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File system | Node.js v24.3.0 Documentation

File system#

Source Code: lib/fs.js

The node:fs module enables interacting with the file system in a way modeled on standard POSIX functions.

To use the promise-based APIs:

import * as fs from 'node:fs/promises';const fs = require('node:fs/promises');

To use the callback and sync APIs:

import * as fs from 'node:fs';const fs = require('node:fs');

All file system operations have synchronous, callback, and promise-based forms, and are accessible using both CommonJS syntax and ES6 Modules (ESM).

Promise example#

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

import { unlink } from 'node: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('node: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 'node:fs';

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

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

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

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

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

Added in: v10.0.0

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'#

Added in: v15.4.0

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

filehandle.chmod(mode)#

Added in: v10.0.0

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

filehandle.chown(uid, gid)#

Added in: v10.0.0

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

filehandle.close()#

Added in: v10.0.0

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

import { open } from 'node:fs/promises';

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

Added in: v16.11.0

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 start is omitted or undefined, filehandle.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by <Buffer>.

If the FileHandle 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. Set the emitClose option to false to change this behavior.

import { open } from 'node:fs/promises';

const fd = await open('/dev/input/event0');

const stream = fd.createReadStream();
setTimeout(() => {
  stream.close(); 
  
  
  
  
  
  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.

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

import { open } from 'node:fs/promises';

const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });
filehandle.createWriteStream([options])#

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 open option to be set to r+ rather than the default r. 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. Set the emitClose option to false to change this behavior.

filehandle.datasync()#

Added in: v10.0.0

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#

Added in: v10.0.0

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

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])#

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(buffer[, options])#

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.readableWebStream([options])#

Returns a byte-oriented ReadableStream that may be used to read the file's contents.

An error will be thrown if this method is called more than once or is called after the FileHandle is closed or closing.

import {
  open,
} from 'node:fs/promises';

const file = await open('./some/file/to/read');

for await (const chunk of file.readableWebStream())
  console.log(chunk);

await file.close();const {
  open,
} = require('node:fs/promises');

(async () => {
  const file = await open('./some/file/to/read');

  for await (const chunk of file.readableWebStream())
    console.log(chunk);

  await file.close();
})();

While the ReadableStream will read the file to completion, it will not close the FileHandle automatically. User code must still call the fileHandle.close() method.

filehandle.readFile(options)#

Added in: v10.0.0

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.readLines([options])#

Added in: v18.11.0

Convenience method to create a readline interface and stream over the file. See filehandle.createReadStream() for the options.

import { open } from 'node:fs/promises';

const file = await open('./some/file/to/read');

for await (const line of file.readLines()) {
  console.log(line);
}const { open } = require('node:fs/promises');

(async () => {
  const file = await open('./some/file/to/read');

  for await (const line of file.readLines()) {
    console.log(line);
  }
})();
filehandle.sync()#

Added in: v10.0.0

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)#

Added in: v10.0.0

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 'node: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.write(buffer, offset[, length[, position]])#

Write buffer to the file.

The promise is fulfilled with an object containing two properties:

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be fulfilled (or rejected). For this scenario, use filehandle.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(buffer[, options])#

Added in: v18.3.0, v16.17.0

Write buffer to the file.

Similar to the above filehandle.write function, this version takes an optional options object. If no options object is specified, it will default with the above values.

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

Write string to the file. If string is not a string, the promise is rejected with an error.

The promise is fulfilled with an object containing two properties:

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

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object. The promise is fulfilled 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 fulfilled (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])#

Added in: v12.9.0

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

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

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

filehandle[Symbol.asyncDispose]()#

Calls filehandle.close() and returns a promise that fulfills when the filehandle is closed.

fsPromises.access(path[, mode])#

Added in: v10.0.0

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. mode should be either the value fs.constants.F_OK or a mask consisting of the bitwise OR of any of fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g. fs.constants.W_OK | fs.constants.R_OK). Check File access constants for possible values of mode.

If the accessibility check is successful, the promise is fulfilled 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, constants } from 'node:fs/promises';

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

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 { copyFile, constants } from 'node:fs/promises';

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


try {
  await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
  console.log('source.txt was copied to destination.txt');
} catch {
  console.error('The file could not be copied');
}
fsPromises.cp(src, dest[, options])#

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.glob(pattern[, options])# 
import { glob } from 'node:fs/promises';

for await (const entry of glob('**/*.js'))
  console.log(entry);const { glob } = require('node:fs/promises');

(async () => {
  for await (const entry of glob('**/*.js'))
    console.log(entry);
})();
fsPromises.lchmod(path, mode)#

Deprecated since: v10.0.0

Changes the permissions on a symbolic link.

