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.

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 EventEmitters.

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 FileHandles. Node.js may change this behavior in the future.

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

filehandle.appendFile(data, options?): Promise

Alias of filehandle.writeFile().

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

filehandle.chmod(mode): Promise

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

filehandle.chown(uid, gid): Promise

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

filehandle.close(): Promise

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?): fs.ReadStream

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

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

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?): fs.WriteStream

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(): Promise

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.pull(...transforms?, options?): AsyncIterable

Return the file contents as an async iterable using the node:stream/iter pull model. Reads are performed in chunkSize-byte chunks (default 128 KB). If transforms are provided, they are applied via stream/iter pull().

The file handle is locked while the iterable is being consumed and unlocked when iteration completes, an error occurs, or the consumer breaks.

This function is only available when the --experimental-stream-iter flag is enabled.

import { open } from 'node:fs/promises';
import { text } from 'node:stream/iter';
import { compressGzip } from 'node:zlib/iter';

const fh = await open('input.txt', 'r');

// Read as text
console.log(await text(fh.pull({ autoClose: true })));

// Read 1 KB starting at byte 100
const fh2 = await open('input.txt', 'r');
console.log(await text(fh2.pull({ start: 100, limit: 1024, autoClose: true })));

// Read with compression
const fh3 = await open('input.txt', 'r');
const compressed = fh3.pull(compressGzip(), { autoClose: true });

const { open } = require('node:fs/promises');
const { text } = require('node:stream/iter');
const { compressGzip } = require('node:zlib/iter');

async function run() {
  const fh = await open('input.txt', 'r');

  // Read as text
  console.log(await text(fh.pull({ autoClose: true })));

  // Read 1 KB starting at byte 100
  const fh2 = await open('input.txt', 'r');
  console.log(await text(fh2.pull({ start: 100, limit: 1024, autoClose: true })));

  // Read with compression
  const fh3 = await open('input.txt', 'r');
  const compressed = fh3.pull(compressGzip(), { autoClose: true });
}

run().catch(console.error);

filehandle.pullSync(...transforms?, options?): Iterable

Synchronous counterpart of filehandle.pull(). Returns a sync iterable that reads the file using synchronous I/O on the main thread. Reads are performed in chunkSize-byte chunks (default 128 KB).

The file handle is locked while the iterable is being consumed. Unlike the async pull(), this method does not support AbortSignal since all operations are synchronous.

This function is only available when the --experimental-stream-iter flag is enabled.

import { open } from 'node:fs/promises';
import { textSync, pipeToSync } from 'node:stream/iter';
import { compressGzipSync, decompressGzipSync } from 'node:zlib/iter';

const fh = await open('input.txt', 'r');

// Read as text (sync)
console.log(textSync(fh.pullSync({ autoClose: true })));

// Sync compress pipeline: file -> gzip -> file
const src = await open('input.txt', 'r');
const dst = await open('output.gz', 'w');
pipeToSync(src.pullSync(compressGzipSync(), { autoClose: true }), dst.writer({ autoClose: true }));

const { open } = require('node:fs/promises');
const { textSync, pipeToSync } = require('node:stream/iter');
const { compressGzipSync, decompressGzipSync } = require('node:zlib/iter');

async function run() {
  const fh = await open('input.txt', 'r');

  // Read as text (sync)
  console.log(textSync(fh.pullSync({ autoClose: true })));

  // Sync compress pipeline: file -> gzip -> file
  const src = await open('input.txt', 'r');
  const dst = await open('output.gz', 'w');
  pipeToSync(
    src.pullSync(compressGzipSync(), { autoClose: true }),
    dst.writer({ autoClose: true }),
  );
}

run().catch(console.error);

filehandle.read(buffer, offset?, length?, position?): Promise

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?): Promise

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?): Promise

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?): ReadableStream

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 unless the autoClose option is set to true.

filehandle.readFile(options): Promise

Asynchronously reads the entire contents of a file.

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

If buffer is provided and no encoding is specified, the returned Buffer is a view over the supplied buffer containing only the bytes read. If the supplied buffer is too small to contain the entire file, the operation will fail.

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.

