod(2) documentation for more detail.

import { chmod } from 'fs';

chmod('my_file.txt', 0o775, (err) => {
  if (err) throw err;
  console.log('The permissions for file "my_file.txt" have been changed!');
});

File modes#

The mode argument used in both the fs.chmod() and fs.chmodSync() methods is a numeric bitmask created using a logical OR of the following constants:

An easier method of constructing the mode is to use a sequence of three octal digits (e.g. 765). The left-most digit (7 in the example), specifies the permissions for the file owner. The middle digit (6 in the example), specifies permissions for the group. The right-most digit (5 in the example), specifies the permissions for others.

For example, the octal value 0o765 means:

• The owner may read, write and execute the file.
• The group may read and write the file.
• Others may read and execute the file.

When using raw numbers where file modes are expected, any value larger than 0o777 may result in platform-specific behaviors that are not supported to work consistently. Therefore constants like S_ISVTX, S_ISGID or S_ISUID are not exposed in fs.constants.

Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner or others is not implemented.

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

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

Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.

See the POSIX chown(2) documentation for more detail.

fs.close(fd[, callback])#

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

Closes the file descriptor. No arguments other than a possible exception are given to the completion callback.

Calling fs.close() on any file descriptor (fd) that is currently in use through any other fs operation may lead to undefined behavior.

See the POSIX close(2) documentation for more detail.

fs.copyFile(src, dest[, mode], callback)#

• src <string> | <Buffer> | <URL> source filename to copy
• dest <string> | <Buffer> | <URL> destination filename of the copy operation
• mode <integer> 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.

mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).

• fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
• fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
• fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.

import { copyFile, constants } from 'fs';

function callback(err) {
  if (err) throw err;
  console.log('source.txt was copied to destination.txt');
}

// destination.txt will be created or overwritten by default.
copyFile('source.txt', 'destination.txt', callback);

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

fs.cp(src, dest[, options], callback)#

• src <string> | <URL> source path to copy.
• dest <string> | <URL> destination path to copy to.
• options <Object>
  • dereference <boolean> dereference symlinks. Default: false.
  • errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.
  • filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. Can also return a Promise that resolves to true or false Default: undefined.
  • force <boolean> overwrite existing file or directory. _The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.
  • preserveTimestamps <boolean> When true timestamps from src will be preserved. Default: false.
  • recursive <boolean> copy directories recursively Default: false
• callback <Function>

Asynchronously copies the entire directory structure from src to dest, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.

fs.createReadStream(path[, options])#

• path <string> | <Buffer> | <URL>
• options <string> | <Object>
  • flags <string> See support of file system flags. Default: 'r'.
  • encoding <string> Default: null
  • fd <integer> | <FileHandle> Default: null
  • mode <integer> Default: 0o666
  • autoClose <boolean> Default: true
  • emitClose <boolean> Default: true
  • start <integer>
  • end <integer> Default: Infinity
  • highWaterMark <integer> Default: 64 * 1024
  • fs <Object> | <null> Default: null
• Returns: <fs.ReadStream> See Readable Stream.

Unlike the 16 kb default highWaterMark for a readable stream, the stream returned by this method has a default highWaterMark of 64 kb.

options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. If fd is specified and start is omitted or undefined, fs.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by <Buffer>.

If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fds should be passed to <net.Socket>.

If fd points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally.

By default, the stream will emit a 'close' event after it has been destroyed, like most Readable streams. Set the emitClose option to false to change this behavior.