This method is only implemented on macOS.

fsPromises.lutimes(path, atime, mtime)#

Added in: v14.5.0, v12.19.0

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)#

Added in: v10.0.0

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

fsPromises.lstat(path[, options])#

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])#

Added in: v10.0.0

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.

import { mkdir } from 'node:fs/promises';

try {
  const projectFolder = new URL('./test/project/', import.meta.url);
  const createDir = await mkdir(projectFolder, { recursive: true });

  console.log(`created ${createDir}`);
} catch (err) {
  console.error(err.message);
}const { mkdir } = require('node:fs/promises');
const { join } = require('node:path');

async function makeDirectory() {
  const projectFolder = join(__dirname, 'test', 'project');
  const dirCreation = await mkdir(projectFolder, { recursive: true });

  console.log(dirCreation);
  return dirCreation;
}

makeDirectory().catch(console.error);
fsPromises.mkdtemp(prefix[, options])#

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 'node:fs/promises';
import { join } from 'node:path';
import { tmpdir } from 'node:os';

try {
  await mkdtemp(join(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('node:path').sep).

fsPromises.opendir(path[, options])#

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 'node: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])#

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 returned array will contain <fs.Dirent> objects.

import { readdir } from 'node: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])#

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.

An example of reading a package.json file located in the same directory of the running code:

import { readFile } from 'node:fs/promises';
try {
  const filePath = new URL('./package.json', import.meta.url);
  const contents = await readFile(filePath, { encoding: 'utf8' });
  console.log(contents);
} catch (err) {
  console.error(err.message);
}const { readFile } = require('node:fs/promises');
const { resolve } = require('node:path');
async function logFile() {
  try {
    const filePath = resolve('./package.json');
    const contents = await readFile(filePath, { encoding: 'utf8' });
    console.log(contents);
  } catch (err) {
    console.error(err.message);
  }
}
logFile();

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 'node:fs/promises';

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

  
  controller.abort();

  await promise;
} catch (err) {
  
  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])#

Added in: v10.0.0

Reads the contents of the symbolic link referred to by path. See the POSIX readlink(2) documentation for more detail. The promise is fulfilled 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])#

Added in: v10.0.0

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.rmdir(path[, options])#

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])#

Added in: v14.14.0

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

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

Creates a symbolic link.

The type argument is only used on Windows platforms and can be one of 'dir', 'file', or 'junction'. If the type argument is null, 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. Junction points on NTFS volumes can only point to directories.

fsPromises.truncate(path[, len])#

Added in: v10.0.0

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

fsPromises.unlink(path)#

Added in: v10.0.0

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)#

Added in: v10.0.0

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

The atime and mtime arguments follow these rules:

fsPromises.watch(filename[, options])#

Added in: v15.9.0, v14.18.0

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

const { watch } = require('node: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])#

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.

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() or filehandle.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 'node:fs/promises';
import { Buffer } from 'node: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 });

  
  controller.abort();

  await promise;
} catch (err) {
  
  console.error(err);
}

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

fsPromises.constants#

Added in: v18.4.0, v16.17.0

Returns an object containing commonly used constants for file system operations. The object is the same as fs.constants. See FS constants for more details.

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)#

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. mode should be either the value fs.constants.F_OK or a mask consisting of the bitwise OR of any of fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g. fs.constants.W_OK | fs.constants.R_OK). Check File access constants for possible values of mode.

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 'node:fs';

const file = 'package.json';


access(file, constants.F_OK, (err) => {
  console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});


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


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


access(file, constants.R_OK | constants.W_OK, (err) => {
  console.log(`${file} ${err ? 'is not' : 'is'} readable and 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 'node: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 'node: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 'node: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 'node: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)#

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 'node: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 'node: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 'node: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)#

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 'node: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:

Constant Octal Description fs.constants.S_IRUSR 0o400 read by owner fs.constants.S_IWUSR 0o200 write by owner fs.constants.S_IXUSR 0o100 execute/search by owner fs.constants.S_IRGRP 0o40 read by group fs.constants.S_IWGRP 0o20 write by group fs.constants.S_IXGRP 0o10 execute/search by group fs.constants.S_IROTH 0o4 read by others fs.constants.S_IWOTH 0o2 write by others fs.constants.S_IXOTH 0o1 execute/search by others

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

Number Description 7 read, write, and execute 6 read and write 5 read and execute 4 read only 3 write and execute 2 write only 1 execute only 0 no permission

For example, the octal value 0o765 means:

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.close(fd[, callback])#

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)#

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).

import { copyFile, constants } from 'node:fs';

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


copyFile('source.txt', 'destination.txt', callback);


copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback);
fs.cp(src, dest[, options], callback)#

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])#

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. 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, an override for read is required. If no fd is provided, an override for open is also required. If autoClose is true, an override for close is also required.

import { createReadStream } from 'node:fs';


const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
  stream.close(); 
  
  
  
  
  
  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 'node:fs';

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

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

fs.createWriteStream(path[, options])#

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. 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 at least one of write and writev are required. If no fd option is supplied, an override for open is also required. If autoClose is true, an override for close is also 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)#

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

import { exists } from 'node: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().

