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:
MJS modules
import * as fs from 'node:fs/promises';
CJS modules
const fs = require('node:fs/promises'); To use the callback and sync APIs:
MJS modules
import * as fs from 'node:fs';
CJS modules
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-based operations return a promise that is fulfilled when the asynchronous operation is complete.
MJS modules
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);
}
CJS modules
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'); 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.
MJS modules
import { unlink } from 'node:fs';
unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
CJS modules
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.
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.
MJS modules
import { unlinkSync } from 'node:fs';
try {
unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
CJS modules
const { unlinkSync } = require('node:fs');
try {
unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
} 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.
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.
'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> | <Stream>
options <Object> | <string> 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)
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.undefined upon success.Changes the ownership of the file. A wrapper for chown(2).
filehandle.close()
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();
} copy filehandle.createReadStream([options])
options <Object> Unlike the 16 KiB default highWaterMark for a <stream.Readable>, the stream returned by this method has a default highWaterMark of 64 KiB.
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); copy 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 }); copy filehandle.createWriteStream([options])
options <Object> 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()
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
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.length <integer> The number of bytes to read.position <integer> | <null> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged.bytesRead <integer> The number of bytes readbuffer <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> | <null> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged. Default:: null
bytesRead <integer> The number of bytes readbuffer <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> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged. Default:: null
bytesRead <integer> The number of bytes readbuffer <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>
type <string> | <undefined> Whether to open a normal or a 'bytes' stream. Default: undefined
Returns: <ReadableStream>
Returns a ReadableStream that may be used to read the files data.
An error will be thrown if this method is called more than once or is called after the FileHandle is closed or closing.
MJS modules
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();
CJS modules
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)
options <Object> | <string> encoding <string> | <null> Default: null
signal <AbortSignal> allows aborting an in-progress readFileoptions.encoding), the data is returned as a <Buffer> object. Otherwise, the data will be a string.Asynchronously reads the entire contents of a file.
If options is a string, then it specifies the encoding.
The <FileHandle> has to support reading.
If one or more filehandle.read() calls are made on a file handle and then a filehandle.readFile() call is made, the data will be read from the current position till the end of the file. It doesn't always read from the beginning of the file.
filehandle.readLines([options])
options <Object> Convenience method to create a readline interface and stream over the file. See filehandle.createReadStream() for the options.
MJS modules
import { open } from 'node:fs/promises';
const file = await open('./some/file/to/read');
for await (const line of file.readLines()) {
console.log(line);
}
CJS modules
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])
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
bytesRead <integer> the number of bytes readbuffers <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.filehandle.sync()
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)
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();
} copy 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)
Change the file system timestamps of the object referenced by the <FileHandle> then resolves the promise with no arguments upon success.
filehandle.write(buffer, offset[, length[, position]])
buffer <Buffer> | <TypedArray> | <DataView>
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
Write buffer to the file.
The promise is resolved with an object containing two properties:
bytesWritten <integer> the number of bytes writtenbuffer <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 resolved (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> 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'
Write string to the file. If string is not a string, the promise is rejected with an error.
The promise is resolved with an object containing two properties:
bytesWritten <integer> the number of bytes writtenbuffer <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 resolved (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> | <Stream>
options <Object> | <string> 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 resolved with no arguments upon success.
If options is a string, then it specifies the encoding.
The <FileHandle> has to support writing.
It is unsafe to use filehandle.writeFile() multiple times on the same file without waiting for the promise to be resolved (or rejected).
If one or more filehandle.write() calls are made on a file handle and then a filehandle.writeFile() call is made, the data will be written from the current position till the end of the file. It doesn't always write from the beginning of the file.
filehandle.writev(buffers[, position])
buffers <Buffer[]> | <TypedArray[]> | <DataView[]>
position <integer> | <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
Write an array of <ArrayBufferView>s to the file.
The promise is resolved with an object containing a two properties:
bytesWritten <integer> the number of bytes writtenbuffers <Buffer[]> | <TypedArray[]> | <DataView[]> a reference to the buffers input.It is unsafe to call writev() multiple times on the same file without waiting for the promise to be resolved (or rejected).
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
filehandle[Symbol.asyncDispose]()
An alias for filehandle.close().
fsPromises.access(path[, mode])
path <string> | <Buffer> | <URL>
mode <integer> Default: fs.constants.F_OK
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 resolved with no value. If any of the accessibility checks fail, the promise is rejected with an <Error> object. The following example checks if the file /etc/passwd can be read and written by the current process.
import { access, 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');
} copy Using fsPromises.access() to check for the accessibility of a file before calling fsPromises.open() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.
fsPromises.appendFile(path, data[, options])
path <string> | <Buffer> | <URL> | <FileHandle> filename or <FileHandle>
data <string> | <Buffer>
options <Object> | <string> encoding <string> | <null> Default: 'utf8'
mode <integer> Default: 0o666
flag <string> See support of file system flags. Default: 'a'.undefined upon success.Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.
