Node.js v26.7.0 documentation

File system#

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');
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To use the callback and sync APIs:

import * as fs from 'node:fs';
const fs = require('node:fs');
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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');
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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');
});
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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) {
  // handle the error
}
const { unlinkSync } = require('node:fs');

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

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

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

Class: FileHandle#

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

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

All <FileHandle> objects are <EventEmitter>s.

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

Event: 'close'#

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

filehandle.appendFile(data[, options])#

• data <string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • signal <AbortSignal> | <undefined> allows aborting an in-progress writeFile. Default: undefined
• Returns: <Promise> Fulfills with undefined upon success.

Alias of filehandle.writeFile().

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

filehandle.chmod(mode)#

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

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

filehandle.chown(uid, gid)#

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

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

filehandle.close()#

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

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

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

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

• options <Object>
  • encoding <string> Default: null
  • autoClose <boolean> Default: true
  • emitClose <boolean> Default: true
  • start <integer>
  • end <integer> Default: Infinity
  • highWaterMark <integer> Default: 64 * 1024
  • signal <AbortSignal> | <undefined> Default: undefined
• Returns: <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);
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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 });
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filehandle.createWriteStream([options])#

• options <Object>
  • encoding <string> Default: 'utf8'
  • autoClose <boolean> Default: true
  • emitClose <boolean> Default: true
  • start <integer>
  • highWaterMark <number> Default: 16384
  • flush <boolean> If true, the underlying file descriptor is flushed prior to closing it. Default: false.
• Returns: <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()#

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

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

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

filehandle.fd#

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

filehandle.pull([...transforms][, options])#

• ...transforms <Function> | <Object> Optional transforms to apply via stream/iter pull().
• options <Object>
  • signal <AbortSignal>
  • autoClose <boolean> Close the file handle when the stream ends. Default: false.
  • start <number> Byte offset to begin reading from. When specified, reads use explicit positioning (pread semantics). Default: current file position.
  • limit <number> Maximum number of bytes to read before ending the iterator. Reads stop when limit bytes have been delivered or EOF is reached, whichever comes first. Default: read until EOF.
  • chunkSize <number> Size in bytes of the buffer allocated for each read operation. Default: 131072 (128 KB).
• Returns: <AsyncIterable> whose chunks fulfill with <Uint8Array>[]

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);
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filehandle.pullSync([...transforms][, options])#

• ...transforms <Function> | <Object> Optional transforms to apply via stream/iter pullSync().
• options <Object>
  • autoClose <boolean> Close the file handle when the stream ends. Default: false.
  • start <number> Byte offset to begin reading from. When specified, reads use explicit positioning. Default: current file position.
  • limit <number> Maximum number of bytes to read before ending the iterator. Default: read until EOF.
  • chunkSize <number> Size in bytes of the buffer allocated for each read operation. Default: 131072 (128 KB).
• Returns: <Iterable> whose chunks return <Uint8Array>[]

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

• buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.
• offset <integer> The location in the buffer at which to start filling. Default: 0
• length <integer> The number of bytes to read. Default: buffer.byteLength - offset
• position <integer> | <bigint> | <null> The location where to begin reading data from the file. If null or -1, data will be read from the current file position, and the position will be updated. If position is a non-negative integer, the current file position will remain unchanged. Default: null
• Returns: <Promise> Fulfills upon success with an object with two properties:
  • bytesRead <integer> The number of bytes read
  • buffer <Buffer> | <TypedArray> | <DataView> A reference to the passed in buffer argument.

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

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

filehandle.read([options])#

• options <Object>
  • buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read. Default: Buffer.alloc(16384)
  • offset <integer> The location in the buffer at which to start filling. Default: 0
  • length <integer> The number of bytes to read. Default: buffer.byteLength - offset
  • position <integer> | <bigint> | <null> The location where to begin reading data from the file. If null or -1, data will be read from the current file position, and the position will be updated. If position is a non-negative integer, the current file position will remain unchanged. Default:: null
• Returns: <Promise> Fulfills upon success with an object with two properties:
  • bytesRead <integer> The number of bytes read
  • buffer <Buffer> | <TypedArray> | <DataView> A reference to the passed in buffer argument.

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

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

filehandle.read(buffer[, options])#

• buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.
• options <Object>
  • offset <integer> The location in the buffer at which to start filling. Default: 0
  • length <integer> The number of bytes to read. Default: buffer.byteLength - offset
  • position <integer> | <bigint> | <null> The location where to begin reading data from the file. If null or -1, data will be read from the current file position, and the position will be updated. If position is a non-negative integer, the current file position will remain unchanged. Default:: null
• Returns: <Promise> Fulfills upon success with an object with two properties:
  • bytesRead <integer> The number of bytes read
  • buffer <Buffer> | <TypedArray> | <DataView> A reference to the passed in buffer argument.

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

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

filehandle.readableWebStream([options])#

• options <Object>
  • autoClose <boolean> When true, causes the <FileHandle> to be closed when the stream is closed. Default: false
• Returns: <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();
})();
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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)#

• options <Object> | <string>
  • encoding <string> | <null> Default: null
  • signal <AbortSignal> allows aborting an in-progress readFile
  • buffer <Buffer> | <TypedArray> | <DataView> | <Function> A buffer to read into, or a function called with the file size that returns the buffer.
• Returns: <Promise> Fulfills upon a successful read with the contents of the file. If no encoding is specified (using options.encoding), the data is returned as a <Buffer> object. Otherwise, the data will be a string.

Asynchronously reads the entire contents of a file.