If path is a symbolic link, it is followed. Thus, if path exists but points to a non-existent element, the callback will receive the value false.

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 'node: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 'node: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 'node: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 'node: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)#

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)#

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)#

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.fsync(fd, callback)#

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)#

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 'node: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.glob(pattern[, options], callback)# 
import { glob } from 'node:fs';

glob('**/*.js', (err, matches) => {
  if (err) throw err;
  console.log(matches);
});const { glob } = require('node:fs');

glob('**/*.js', (err, matches) => {
  if (err) throw err;
  console.log(matches);
});
fs.lutimes(path, atime, mtime, callback)#

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)#

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)#

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)#

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 (for instance, if it was previously 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. If recursive is false and the directory exists, an EEXIST error occurs.

import { mkdir } from 'node:fs';


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 'node:fs';

mkdir('/', { recursive: true }, (err) => {
  
});

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

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

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 'node:fs';
import { join } from 'node:path';
import { tmpdir } from 'node:os';

mkdtemp(join(tmpdir(), 'foo-'), (err, directory) => {
  if (err) throw err;
  console.log(directory);
  
});

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('node:path').sep).

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


const tmpDir = tmpdir();


mkdtemp(tmpDir, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  
  
  
});


import { sep } from 'node:path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  
  
  
});
fs.open(path[, flags[, mode]], callback)#

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.openAsBlob(path[, options])#

Returns a <Blob> whose data is backed by the given file.

The file must not be modified after the <Blob> is created. Any modifications will cause reading the <Blob> data to fail with a DOMException error. Synchronous stat operations on the file when the Blob is created, and before each read in order to detect whether the file data has been modified on disk.

import { openAsBlob } from 'node:fs';

const blob = await openAsBlob('the.file.txt');
const ab = await blob.arrayBuffer();
blob.stream();const { openAsBlob } = require('node:fs');

(async () => {
  const blob = await openAsBlob('the.file.txt');
  const ab = await blob.arrayBuffer();
  blob.stream();
})();
fs.opendir(path[, options], callback)#

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)#

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.

The fs.read() method reads data from the file specified by the file descriptor (fd). The length argument indicates the maximum number of bytes that Node.js will attempt to read from the kernel. However, the actual number of bytes read (bytesRead) can be lower than the specified length for various reasons.

For example:

Therefore, when using fs.read(), it's important to check the bytesRead value to determine how many bytes were actually read from the file. Depending on your application logic, you may need to handle cases where bytesRead is lower than the specified length, such as by wrapping the read call in a loop if you require a minimum amount of bytes.

This behavior is similar to the POSIX preadv2 function.

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

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)#

Asynchronously reads the entire contents of a file.

import { readFile } from 'node: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 'node: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 'node:fs';


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


readFile('<directory>', (err, 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 'node:fs';

const controller = new AbortController();
const signal = controller.signal;
readFile(fileInfo[0].name, { signal }, (err, buf) => {
  
});

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 KiB of data. For regular files, each read will process 512 KiB 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)#

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)#

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)#

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.

A path that does not exist results in an ENOENT error. error.path is the absolute file path.

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

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)#

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 'node:fs';

rename('oldFile.txt', 'newFile.txt', (err) => {
  if (err) throw err;
  console.log('Rename complete!');
});
fs.rmdir(path[, options], callback)#

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)#

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)#

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.

fs.stat() follows symbolic links. Use fs.lstat() to look at the links themselves.

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 'node: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)#

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 null, 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. Junction points on NTFS volumes can only point to directories.

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

import { symlink } from 'node:fs';

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

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

$ tree .
.
├── mew
└── mewtwo -> ./mew
fs.truncate(path[, len], callback)#

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 'node:fs';

truncate('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was truncated');
});const { truncate } = require('node:fs');

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)#

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

import { unlink } from 'node:fs';

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])#

Added in: v0.1.31

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)#

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

The atime and mtime arguments follow these rules:

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

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.

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.

The fs.watch API does not provide any protection with respect to malicious actions on the file system. For example, on Windows it is implemented by monitoring changes in a directory versus specific files. This allows substitution of a file and fs reporting changes on the new file with the same filename.

Availability#

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

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 'node: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)#

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 'node: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.mtimeMs and prev.mtimeMs.

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:

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

Write buffer to the file specified by fd.

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

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

The callback will be given three arguments (err, 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)#

Write string to the file specified by fd. If string is not a string, 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)#

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.

import { writeFile } from 'node:fs';
import { Buffer } from 'node: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 'node:fs';

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

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

Similarly to fs.readFile - fs.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 fs.writeFile(). Cancelation is "best effort", and some amount of data is likely still to be written.