By providing the fs option, it is possible to override the corresponding fs implementations for open, read, and close. When providing the fs option, overrides for open, read, and close are required.

import { createReadStream } from 'fs';

// Create a stream from some character device.
const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
  stream.close(); // This may not close the stream.
  // Artificially marking end-of-stream, as if the underlying resource had
  // indicated end-of-file by itself, allows the stream to close.
  // This does not cancel pending read operations, and if there is such an
  // operation, the process may still not be able to exit successfully
  // until it finishes.
  stream.push(null);
  stream.read(0);
}, 100);

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:

import { createReadStream } from '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> See support of file system flags. Default: 'w'.
  • encoding <string> Default: 'utf8'
  • fd <integer> | <FileHandle> Default: null
  • mode <integer> Default: 0o666
  • autoClose <boolean> Default: true
  • emitClose <boolean> Default: true
  • start <integer>
  • fs <Object> | <null> Default: null
• Returns: <fs.WriteStream> See Writable Stream.

options may also include a start option to allow writing data at some position past the beginning of the file, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than replacing it may require the flags option to be set to r+ rather than the default w. The encoding can be any one of those accepted by <Buffer>.

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

By default, the stream will emit a 'close' event after it has been destroyed, like most Writable streams. Set the emitClose option to false to change this behavior.

By providing the fs option it is possible to override the corresponding fs implementations for open, write, writev and close. Overriding write() without writev() can reduce performance as some optimizations (_writev()) will be disabled. When providing the fs option, overrides for open, close, and at least one of write and writev are required.

Like <fs.ReadStream>, if fd is specified, <fs.WriteStream> will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking 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:

import { exists } from 'fs';

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

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

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

write (NOT RECOMMENDED)

import { exists, open, close } from 'fs';

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

      try {
        writeMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  }
});

write (RECOMMENDED)

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

    throw err;
  }

  try {
    writeMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

read (NOT RECOMMENDED)

import { open, close, exists } from 'fs';

exists('myfile', (e) => {
  if (e) {
    open('myfile', 'r', (err, fd) => {
      if (err) throw err;

      try {
        readMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

import { open, close } from 'fs';

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

    throw err;
  }

  try {
    readMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

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.fchmod(fd, mode, callback)#

• fd <integer>
• mode <string> | <integer>
• callback <Function>
  • err <Error>

Sets the permissions on the file. No arguments other than a possible exception are given to the completion callback.

See the POSIX fchmod(2) documentation for more detail.

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

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

Sets the owner of the file. No arguments other than a possible exception are given to the completion callback.

See the POSIX fchown(2) documentation for more detail.

fs.fdatasync(fd, callback)#

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

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX fdatasync(2) documentation for details. No arguments other than a possible exception are given to the completion callback.

fs.fstat(fd[, options], callback)#

• fd <integer>
• options <Object>
  • bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.
• callback <Function>
  • err <Error>
  • stats <fs.Stats>

Invokes the callback with the <fs.Stats> for the file descriptor.

See the POSIX fstat(2) documentation for more detail.

fs.fsync(fd, callback)#

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

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX fsync(2) documentation for more detail. No arguments other than a possible exception are given to the completion callback.

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

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

Truncates the file descriptor. No arguments other than a possible exception are given to the completion callback.

See the POSIX ftruncate(2) documentation for more detail.

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:

import { open, close, ftruncate } from 'fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('temp.txt', 'r+', (err, fd) => {
  if (err) throw err;

  try {
    ftruncate(fd, 4, (err) => {
      closeFd(fd);
      if (err) throw err;
    });
  } catch (err) {
    closeFd(fd);
    if (err) throw err;
  }
});

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

If len is negative then 0 will be used.

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().

fs.lchmod(path, mode, callback)#

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

Changes the permissions on a symbolic link. No arguments other than a possible exception are given to the completion callback.

This method is only implemented on macOS.

See the POSIX lchmod(2) documentation for more detail.

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

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

Set the owner of the symbolic link. No arguments other than a possible exception are given to the completion callback.

See the POSIX lchown(2) documentation for more detail.

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

• path <string> | <Buffer> | <URL>
• atime <number> | <string> | <Date>
• mtime <number> | <string> | <Date>
• callback <Function>
  • err <Error>

Changes the access and modification times of a file in the same way as fs.utimes(), with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.

No arguments other than a possible exception are given to the completion callback.

fs.link(existingPath, newPath, callback)#

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

Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail. No arguments other than a possible exception are given to the completion callback.

fs.lstat(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <Object>
  • bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.
• callback <Function>
  • err <Error>
  • stats <fs.Stats>

Retrieves the <fs.Stats> for the symbolic link referred to by the path. 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.