An example using the buffer option with a pre-allocated buffer:

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

const file = await open('./some/file/to/read');
try {
  const buf = Buffer.alloc(16384);
  const contents = await file.readFile({ buffer: buf });
  console.log(contents); // A view over `buf` containing only the bytes read
} finally {
  await file.close();
}

An example using the buffer option with a function returning a buffer:

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

const file = await open('./some/file/to/read');
try {
  const contents = await file.readFile({
    buffer: (size) => Buffer.alloc(size),
  });
  console.log(contents);
} finally {
  await file.close();
}

filehandle.readLines(options?): readline.InterfaceConstructor

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.readv(buffers, position?): Promise

Read from a file and write to an array of ArrayBufferViews

filehandle.stat(options?): Promise

filehandle.sync(): Promise

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?): Promise

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.utimes(atime, mtime): Promise

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

filehandle.write(buffer, offset, length?, position?): Promise

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?): Promise

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?): Promise

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): Promise

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?): Promise

Write an array of ArrayBufferViews 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.writer(options?): Object

Return a node:stream/iter writer backed by this file handle.

The writer supports both Symbol.asyncDispose and Symbol.dispose:

• await using w = fh.writer() — if the writer is still open (no end() called), asyncDispose calls fail(). If end() is pending, it waits for it to complete.
• using w = fh.writer() — calls fail() unconditionally.

The writeSync() and writevSync() methods enable the try-sync fast path used by stream/iter pipeTo(). When the reader's chunk size matches the writer's chunkSize, all writes in a pipeTo() pipeline complete synchronously with zero promise overhead.

This function is only available when the --experimental-stream-iter flag is enabled.

import { open } from 'node:fs/promises';
import { from, pipeTo } from 'node:stream/iter';
import { compressGzip } from 'node:zlib/iter';

// Async pipeline
const fh = await open('output.gz', 'w');
await pipeTo(from('Hello!'), compressGzip(), fh.writer({ autoClose: true }));

// Sync pipeline with limit
const src = await open('input.txt', 'r');
const dst = await open('output.txt', 'w');
const w = dst.writer({ limit: 1024 * 1024 }); // Max 1 MB
await pipeTo(src.pull({ autoClose: true }), w);
await w.end();
await dst.close();

const { open } = require('node:fs/promises');
const { from, pipeTo } = require('node:stream/iter');
const { compressGzip } = require('node:zlib/iter');

async function run() {
  // Async pipeline
  const fh = await open('output.gz', 'w');
  await pipeTo(from('Hello!'), compressGzip(), fh.writer({ autoClose: true }));

  // Sync pipeline with limit
  const src = await open('input.txt', 'r');
  const dst = await open('output.txt', 'w');
  const w = dst.writer({ limit: 1024 * 1024 }); // Max 1 MB
  await pipeTo(src.pull({ autoClose: true }), w);
  await w.end();
  await dst.close();
}

run().catch(console.error);

filehandle[Symbol.asyncDispose](): Promise

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

This method enables the filehandle to be used with await using, which will automatically close the file when the scope exits. For more information, see the MDN documentation on using statements.

fsPromises.access(path, mode?): Promise

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.appendFile(path, data, options?): Promise

Asynchronously append data to a file, creating the file if it does not yet data can be a string, a buffer, an AsyncIterable, or an Iterable object.

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

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

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

fsPromises.chmod(path, mode): Promise

Changes the permissions of a file.

fsPromises.chown(path, uid, gid): Promise

Changes the ownership of a file.

fsPromises.copyFile(src, dest, mode?): Promise

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

Symbolic links are followed. If src is a symbolic link, the target file is copied. If dest is a symbolic link, the target file is overwritten unless mode contains fs.constants.COPYFILE_EXCL.

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');
}

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

fsPromises.cp(src, dest, options?): Promise

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?): AsyncIterator

When followSymlinks is enabled, detected symbolic link cycles are not traversed recursively.

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): Promise

Changes the permissions on a symbolic link.

This method is only implemented on macOS.

fsPromises.lchown(path, uid, gid): Promise

Changes the ownership on a symbolic link.

fsPromises.lutimes(path, atime, mtime): Promise

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): Promise

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

fsPromises.lstat(path, options?): Promise

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?): Promise

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?): Promise

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.mkdtempDisposable(prefix, options?): Promise

The resulting Promise holds an async-disposable object whose path property holds the created directory path. When the object is disposed, the directory and its contents will be removed asynchronously if it still exists. If the directory cannot be deleted, disposal will throw an error. The object has an async remove() method which will perform the same task.

Both this function and the disposal function on the resulting object are async, so it should be used with await + await using as in await using dir = await fsPromises.mkdtempDisposable('prefix').

See the MDN documentation on using statements for more information about explicit resource management.

For detailed information, see the documentation of fsPromises.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.

fsPromises.open(path, flags?, mode?): Promise

Opens a FileHandle.

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

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

fsPromises.opendir(path, options?): Promise

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.