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

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

controller.abort();

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

Using fs.writeFile() with file descriptors#

When file is a file descriptor, the behavior is almost identical to directly calling fs.write() like:

import { write } from 'node:fs';
import { Buffer } from 'node:buffer';

write(fd, Buffer.from(data, options.encoding), callback);

The difference from directly calling fs.write() is that under some unusual conditions, fs.write() might write only part of the buffer and need to be retried to write the remaining data, whereas fs.writeFile() retries until the data is entirely written (or an error occurs).

The implications of this are a common source of confusion. In the file descriptor case, the file is not replaced! The data is not necessarily written to the beginning of the file, and the file's original data may remain before and/or after the newly written data.

For example, if fs.writeFile() is called twice in a row, first to write the string 'Hello', then to write the string ', World', the file would contain 'Hello, World', and might contain some of the file's original data (depending on the size of the original file, and the position of the file descriptor). If a file name had been used instead of a descriptor, the file would be guaranteed to contain only ', World'.

fs.writev(fd, buffers[, position], callback)#

Write an array of ArrayBufferViews to the file specified by fd using writev().

position is 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.

The callback will be given three arguments: err, bytesWritten, and buffers. bytesWritten is how many bytes were written from buffers.

If this method is util.promisify()ed, it returns a promise for an Object with bytesWritten and buffers properties.

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

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.

Synchronous API#

The synchronous APIs perform all operations synchronously, blocking the event loop until the operation completes or fails.

fs.accessSync(path[, mode])#

Synchronously tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. mode should be either the value fs.constants.F_OK or a mask consisting of the bitwise OR of any of fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g. fs.constants.W_OK | fs.constants.R_OK). Check File access constants for possible values of mode.

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

import { accessSync, constants } from 'node:fs';

try {
  accessSync('etc/passwd', constants.R_OK | constants.W_OK);
  console.log('can read/write');
} catch (err) {
  console.error('no access!');
}
fs.appendFileSync(path, data[, options])#

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

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

import { appendFileSync } from 'node:fs';

try {
  appendFileSync('message.txt', 'data to append');
  console.log('The "data to append" was appended to file!');
} catch (err) {
  
}

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

import { appendFileSync } from 'node:fs';

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

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

import { openSync, closeSync, appendFileSync } from 'node:fs';

let fd;

try {
  fd = openSync('message.txt', 'a');
  appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
  
} finally {
  if (fd !== undefined)
    closeSync(fd);
}
fs.closeSync(fd)#

Added in: v0.1.21

Closes the file descriptor. Returns undefined.

Calling fs.closeSync() 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.copyFileSync(src, dest[, mode])#

Synchronously copies src to dest. By default, dest is overwritten if it already exists. Returns undefined. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

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).

import { copyFileSync, constants } from 'node:fs';


copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');


copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
fs.cpSync(src, dest[, options])#

Synchronously 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.existsSync(path)#

Returns true if the path exists, false otherwise.

For detailed information, see the documentation of the asynchronous version of this API: fs.exists().

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

import { existsSync } from 'node:fs';

if (existsSync('/etc/passwd'))
  console.log('The path exists.');
fs.fchmodSync(fd, mode)#

Added in: v0.4.7

Sets the permissions on the file. Returns undefined.

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

fs.fchownSync(fd, uid, gid)#

Added in: v0.4.7

Sets the owner of the file. Returns undefined.

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

fs.fdatasyncSync(fd)#

Added in: v0.1.96

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. Returns undefined.

fs.fsyncSync(fd)#

Added in: v0.1.96

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. Returns undefined.

fs.ftruncateSync(fd[, len])#

Added in: v0.8.6

Truncates the file descriptor. Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.ftruncate().

fs.globSync(pattern[, options])# 
import { globSync } from 'node:fs';

console.log(globSync('**/*.js'));const { globSync } = require('node:fs');

console.log(globSync('**/*.js'));
fs.lchmodSync(path, mode)#

Deprecated since: v0.4.7

Changes the permissions on a symbolic link. Returns undefined.

This method is only implemented on macOS.

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

fs.lchownSync(path, uid, gid)#

Set the owner for the path. Returns undefined.

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

fs.lstatSync(path[, options])#

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

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

fs.mkdtempSync(prefix[, options])#

Returns the created directory path.

For detailed information, see the documentation of the asynchronous version of this API: fs.mkdtemp().

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

fs.opendirSync(path[, options])#

Synchronously open a directory. See opendir(3).

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.readdirSync(path[, options])#

Reads the contents of the directory.

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 returned. If the encoding is set to 'buffer', the filenames returned will be passed as <Buffer> objects.

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

fs.readFileSync(path[, options])#

Returns the contents of the path.