See the POSIX lstat(2) documentation for more details.

fs.mkdir(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <Object> | <integer>
  • recursive <boolean> Default: false
  • mode <string> | <integer> Not supported on Windows. Default: 0o777.
• callback <Function>
  • err <Error>

Asynchronously creates a directory.

The callback is given a possible exception and, if recursive is true, the first directory path created, (err, [path]). path can still be undefined when recursive is true, if no directory was created.

The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fs.mkdir() when path is a directory that exists results in an error only when recursive is false.

import { mkdir } from 'fs';

// Creates /tmp/a/apple, regardless of whether `/tmp` and /tmp/a exist.
mkdir('/tmp/a/apple', { recursive: true }, (err) => {
  if (err) throw err;
});

On Windows, using fs.mkdir() on the root directory even with recursion will result in an error:

import { mkdir } from 'fs';

mkdir('/', { recursive: true }, (err) => {
  // => [Error: EPERM: operation not permitted, mkdir 'C:\']
});

See the POSIX mkdir(2) documentation for more details.

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

• prefix <string>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
• callback <Function>
  • err <Error>
  • directory <string>

Creates a unique temporary directory.

Generates six random characters to be appended behind a required prefix to create a unique temporary directory. Due to platform inconsistencies, avoid trailing X characters in prefix. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing X characters in prefix with random characters.

The created directory 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.

import { mkdtemp } from 'fs';

mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});

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

import { tmpdir } from 'os';
import { mkdtemp } from 'fs';

// The parent directory for the new temporary directory
const tmpDir = tmpdir();

// This method is *INCORRECT*:
mkdtemp(tmpDir, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Will print something similar to `/tmpabc123`.
  // A new temporary directory is created at the file system root
  // rather than *within* the /tmp directory.
});

// This method is *CORRECT*:
import { sep } from 'path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Will print something similar to `/tmp/abc123`.
  // A new temporary directory is created within
  // the /tmp directory.
});

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

• path <string> | <Buffer> | <URL>
• flags <string> | <number> See support of file system flags. Default: 'r'.
• mode <string> | <integer> Default: 0o666 (readable and writable)
• callback <Function>
  • err <Error>
  • fd <integer>

Asynchronous file open. See the POSIX open(2) documentation for more details.

mode sets the file mode (permission and sticky bits), but only if the file was created. On Windows, only the write permission can be manipulated; see fs.chmod().

The callback gets two arguments (err, fd).

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

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

fs.opendir(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <Object>
  • encoding <string> | <null> Default: 'utf8'
  • bufferSize <number> Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: 32
• callback <Function>
  • err <Error>
  • dir <fs.Dir>

Asynchronously open a directory. See the POSIX opendir(3) documentation for more details.

Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

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

• fd <integer>
• buffer <Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to. Default: Buffer.alloc(16384)
• offset <integer> The position in buffer to write the data to. Default: 0
• length <integer> The number of bytes to read. Default: buffer.byteLength
• position <integer> | <bigint> Specifies where to begin reading from in the file. If position is null or -1 , 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 be unchanged.
• callback <Function>
  • err <Error>
  • bytesRead <integer>
  • buffer <Buffer>

Read data from the file specified by fd.

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

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesRead and buffer properties.

fs.read(fd, [options,] callback)#

• fd <integer>
• options <Object>
  • buffer <Buffer> | <TypedArray> | <DataView> Default: Buffer.alloc(16384)
  • offset <integer> Default: 0
  • length <integer> Default: buffer.byteLength
  • position <integer> | <bigint> Default: null
• callback <Function>
  • err <Error>
  • bytesRead <integer>
  • buffer <Buffer>

Similar to the fs.read() function, this version takes an optional options object. If no options object is specified, it will default with the above values.

fs.readdir(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
  • withFileTypes <boolean> Default: false
• callback <Function>
  • err <Error>
  • files <string[]> | <Buffer[]> | <fs.Dirent[]>

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 '..'.