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

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();
}
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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();
}
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filehandle.readLines([options])#

• options <Object>
  • encoding <string> Default: null
  • autoClose <boolean> Default: true
  • emitClose <boolean> Default: true
  • start <integer>
  • end <integer> Default: Infinity
  • highWaterMark <integer> Default: 64 * 1024
• Returns: <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);
  }
})();
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filehandle.readv(buffers[, position])#

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

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

filehandle.stat([options])#

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

filehandle.sync()#

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

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

filehandle.truncate(len)#

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

Truncates the file.

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

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

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

let filehandle = null;
try {
  filehandle = await open('temp.txt', 'r+');
  await filehandle.truncate(4);
} finally {
  await filehandle?.close();
}
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If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'):

If len is negative then 0 will be used.

filehandle.utimes(atime, mtime)#

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

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

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

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

Write buffer to the file.

The promise is fulfilled with an object containing two properties:

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

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be 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])#

• buffer <Buffer> | <TypedArray> | <DataView>
• options <Object>
  • offset <integer> Default: 0
  • length <integer> Default: buffer.byteLength - offset
  • position <integer> | <null> Default: null
• Returns: <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]])#

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

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

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be 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)#

• data <string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable>
• options <Object> | <string>
  • encoding <string> | <null> The expected character encoding when data is a string. Default: 'utf8'
  • signal <AbortSignal> | <undefined> allows aborting an in-progress writeFile. Default: undefined
• Returns: <Promise>

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

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

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

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

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

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

• options <Object>
  • autoClose <boolean> Close the file handle when the writer ends or fails. Default: false.
  • start <number> Byte offset to start writing at. When specified, writes use explicit positioning. Default: current file position.
  • limit <number> Maximum number of bytes the writer will accept. Async writes (write(), writev()) that would exceed the limit reject with ERR_OUT_OF_RANGE. Sync writes (writeSync(), writevSync()) return false. Default: no limit.
  • chunkSize <number> Maximum chunk size in bytes for synchronous write operations. Writes larger than this threshold fall back to async I/O. Set this to match the reader's chunkSize for optimal pipeTo() performance. Default: 131072 (128 KB).
• Returns: <Object>
  • write(chunk[, options]) <Function> Returns <Promise>. Accepts Uint8Array, Buffer, or string (UTF-8 encoded).
    • chunk <Buffer> | <TypedArray> | <DataView> | <string>
    • options <Object>
      • signal <AbortSignal> If the signal is already aborted, the write rejects with AbortError without performing I/O.
  • writev(chunks[, options]) <Function> Returns <Promise>. Uses scatter/gather I/O via a single writev() syscall. Accepts mixed Uint8Array/string arrays.
    • chunks <Buffer>[] | <TypedArray>[] | <DataView>[] | <string>[]
    • options <Object>
      • signal <AbortSignal> If the signal is already aborted, the write rejects with AbortError without performing I/O.
  • writeSync(chunk) <Function> Returns <boolean>. Attempts a synchronous write. Returns true if the write succeeded, false if the caller should fall back to async write(). Returns false when: the writer is closed/errored, an async operation is in flight, the chunk exceeds chunkSize, or the write would exceed limit.
    • chunk <Buffer> | <TypedArray> | <DataView> | <string>
  • writevSync(chunks) <Function> Returns <boolean>. Synchronous batch write. Same fallback semantics as writeSync().
    • chunks <Buffer>[] | <TypedArray>[] | <DataView>[] | <string>[]
  • end([options]) <Function> Returns <Promise>, fulfills with the total number of bytes written. Idempotent: returns totalBytesWritten if already closed, returns the pending promise if already closing. Rejects if the writer is in an errored state.
    • options <Object>
      • signal <AbortSignal> If the signal is already aborted, end() rejects with AbortError and the writer remains open.
  • endSync() <Function> Returns <number> | <number> total bytes written on success, -1 if the writer is errored or an async operation is in flight. Idempotent when already closed.
  • fail(reason) <Function> Puts the writer into a terminal error state. Synchronous. If the writer is already closed or errored, this is a no-op. If autoClose is true, closes the file handle synchronously.

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);
javascriptcopy

filehandle[Symbol.asyncDispose]()#

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])#

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

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. 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');
}
mjscopy

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

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

• path <string> | <Buffer> | <URL> | <FileHandle> filename or <FileHandle>
• data <string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable>
• options <Object> | <string>
  • encoding <string> | <null> Default: 'utf8'
  • mode <integer> Default: 0o666
  • flag <string> See support of file system flags. Default: 'a'.
  • flush <boolean> If true, the underlying file descriptor is flushed prior to closing it. Default: false.
• Returns: <Promise> Fulfills with undefined upon success.

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)#

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

Changes the permissions of a file.

fsPromises.chown(path, uid, gid)#

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

Changes the ownership of a file.

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

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

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

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

import { 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');
}
mjscopy

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

• src <string> | <URL> source path to copy.
• dest <string> | <URL> destination path to copy to.
• options <Object>
  • dereference <boolean> dereference symlinks. Default: false.
  • errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.
  • filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. When ignoring a directory, all of its contents will be skipped as well. Can also return a Promise that resolves to true or false Default: undefined.
    • src <string> source path to copy.
    • dest <string> destination path to copy to.
    • Returns: <boolean> | <Promise> A value that is coercible to boolean or a Promise that fulfils with such value.
  • force <boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.
  • mode <integer> modifiers for copy operation. Default: 0. See mode flag of fsPromises.copyFile().
  • preserveTimestamps <boolean> When true timestamps from src will be preserved. Default: false.
  • recursive <boolean> copy directories recursively Default: false
  • verbatimSymlinks <boolean> When true, path resolution for symlinks will be skipped. Default: false
• Returns: <Promise> Fulfills with undefined upon success.

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

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

fsPromises.glob(pattern[, options])