For detailed information, see the documentation of the asynchronous version of this API: fs.readFile().

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

Similar to fs.readFile(), when the path is a directory, the behavior of fs.readFileSync() is platform-specific.

import { readFileSync } from 'node:fs';


readFileSync('<directory>');



readFileSync('<directory>');
fs.readlinkSync(path[, options])#

Returns the symbolic link's string value.

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 returned. If the encoding is set to 'buffer', the link path returned will be passed as a <Buffer> object.

fs.readSync(fd, buffer[, options])#

Returns the number of bytesRead.

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

For detailed information, see the documentation of the asynchronous version of this API: fs.read().

fs.realpathSync.native(path[, options])#

Added in: v9.2.0

Synchronous realpath(3).

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

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path returned. 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.rmdirSync(path[, options])#

Synchronous rmdir(2). Returns undefined.

Using fs.rmdirSync() 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.rmSync() with options { recursive: true, force: true }.

fs.rmSync(path[, options])#

Synchronously removes files and directories (modeled on the standard POSIX rm utility). Returns undefined.

fs.statSync(path[, options])#

Retrieves the <fs.Stats> for the path.

fs.truncateSync(path[, len])#

Added in: v0.8.6

Truncates the file. Returns undefined. A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync() is called.

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

Common Objects#

The common objects are shared by all of the file system API variants (promise, callback, and synchronous).

Class: fs.Dir#

Added in: v12.12.0

A class representing a directory stream.

Created by fs.opendir(), fs.opendirSync(), or fsPromises.opendir().

import { opendir } from 'node: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.

dir.close()#

Added in: v12.12.0

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

A promise is returned that will be fulfilled after the resource has been closed.

dir.close(callback)#

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

The callback will be called after the resource handle has been closed.

dir.closeSync()#

Added in: v12.12.0

Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

dir.read()#

Added in: v12.12.0

Asynchronously read the next directory entry via readdir(3) as an <fs.Dirent>.

A promise is returned that will be fulfilled with an <fs.Dirent>, or null if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir.read(callback)#

Added in: v12.12.0

Asynchronously read the next directory entry via readdir(3) as an <fs.Dirent>.

After the read is completed, the callback will be called with an <fs.Dirent>, or null if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir.readSync()#

Added in: v12.12.0

Synchronously read the next directory entry as an <fs.Dirent>. See the POSIX readdir(3) documentation for more detail.

If there are no more directory entries to read, null will be returned.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir[Symbol.asyncIterator]()#

Added in: v12.12.0

Asynchronously iterates over the directory until all entries have been read. Refer to the POSIX readdir(3) documentation for more detail.

Entries returned by the async iterator are always an <fs.Dirent>. The null case from dir.read() is handled internally.

See <fs.Dir> for an example.

Directory entries returned by this iterator are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir[Symbol.asyncDispose]()#

Calls dir.close() if the directory handle is open, and returns a promise that fulfills when disposal is complete.

dir[Symbol.dispose]()#

Calls dir.closeSync() if the directory handle is open, and returns undefined.

Class: fs.Dirent#

Added in: v10.10.0

A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by reading from an <fs.Dir>. The directory entry is a combination of the file name and file type pairs.

Additionally, when fs.readdir() or fs.readdirSync() is called with the withFileTypes option set to true, the resulting array is filled with <fs.Dirent> objects, rather than strings or <Buffer>s.

dirent.isBlockDevice()#

Added in: v10.10.0

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

dirent.isCharacterDevice()#

Added in: v10.10.0

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

dirent.isDirectory()#

Added in: v10.10.0

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

dirent.isFIFO()#

Added in: v10.10.0

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

dirent.isFile()#

Added in: v10.10.0

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

dirent.isSocket()#

Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a socket.

dirent.isSymbolicLink()#

Added in: v10.10.0

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

dirent.parentPath#

The path to the parent directory of the file this <fs.Dirent> object refers to.

Class: fs.FSWatcher#

Added in: v0.5.8

A successful call to fs.watch() method will return a new <fs.FSWatcher> object.

All <fs.FSWatcher> objects emit a 'change' event whenever a specific watched file is modified.

Event: 'change'#

Added in: v0.5.8

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

The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a <Buffer> if fs.watch() is called with its encoding option set to 'buffer', otherwise filename will be a UTF-8 string.

import { watch } from 'node:fs';

watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
  if (filename) {
    console.log(filename);
    
  }
});
Event: 'close'#

Added in: v10.0.0

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

Event: 'error'#

Added in: v0.5.8

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

watcher.close()#

Added in: v0.5.8

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

watcher.ref()#

Added in: v14.3.0, v12.20.0

When called, requests that the Node.js event loop not exit so long as the <fs.FSWatcher> is active. Calling watcher.ref() multiple times will have no effect.