See the POSIX readdir(3) documentation for more details.

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.

If options.withFileTypes is set to true, the files array will contain <fs.Dirent> 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> See support of file system flags. Default: 'r'.
  • signal <AbortSignal> allows aborting an in-progress readFile
• callback <Function>
  • err <Error> | <AggregateError>
  • data <string> | <Buffer>

Asynchronously reads the entire contents of a file.

import { readFile } from '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:

import { readFile } from 'fs';

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

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

import { readFile } from 'fs';

// macOS, Linux, and Windows
readFile('<directory>', (err, data) => {
  // => [Error: EISDIR: illegal operation on a directory, read <directory>]
});

//  FreeBSD
readFile('<directory>', (err, data) => {
  // => null, <data>
});

It is possible to abort an ongoing request using an AbortSignal. If a request is aborted the callback is called with an AbortError:

import { readFile } from 'fs';

const controller = new AbortController();
const signal = controller.signal;
readFile(fileInfo[0].name, { signal }, (err, buf) => {
  // ...
});
// When you want to abort the request
controller.abort();

The fs.readFile() function buffers the entire file. To minimize memory costs, when possible prefer streaming via fs.createReadStream().

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.readFile performs.

File descriptors#

1. Any specified file descriptor has to support reading.
2. If a file descriptor is specified as the path, it will not be closed automatically.
3. The reading will begin at the current position. For example, if the file already had 'Hello World' and six bytes are read with the file descriptor, the call to fs.readFile() with the same file descriptor, would give 'World', rather than 'Hello World'.

Performance Considerations#

The fs.readFile() method asynchronously reads the contents of a file into memory one chunk at a time, allowing the event loop to turn between each chunk. This allows the read operation to have less impact on other activity that may be using the underlying libuv thread pool but means that it will take longer to read a complete file into memory.

The additional read overhead can vary broadly on different systems and depends on the type of file being read. If the file type is not a regular file (a pipe for instance) and Node.js is unable to determine an actual file size, each read operation will load on 64 KB of data. For regular files, each read will process 512 KB of data.

For applications that require as-fast-as-possible reading of file contents, it is better to use fs.read() directly and for application code to manage reading the full contents of the file itself.

The Node.js GitHub issue #25741 provides more information and a detailed analysis on the performance of fs.readFile() for multiple file sizes in different Node.js versions.

fs.readlink(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <string> | <Object>
  • encoding <string> Default: 'utf8'
• callback <Function>
  • err <Error>
  • linkString <string> | <Buffer>

Reads the contents of the symbolic link referred to by path. The callback gets two arguments (err, linkString).

See the POSIX readlink(2) documentation for more details.

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.readv(fd, buffers[, position], callback)#

• fd <integer>
• buffers <ArrayBufferView[]>
• position <integer>
• callback <Function>
  • err <Error>
  • bytesRead <integer>
  • buffers <ArrayBufferView[]>

Read from a file specified by fd and write to an array of ArrayBufferViews using readv().

position is the offset from the beginning of the file from where data should be read. If typeof position !== 'number', the data will be read from the current position.

The callback will be given three arguments: err, bytesRead, and buffers. bytesRead is how many bytes were read from the file.

If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesRead and buffers properties.

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.

A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.

This function behaves like realpath(3), with some exceptions:

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.

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.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fs.rename(oldPath, newPath, callback)#

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

Asynchronously rename file at oldPath to the pathname provided as newPath. In the case that newPath already exists, it will be overwritten. If there is a directory at newPath, an error will be raised instead. No arguments other than a possible exception are given to the completion callback.