If options is a string, then it specifies the encoding.
The mode option only affects the newly created file. See fs.open() for more details.
The path may be specified as a <FileHandle> that has been opened for appending (using fsPromises.open()).
fsPromises.chmod(path, mode)
path <string> | <Buffer> | <URL>
mode <string> | <integer>
undefined upon success.Changes the permissions of a file.
fsPromises.chown(path, uid, gid)
path <string> | <Buffer> | <URL>
uid <integer>
gid <integer>
undefined upon success.Changes the ownership of a file.
fsPromises.copyFile(src, dest[, mode])
src <string> | <Buffer> | <URL> source filename to copydest <string> | <Buffer> | <URL> destination filename of the copy operationmode <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.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');
} copy 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. 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
undefined upon success.Asynchronously copies the entire directory structure from src to dest, including subdirectories and files.
When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.
fsPromises.lchmod(path, mode)
path <string> | <Buffer> | <URL>
mode <integer>
undefined upon success.Changes the permissions on a symbolic link.
This method is only implemented on macOS.
fsPromises.lchown(path, uid, gid)
path <string> | <Buffer> | <URL>
uid <integer>
gid <integer>
undefined upon success.Changes the ownership on a symbolic link.
fsPromises.lutimes(path, atime, mtime)
path <string> | <Buffer> | <URL>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
undefined upon success.Changes the access and modification times of a file in the same way as fsPromises.utimes(), with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.
fsPromises.link(existingPath, newPath)
existingPath <string> | <Buffer> | <URL>
newPath <string> | <Buffer> | <URL>
undefined upon success.Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail.
fsPromises.lstat(path[, options])
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.path.Equivalent to fsPromises.stat() unless path refers to a symbolic link, in which case the link itself is stat-ed, not the file that it refers to. Refer to the POSIX lstat(2) document for more detail.
fsPromises.mkdir(path[, options])
path <string> | <Buffer> | <URL>
options <Object> | <integer> undefined if recursive is false, or the first directory path created if recursive is true.Asynchronously creates a directory.
The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fsPromises.mkdir() when path is a directory that exists results in a rejection only when recursive is false.
MJS modules
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);
}
CJS modules
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])
prefix <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
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);
} copy 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.open(path, flags[, mode])
path <string> | <Buffer> | <URL>
flags <string> | <number> See support of file system flags. Default: 'r'.mode <string> | <integer> Sets the file mode (permission and sticky bits) if the file is created. Default: 0o666 (readable and writable)Opens a <FileHandle>.
Refer to the POSIX open(2) documentation for more detail.
Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.
fsPromises.opendir(path[, options])
path <string> | <Buffer> | <URL>
options <Object> encoding <string> | <null> Default: 'utf8'
bufferSize <number> Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: 32
recursive <boolean> Resolved Dir will be an <AsyncIterable> containing all sub files and directories. Default: false
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);
} copy When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.
fsPromises.readdir(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> '.' and '..'.Reads the contents of a directory.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames. If the encoding is set to 'buffer', the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes is set to true, the resolved array will contain <fs.Dirent> objects.
import { readdir } from 'node:fs/promises';
try {
const files = await readdir(path);
for (const file of files)
console.log(file);
} catch (err) {
console.error(err);
} copy fsPromises.readFile(path[, options])
path <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle
options <Object> | <string> encoding <string> | <null> Default: null
flag <string> See support of file system flags. Default: 'r'.signal <AbortSignal> allows aborting an in-progress readFileAsynchronously 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:
MJS modules
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);
}
CJS modules
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 });
// Abort the request before the promise settles.
controller.abort();
await promise;
} catch (err) {
// When a request is aborted - err is an AbortError
console.error(err);
} copy Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.readFile performs.
Any specified <FileHandle> has to support reading.
fsPromises.readlink(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
linkString upon success.Reads the contents of the symbolic link referred to by path. See the POSIX readlink(2) documentation for more detail. The promise is resolved with the linkString upon success.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer', the link path returned will be passed as a <Buffer> object.
fsPromises.realpath(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
Determines the actual location of path using the same semantics as the fs.realpath.native() function.