By default, all <fs.FSWatcher> objects are "ref'ed", making it normally unnecessary to call watcher.ref() unless watcher.unref() had been called previously.

watcher.unref()#

Added in: v14.3.0, v12.20.0

When called, the active <fs.FSWatcher> object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the <fs.FSWatcher> object's callback is invoked. Calling watcher.unref() multiple times will have no effect.

Class: fs.StatWatcher#

Added in: v14.3.0, v12.20.0

A successful call to fs.watchFile() method will return a new <fs.StatWatcher> object.

watcher.ref()#

Added in: v14.3.0, v12.20.0

When called, requests that the Node.js event loop not exit so long as the <fs.StatWatcher> is active. Calling watcher.ref() multiple times will have no effect.

By default, all <fs.StatWatcher> objects are "ref'ed", making it normally unnecessary to call watcher.ref() unless watcher.unref() had been called previously.

watcher.unref()#

Added in: v14.3.0, v12.20.0

When called, the active <fs.StatWatcher> object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the <fs.StatWatcher> object's callback is invoked. Calling watcher.unref() multiple times will have no effect.

Class: fs.ReadStream#

Added in: v0.1.93

Instances of <fs.ReadStream> are created and returned using the fs.createReadStream() function.

Event: 'close'#

Added in: v0.1.93

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

Event: 'ready'#

Added in: v9.11.0

Emitted when the <fs.ReadStream> is ready to be used.

Fires immediately after 'open'.

readStream.bytesRead#

Added in: v6.4.0

The number of bytes that have been read so far.

readStream.path#

Added in: v0.1.93

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

readStream.pending#

Added in: v11.2.0, v10.16.0

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

Class: fs.Stats#

A <fs.Stats> object provides information about a file.

Objects returned from fs.stat(), fs.lstat(), fs.fstat(), and their synchronous counterparts are of this type. If bigint in the options passed to those methods is true, the numeric values will be bigint instead of number, and the object will contain additional nanosecond-precision properties suffixed with Ns. Stat objects are not to be created directly using the new keyword.

Stats {
  dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  atimeMs: 1318289051000.1,
  mtimeMs: 1318289051000.1,
  ctimeMs: 1318289051000.1,
  birthtimeMs: 1318289051000.1,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }

bigint version:

BigIntStats {
  dev: 2114n,
  ino: 48064969n,
  mode: 33188n,
  nlink: 1n,
  uid: 85n,
  gid: 100n,
  rdev: 0n,
  size: 527n,
  blksize: 4096n,
  blocks: 8n,
  atimeMs: 1318289051000n,
  mtimeMs: 1318289051000n,
  ctimeMs: 1318289051000n,
  birthtimeMs: 1318289051000n,
  atimeNs: 1318289051000000000n,
  mtimeNs: 1318289051000000000n,
  ctimeNs: 1318289051000000000n,
  birthtimeNs: 1318289051000000000n,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
stats.isBlockDevice()#

Added in: v0.1.10

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

stats.isCharacterDevice()#

Added in: v0.1.10

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

stats.isDirectory()#

Added in: v0.1.10

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

If the <fs.Stats> object was obtained from calling fs.lstat() on a symbolic link which resolves to a directory, this method will return false. This is because fs.lstat() returns information about a symbolic link itself and not the path it resolves to.

stats.isFIFO()#

Added in: v0.1.10

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

stats.isFile()#

Added in: v0.1.10

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

stats.isSocket()#

Added in: v0.1.10

Returns true if the <fs.Stats> object describes a socket.

stats.isSymbolicLink()#

Added in: v0.1.10

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

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

stats.dev#

The numeric identifier of the device containing the file.

stats.uid#

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

stats.gid#

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

stats.rdev#

A numeric device identifier if the file represents a device.

stats.size#

The size of the file in bytes.

If the underlying file system does not support getting the size of the file, this will be 0.

stats.atimeMs#

Added in: v8.1.0

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

stats.mtimeMs#

Added in: v8.1.0

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

stats.ctimeMs#

Added in: v8.1.0

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

stats.birthtimeMs#

Added in: v8.1.0

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

stats.atimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time this file was accessed expressed in nanoseconds since the POSIX Epoch.

stats.mtimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time this file was modified expressed in nanoseconds since the POSIX Epoch.

stats.ctimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time the file status was changed expressed in nanoseconds since the POSIX Epoch.

stats.birthtimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the creation time of this file expressed in nanoseconds since the POSIX Epoch.

stats.atime#

Added in: v0.11.13

The timestamp indicating the last time this file was accessed.

stats.mtime#

Added in: v0.11.13

The timestamp indicating the last time this file was modified.

stats.ctime#

Added in: v0.11.13

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

stats.birthtime#

Added in: v0.11.13

The timestamp indicating the creation time of this file.