See also: rename(2).

import { rename } from 'fs';

rename('oldFile.txt', 'newFile.txt', (err) => {
  if (err) throw err;
  console.log('Rename complete!');
});

fs.rmdir(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <Object>
  • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
  • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
  • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
• callback <Function>
  • err <Error>

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

Using fs.rmdir() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

To get a behavior similar to the rm -rf Unix command, use fs.rm() with options { recursive: true, force: true }.

fs.rm(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <Object>
  • force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.
  • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
  • recursive <boolean> If true, perform a recursive removal. In recursive mode operations are retried on failure. Default: false.
  • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
• callback <Function>
  • err <Error>

Asynchronously removes files and directories (modeled on the standard POSIX rm utility). No arguments other than a possible exception are given to the completion callback.

fs.stat(path[, options], callback)#

• path <string> | <Buffer> | <URL>
• options <Object>
  • bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.
• 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.

For example, given the following directory structure:

- txtDir
-- file.txt
- app.js

The next program will check for the stats of the given paths:

import { stat } from 'fs';

const pathsToCheck = ['./txtDir', './txtDir/file.txt'];

for (let i = 0; i < pathsToCheck.length; i++) {
  stat(pathsToCheck[i], (err, stats) => {
    console.log(stats.isDirectory());
    console.log(stats);
  });
}

The resulting output will resemble:

true
Stats {
  dev: 16777220,
  mode: 16877,
  nlink: 3,
  uid: 501,
  gid: 20,
  rdev: 0,
  blksize: 4096,
  ino: 14214262,
  size: 96,
  blocks: 0,
  atimeMs: 1561174653071.963,
  mtimeMs: 1561174614583.3518,
  ctimeMs: 1561174626623.5366,
  birthtimeMs: 1561174126937.2893,
  atime: 2019-06-22T03:37:33.072Z,
  mtime: 2019-06-22T03:36:54.583Z,
  ctime: 2019-06-22T03:37:06.624Z,
  birthtime: 2019-06-22T03:28:46.937Z
}
false
Stats {
  dev: 16777220,
  mode: 33188,
  nlink: 1,
  uid: 501,
  gid: 20,
  rdev: 0,
  blksize: 4096,
  ino: 14214074,
  size: 8,
  blocks: 8,
  atimeMs: 1561174616618.8555,
  mtimeMs: 1561174614584,
  ctimeMs: 1561174614583.8145,
  birthtimeMs: 1561174007710.7478,
  atime: 2019-06-22T03:36:56.619Z,
  mtime: 2019-06-22T03:36:54.584Z,
  ctime: 2019-06-22T03:36:54.584Z,
  birthtime: 2019-06-22T03:26:47.711Z
}

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

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

Creates the link called path pointing to target. No arguments other than a possible exception are given to the completion callback.

See the POSIX symlink(2) documentation for more details.

The type argument is only available on Windows and ignored on other platforms. It can be set to 'dir', 'file', or 'junction'. If the type argument is not set, Node.js will autodetect target type and use 'file' or 'dir'. If the target does not exist, 'file' will be used. Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Relative targets are relative to the link’s parent directory.

import { symlink } from 'fs';

symlink('./mew', './example/mewtwo', callback);

The above example creates a symbolic link mewtwo in the example which points to mew in the same directory:

$ tree example/
example/
├── mew
└── mewtwo -> ./mew

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

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

Truncates the file. 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.

import { truncate } from 'fs';
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was truncated');
});const { truncate } = require('fs');
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was truncated');
});

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

See the POSIX truncate(2) documentation for more details.

fs.unlink(path, callback)#

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

Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.

import { unlink } from 'fs';
// Assuming that 'path/file.txt' is a regular file.
unlink('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was deleted');
});

fs.unlink() will not work on a directory, empty or otherwise. To remove a directory, use fs.rmdir().

See the POSIX unlink(2) documentation for more details.

fs.unwatchFile(filename[, listener])#

• filename <string> | <Buffer> | <URL>
• listener <Function> Optional, a listener previously attached using fs.watchFile()

Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed, effectively stopping watching of filename.

Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error.