Only paths that can be converted to UTF8 strings are supported.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.
fsPromises.rename(oldPath, newPath)
oldPath <string> | <Buffer> | <URL>
newPath <string> | <Buffer> | <URL>
undefined upon success.Renames oldPath to newPath.
fsPromises.rmdir(path[, options])
path <string> | <Buffer> | <URL>
options <Object> maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.undefined upon success.Removes the directory identified by path.
Using fsPromises.rmdir() on a file (not a directory) results in the promise being rejected with an ENOENT error on Windows and an ENOTDIR error on POSIX.
To get a behavior similar to the rm -rf Unix command, use fsPromises.rm() with options { recursive: true, force: true }.
fsPromises.rm(path[, options])
path <string> | <Buffer> | <URL>
options <Object> force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.recursive <boolean> If true, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: false.retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.undefined upon success.Removes files and directories (modeled on the standard POSIX rm utility).
fsPromises.stat(path[, options])
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.path.fsPromises.statfs(path[, options])
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.StatFs> object should be bigint. Default: false.path.fsPromises.symlink(target, path[, type])
target <string> | <Buffer> | <URL>
path <string> | <Buffer> | <URL>
type <string> | <null> Default: null
undefined upon success.Creates a symbolic link.
The type argument is only used on Windows platforms and can be one of 'dir', 'file', or 'junction'. If the type argument is not a string, 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])
path <string> | <Buffer> | <URL>
len <integer> Default: 0
undefined upon success.Truncates (shortens or extends the length) of the content at path to len bytes.
fsPromises.unlink(path)
If path refers to a symbolic link, then the link is removed without affecting the file or directory to which that link refers. If the path refers to a file path that is not a symbolic link, the file is deleted. See the POSIX unlink(2) documentation for more detail.
fsPromises.utimes(path, atime, mtime)
path <string> | <Buffer> | <URL>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
undefined upon success.Change the file system timestamps of the object referenced by path.
The atime and mtime arguments follow these rules:
Dates, or a numeric string like '123456789.0'.NaN, Infinity, or -Infinity, an Error will be thrown.fsPromises.watch(filename[, options])
filename <string> | <Buffer> | <URL>
options <string> | <Object> persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. Default: true.recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default: false.encoding <string> Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8'.signal <AbortSignal> An <AbortSignal> used to signal when the watcher should stop.Returns 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;
}
})(); copy On most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.
All the caveats for fs.watch() also apply to fsPromises.watch().
fsPromises.writeFile(file, data[, options])
file <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle
data <string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>
options <Object> | <string> encoding <string> | <null> Default: 'utf8'
mode <integer> Default: 0o666
flag <string> See support of file system flags. Default: 'w'.signal <AbortSignal> allows aborting an in-progress writeFileundefined upon success.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 });
// Abort the request before the promise settles.
controller.abort();
await promise;
} catch (err) {
// When a request is aborted - err is an AbortError
console.error(err);
} copy Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.writeFile performs.
fsPromises.constants
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.
The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.
The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.
fs.access(path[, mode], callback)
path <string> | <Buffer> | <URL>
mode <integer> Default: fs.constants.F_OK
callback <Function> err <Error>
Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. 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';
// Check if the file exists in the current directory.
access(file, constants.F_OK, (err) => {
console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});
// Check if the file is readable.
access(file, constants.R_OK, (err) => {
console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});
// Check if the file is writable.
access(file, constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});
// Check if the file is readable and writable.
access(file, constants.R_OK | constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not' : 'is'} readable and writable`);
}); copy 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;
});
}
});
}); copy 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;
});
}
}); copy 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;
});
}
});
}); copy 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;
});
}
}); copy The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the accessibility of a file only if the file will not be used directly, for example when its accessibility is a signal from another process.
On Windows, access-control policies (ACLs) on a directory may limit access to a file or directory. The fs.access() function, however, does not check the ACL and therefore may report that a path is accessible even if the ACL restricts the user from reading or writing to it.
fs.appendFile(path, data[, options], callback)
path <string> | <Buffer> | <URL> | <number> filename or file descriptordata <string> | <Buffer>
options <Object> | <string> encoding <string> | <null> Default: 'utf8'
mode <integer> Default: 0o666
flag <string> See support of file system flags. Default: 'a'.callback <Function> err <Error>
Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.
The mode option only affects the newly created file. See fs.open() for more details.
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', (err) => {
if (err) throw err;
console.log('The "data to append" was appended to file!');
}); copy If options is a string, then it specifies the encoding:
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', 'utf8', callback); copy 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;
}
}); copy 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!');
}); copy 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.chown(path, uid, gid, callback)
Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.
See the POSIX chown(2) documentation for more detail.
fs.close(fd[, callback])
fd <integer>
callback <Function> err <Error>
Closes the file descriptor. No arguments other than a possible exception are given to the completion callback.