Stat time values#

The atimeMs, mtimeMs, ctimeMs, birthtimeMs properties are numeric values that hold the corresponding times in milliseconds. Their precision is platform specific. When bigint: true is passed into the method that generates the object, the properties will be bigints, otherwise they will be numbers.

The atimeNs, mtimeNs, ctimeNs, birthtimeNs properties are bigints that hold the corresponding times in nanoseconds. They are only present when bigint: true is passed into the method that generates the object. Their precision is platform specific.

atime, mtime, ctime, and birthtime are Date object alternate representations of the various times. The Date and number values are not connected. Assigning a new number value, or mutating the Date value, will not be reflected in the corresponding alternate representation.

The times in the stat object have the following semantics:

Prior to Node.js 0.12, the ctime held the birthtime on Windows systems. As of 0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.StatFs#

Added in: v19.6.0, v18.15.0

Provides information about a mounted file system.

Objects returned from fs.statfs() and its synchronous counterpart are of this type. If bigint in the options passed to those methods is true, the numeric values will be bigint instead of number.

StatFs {
  type: 1397114950,
  bsize: 4096,
  blocks: 121938943,
  bfree: 61058895,
  bavail: 61058895,
  files: 999,
  ffree: 1000000
}

bigint version:

StatFs {
  type: 1397114950n,
  bsize: 4096n,
  blocks: 121938943n,
  bfree: 61058895n,
  bavail: 61058895n,
  files: 999n,
  ffree: 1000000n
}
statfs.bavail#

Added in: v19.6.0, v18.15.0

Free blocks available to unprivileged users.

statfs.bfree#

Added in: v19.6.0, v18.15.0

Free blocks in file system.

statfs.blocks#

Added in: v19.6.0, v18.15.0

Total data blocks in file system.

statfs.bsize#

Added in: v19.6.0, v18.15.0

Optimal transfer block size.

statfs.ffree#

Added in: v19.6.0, v18.15.0

Free file nodes in file system.

statfs.files#

Added in: v19.6.0, v18.15.0

Total file nodes in file system.

Class: fs.WriteStream#

Added in: v0.1.93

Instances of <fs.WriteStream> are created and returned using the fs.createWriteStream() function.

Event: 'close'#

Added in: v0.1.93

Emitted when the <fs.WriteStream>'s underlying file descriptor has been closed.

Event: 'ready'#

Added in: v9.11.0

Emitted when the <fs.WriteStream> is ready to be used.

Fires immediately after 'open'.

writeStream.bytesWritten#

Added in: v0.4.7

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

writeStream.close([callback])#

Added in: v0.9.4

Closes writeStream. Optionally accepts a callback that will be executed once the writeStream is closed.

writeStream.path#

Added in: v0.1.93

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

writeStream.pending#

Added in: v11.2.0

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

fs.constants#

Returns an object containing commonly used constants for file system operations.

FS constants#

The following constants are exported by fs.constants and fsPromises.constants.

Not every constant will be available on every operating system; this is especially important for Windows, where many of the POSIX specific definitions are not available. For portable applications it is recommended to check for their presence before use.

To use more than one constant, use the bitwise OR | operator.

Example:

import { open, constants } from 'node:fs';

const {
  O_RDWR,
  O_CREAT,
  O_EXCL,
} = constants;

open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
  
});
File access constants#

The following constants are meant for use as the mode parameter passed to fsPromises.access(), fs.access(), and fs.accessSync().

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

The definitions are also available on Windows.

File copy constants#

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

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

The definitions are also available on Windows.

File open constants#

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

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

On Windows, only O_APPEND, O_CREAT, O_EXCL, O_RDONLY, O_RDWR, O_TRUNC, O_WRONLY, and UV_FS_O_FILEMAP are available.

File type constants#

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

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

On Windows, only S_IFCHR, S_IFDIR, S_IFLNK, S_IFMT, and S_IFREG, are available.

File mode constants#

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

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

On Windows, only S_IRUSR and S_IWUSR are available.

Notes# Ordering of callback and promise-based operations#

Because they are executed asynchronously by the underlying thread pool, there is no guaranteed ordering when using either the callback or promise-based methods.

For example, the following is prone to error because the fs.stat() operation might complete before the fs.rename() operation:

const fs = require('node:fs');

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

It is important to correctly order the operations by awaiting the results of one before invoking the other:

import { rename, stat } from 'node:fs/promises';

const oldPath = '/tmp/hello';
const newPath = '/tmp/world';

try {
  await rename(oldPath, newPath);
  const stats = await stat(newPath);
  console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
  console.error('there was an error:', error.message);
}const { rename, stat } = require('node:fs/promises');

(async function(oldPath, newPath) {
  try {
    await rename(oldPath, newPath);
    const stats = await stat(newPath);
    console.log(`stats: ${JSON.stringify(stats)}`);
  } catch (error) {
    console.error('there was an error:', error.message);
  }
})('/tmp/hello', '/tmp/world');

Or, when using the callback APIs, move the fs.stat() call into the callback of the fs.rename() operation:

import { rename, stat } from 'node:fs';

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

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

Most fs operations accept file paths that may be specified in the form of a string, a <Buffer>, or a <URL> object using the file: protocol.