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

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

• path <string> | <Buffer> | <URL>
• atime <number> | <string> | <Date>
• mtime <number> | <string> | <Date>
• callback <Function>
  • err <Error>

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

The atime and mtime arguments follow these rules:

• Values can be either numbers representing Unix epoch time in seconds, Dates, or a numeric string like '123456789.0'.
• If the value can not be converted to a number, or is NaN, Infinity or -Infinity, an Error will be thrown.

fs.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'.
  • signal <AbortSignal> allows closing the watcher with an AbortSignal.
• listener <Function> | <undefined> Default: undefined
  • eventType <string>
  • filename <string> | <Buffer>
• Returns: <fs.FSWatcher>

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

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

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

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

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

If a signal is passed, aborting the corresponding AbortController will close the returned <fs.FSWatcher>.

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. An ERR_FEATURE_UNAVAILABLE_ON_PLATFORM exception will be thrown when the option is used on a platform that does not support it.

On Windows, no events will be emitted if the watched directory is moved or renamed. An EPERM error is reported when the watched directory is deleted.

Availability#

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

• On Linux systems, this uses inotify(7).
• On BSD systems, this uses kqueue(2).
• On macOS, this uses kqueue(2) for files and FSEvents for directories.
• On SunOS systems (including Solaris and SmartOS), this uses event ports.
• On Windows systems, this feature depends on ReadDirectoryChangesW.
• On AIX systems, this feature depends on AHAFS, which must be enabled.
• On IBM i systems, this feature is not supported.

If the underlying functionality is not available for some reason, then fs.watch() will not be able to function and may throw an exception. 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 or Docker.

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.

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

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

• filename <string> | <Buffer> | <URL>
• options <Object>
  • bigint <boolean> Default: false
  • persistent <boolean> Default: true
  • interval <integer> Default: 5007
• listener <Function>
  • current <fs.Stats>
  • previous <fs.Stats>
• Returns: <fs.StatWatcher>

Watch for changes on filename. The callback listener will be called each time the file is accessed.

The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds.

The listener gets two arguments the current stat object and the previous stat object:

import { watchFile } from 'fs';

watchFile('message.text', (curr, prev) => {
  console.log(`the current mtime is: ${curr.mtime}`);
  console.log(`the previous mtime was: ${prev.mtime}`);
});

These stat objects are instances of fs.Stat. If the bigint option is true, the numeric values in these objects are specified as BigInts.

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

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

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

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

This happens when:

• the file is deleted, followed by a restore
• the file is renamed and then renamed a second time back to its original name

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

• fd <integer>
• buffer <Buffer> | <TypedArray> | <DataView> | <string> | <Object>
• offset <integer>
• length <integer>
• position <integer>
• callback <Function>
  • err <Error>
  • bytesWritten <integer>
  • buffer <Buffer> | <TypedArray> | <DataView>

Write buffer to the file specified by fd. If buffer is a normal object, it must have an own toString function property.

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

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

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

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

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

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

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

• fd <integer>
• string <string> | <Object>
• position <integer>
• encoding <string> Default: 'utf8'
• callback <Function>
  • err <Error>
  • written <integer>
  • string <string>

Write string to the file specified by fd. If string is not a string, or an object with an own toString function property, then an exception is thrown.

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

encoding is the expected string encoding.

The callback will receive the arguments (err, written, string) where written specifies how many bytes the passed string required to be written. Bytes written is not necessarily the same as string characters written. See Buffer.byteLength.

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

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

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

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

• file <string> | <Buffer> | <URL> | <integer> filename or file descriptor
• data <string> | <Buffer> | <TypedArray> | <DataView> | <Object>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • mode <integer> Default: 0o666
  • flag <string> See support of file system flags. Default: 'w'.
  • signal <AbortSignal> allows aborting an in-progress writeFile
• callback <Function>
  • err <Error> | <AggregateError>

When file is a filename, asynchronously writes data to the file, replacing the file if it already exists. data can be a string or a buffer.

When file is a file descriptor, the behavior is similar to calling fs.write() directly (which is recommended). See the notes below on using a file descriptor.

The encoding option is ignored if data is a buffer.

The mode option only affects the newly created file. See fs.open() for more details.

If data is a plain object, it must have an own (not inherited) toString function property.

import { writeFile } from 'fs';
import { Buffer } from 'buffer';

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

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

import { writeFile } from 'fs';

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