Calling fs.close() on any file descriptor (fd) that is currently in use through any other fs operation may lead to undefined behavior.
See the POSIX close(2) documentation for more detail.
fs.copyFile(src, dest[, mode], callback)
src <string> | <Buffer> | <URL> source filename to copydest <string> | <Buffer> | <URL> destination filename of the copy operationmode <integer> modifiers for copy operation. Default: 0.callback <Function>
Asynchronously copies src to dest. By default, dest is overwritten if it already exists. No arguments other than a possible exception are given to the callback function. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.
mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).
fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.import { copyFile, constants } from 'node:fs';
function callback(err) {
if (err) throw err;
console.log('source.txt was copied to destination.txt');
}
// destination.txt will be created or overwritten by default.
copyFile('source.txt', 'destination.txt', callback);
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback); copy fs.cp(src, dest[, options], callback)
src <string> | <URL> source path to copy.dest <string> | <URL> destination path to copy to.options <Object> dereference <boolean> dereference symlinks. Default: false.errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. 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. 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 fs.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
callback <Function>
Asynchronously copies the entire directory structure from src to dest, including subdirectories and files.
When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.
fs.createReadStream(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> flags <string> See support of file system flags. Default: 'r'.encoding <string> Default: null
fd <integer> | <FileHandle> Default: null
mode <integer> Default: 0o666
autoClose <boolean> Default: true
emitClose <boolean> Default: true
start <integer>
end <integer> Default: Infinity
highWaterMark <integer> Default: 64 * 1024
fs <Object> | <null> Default: null
signal <AbortSignal> | <null> Default: null
Unlike the 16 KiB default highWaterMark for a <stream.Readable>, the stream returned by this method has a default highWaterMark of 64 KiB.
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';
// Create a stream from some character device.
const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0);
}, 100); copy 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 }); copy If options is a string, then it specifies the encoding.
fs.createWriteStream(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> flags <string> See support of file system flags. Default: 'w'.encoding <string> Default: 'utf8'
fd <integer> | <FileHandle> Default: null
mode <integer> Default: 0o666
autoClose <boolean> Default: true
emitClose <boolean> Default: true
start <integer>
fs <Object> | <null> Default: null
signal <AbortSignal> | <null> Default: null
highWaterMark <number> Default: 16384
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)
path <string> | <Buffer> | <URL>
callback <Function> exists <boolean>
Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false:
import { exists } from 'node:fs';
exists('/etc/passwd', (e) => {
console.log(e ? 'it exists' : 'no passwd!');
}); copy The parameters for this callback are not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().
Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile(), or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.
write (NOT RECOMMENDED)
import { exists, open, close } from '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;
});
}
});
}
}); copy 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;
});
}
}); copy 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');
}
}); copy 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;
});
}
}); copy The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the existence of a file only if the file won't be used directly, for example when its existence is a signal from another process.
fs.fchmod(fd, mode, callback)
fd <integer>
mode <string> | <integer>
callback <Function> err <Error>
Sets the permissions on the file. No arguments other than a possible exception are given to the completion callback.
See the POSIX fchmod(2) documentation for more detail.
fs.fchown(fd, uid, gid, callback)
fd <integer>
uid <integer>
gid <integer>
callback <Function> err <Error>
Sets the owner of the file. No arguments other than a possible exception are given to the completion callback.
See the POSIX fchown(2) documentation for more detail.
fs.fdatasync(fd, callback)
fd <integer>
callback <Function> err <Error>
Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX fdatasync(2) documentation for details. No arguments other than a possible exception are given to the completion callback.
fs.fstat(fd[, options], callback)
fd <integer>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.callback <Function> err <Error>
stats <fs.Stats>
Invokes the callback with the <fs.Stats> for the file descriptor.
See the POSIX fstat(2) documentation for more detail.
fs.fsync(fd, callback)
fd <integer>
callback <Function> err <Error>
Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX fsync(2) documentation for more detail. No arguments other than a possible exception are given to the completion callback.
fs.ftruncate(fd[, len], callback)
fd <integer>
len <integer> Default: 0
callback <Function> err <Error>
Truncates the file descriptor. No arguments other than a possible exception are given to the completion callback.
See the POSIX ftruncate(2) documentation for more detail.
If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file.
For example, the following program retains only the first four bytes of the file:
import { open, close, ftruncate } from '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;
}
}); copy If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'):
If len is negative then 0 will be used.
fs.futimes(fd, atime, mtime, callback)
fd <integer>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
callback <Function> err <Error>
Change the file system timestamps of the object referenced by the supplied file descriptor. See fs.utimes().
fs.lchmod(path, mode, callback)
path <string> | <Buffer> | <URL>
mode <integer>
callback <Function> err <Error> | <AggregateError>
Changes the permissions on a symbolic link. No arguments other than a possible exception are given to the completion callback.