String paths#

String paths are interpreted as UTF-8 character sequences identifying the absolute or relative filename. Relative paths will be resolved relative to the current working directory as determined by calling process.cwd().

Example using an absolute path on POSIX:

import { open } from 'node:fs/promises';

let fd;
try {
  fd = await open('/open/some/file.txt', 'r');
  
} finally {
  await fd?.close();
}

Example using a relative path on POSIX (relative to process.cwd()):

import { open } from 'node:fs/promises';

let fd;
try {
  fd = await open('file.txt', 'r');
  
} finally {
  await fd?.close();
}
File URL paths#

Added in: v7.6.0

For most node:fs module functions, the path or filename argument may be passed as a <URL> object using the file: protocol.

import { readFileSync } from 'node:fs';

readFileSync(new URL('file:///tmp/hello'));

file: URLs are always absolute paths.

Platform-specific considerations#

On Windows, file: <URL>s with a host name convert to UNC paths, while file: <URL>s with drive letters convert to local absolute paths. file: <URL>s with no host name and no drive letter will result in an error:

import { readFileSync } from 'node:fs';




readFileSync(new URL('file://hostname/p/a/t/h/file'));



readFileSync(new URL('file:///C:/tmp/hello'));


readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
readFileSync(new URL('file:///c/p/a/t/h/file'));

file: <URL>s with drive letters must use : as a separator just after the drive letter. Using another separator will result in an error.

On all other platforms, file: <URL>s with a host name are unsupported and will result in an error:

import { readFileSync } from 'node:fs';




readFileSync(new URL('file://hostname/p/a/t/h/file'));




readFileSync(new URL('file:///tmp/hello'));

A file: <URL> having encoded slash characters will result in an error on all platforms:

import { readFileSync } from 'node:fs';


readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
readFileSync(new URL('file:///C:/p/a/t/h/%2f'));



readFileSync(new URL('file:///p/a/t/h/%2F'));
readFileSync(new URL('file:///p/a/t/h/%2f'));

On Windows, file: <URL>s having encoded backslash will result in an error:

import { readFileSync } from 'node:fs';


readFileSync(new URL('file:///C:/path/%5C'));
readFileSync(new URL('file:///C:/path/%5c'));
Buffer paths#

Paths specified using a <Buffer> are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, <Buffer> paths may be relative or absolute:

Example using an absolute path on POSIX:

import { open } from 'node:fs/promises';
import { Buffer } from 'node:buffer';

let fd;
try {
  fd = await open(Buffer.from('/open/some/file.txt'), 'r');
  
} finally {
  await fd?.close();
}
Per-drive working directories on Windows#

On Windows, Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example fs.readdirSync('C:\\') can potentially return a different result than fs.readdirSync('C:'). For more information, see this MSDN page.

File descriptors#

On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the differences between operating systems and assigns all open files a numeric file descriptor.

The callback-based fs.open(), and synchronous fs.openSync() methods open a file and allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file.

Operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.

import { open, close, fstat } from 'node:fs';

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

open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  try {
    fstat(fd, (err, stat) => {
      if (err) {
        closeFd(fd);
        throw err;
      }

      

      closeFd(fd);
    });
  } catch (err) {
    closeFd(fd);
    throw err;
  }
});

The promise-based APIs use a <FileHandle> object in place of the numeric file descriptor. These objects are better managed by the system to ensure that resources are not leaked. However, it is still required that they are closed when operations are completed:

import { open } from 'node:fs/promises';

let file;
try {
  file = await open('/open/some/file.txt', 'r');
  const stat = await file.stat();
  
} finally {
  await file.close();
}
Threadpool usage#

All callback and promise-based file system APIs (with the exception of fs.FSWatcher()) use libuv's threadpool. This can have surprising and negative performance implications for some applications. See the UV_THREADPOOL_SIZE documentation for more information.

File system flags#

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

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

The exclusive flag 'x' (O_EXCL flag in open(2)) causes the operation to return an error if the path already exists. On POSIX, if the path is a symbolic link, using O_EXCL returns an error even if the link is to a path that does not exist. The exclusive flag might not work with network file systems.

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

Modifying a file rather than replacing it may require the flag option to be set to 'r+' rather than the default 'w'.

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


fs.open('<directory>', 'a+', (err, fd) => {
  
});


fs.open('<directory>', 'a+', (err, fd) => {
  
});

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

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

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