This method is only implemented on macOS.
See the POSIX lchmod(2) documentation for more detail.
fs.lchown(path, uid, gid, callback)
Set the owner of the symbolic link. No arguments other than a possible exception are given to the completion callback.
See the POSIX lchown(2) documentation for more detail.
fs.lutimes(path, atime, mtime, callback)
path <string> | <Buffer> | <URL>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
callback <Function> err <Error>
Changes the access and modification times of a file in the same way as fs.utimes(), with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.
No arguments other than a possible exception are given to the completion callback.
fs.link(existingPath, newPath, callback)
existingPath <string> | <Buffer> | <URL>
newPath <string> | <Buffer> | <URL>
callback <Function> err <Error>
Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail. No arguments other than a possible exception are given to the completion callback.
fs.lstat(path[, options], callback)
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.callback <Function> err <Error>
stats <fs.Stats>
Retrieves the <fs.Stats> for the symbolic link referred to by the path. The callback gets two arguments (err, stats) where stats is a <fs.Stats> object. lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to.
See the POSIX lstat(2) documentation for more details.
fs.mkdir(path[, options], callback)
path <string> | <Buffer> | <URL>
options <Object> | <integer> callback <Function> err <Error>
path <string> | <undefined> Present only if a directory is created with recursive set to true.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';
// Create ./tmp/a/apple, regardless of whether ./tmp and ./tmp/a exist.
mkdir('./tmp/a/apple', { recursive: true }, (err) => {
if (err) throw err;
}); copy 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) => {
// => [Error: EPERM: operation not permitted, mkdir 'C:\']
}); copy See the POSIX mkdir(2) documentation for more details.
fs.mkdtemp(prefix[, options], callback)
prefix <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
callback <Function> 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);
// Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
}); copy 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';
// The parent directory for the new temporary directory
const tmpDir = tmpdir();
// This method is *INCORRECT*:
mkdtemp(tmpDir, (err, directory) => {
if (err) throw err;
console.log(directory);
// Will print something similar to `/tmpabc123`.
// A new temporary directory is created at the file system root
// rather than *within* the /tmp directory.
});
// This method is *CORRECT*:
import { sep } from 'node:path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
if (err) throw err;
console.log(directory);
// Will print something similar to `/tmp/abc123`.
// A new temporary directory is created within
// the /tmp directory.
}); copy fs.open(path[, flags[, mode]], callback)
path <string> | <Buffer> | <URL>
flags <string> | <number> See support of file system flags. Default: 'r'.mode <string> | <integer> Default: 0o666 (readable and writable)callback <Function> 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])
path <string> | <Buffer> | <URL>
options <Object> type <string> An optional mime type for the blob.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.
MJS modules
import { openAsBlob } from 'node:fs';
const blob = await openAsBlob('the.file.txt');
const ab = await blob.arrayBuffer();
blob.stream();
CJS modules
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)
path <string> | <Buffer> | <URL>
options <Object> callback <Function> Asynchronously open a directory. See the POSIX opendir(3) documentation for more details.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding option sets the encoding for the path while opening the directory and subsequent read operations.
fs.read(fd, buffer, offset, length, position, callback)
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to.offset <integer> The position in buffer to write the data to.length <integer> The number of bytes to read.position <integer> | <bigint> | <null> Specifies where to begin reading from in the file. If position is null or -1 , data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will be unchanged.callback <Function> Read data from the file specified by fd.
The callback is given the three arguments, (err, bytesRead, buffer).
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesRead and buffer properties.
fs.read(fd[, options], callback)
fd <integer>
options <Object> buffer <Buffer> | <TypedArray> | <DataView> Default: Buffer.alloc(16384)
offset <integer> Default: 0
length <integer> Default: buffer.byteLength - offset
position <integer> | <bigint> | <null> Default: null
callback <Function> Similar to the fs.read() function, this version takes an optional options object. If no options object is specified, it will default with the above values.
fs.read(fd, buffer[, options], callback)
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to.options <Object> callback <Function> Similar to the fs.read() function, this version takes an optional options object. If no options object is specified, it will default with the above values.
fs.readdir(path[, options], callback)
path <string> | <Buffer> | <URL>
options <string> | <Object> callback <Function> err <Error>
files <string[]> | <Buffer[]> | <fs.Dirent[]>
Reads the contents of a directory. The callback gets two arguments (err, files) where files is an array of the names of the files in the directory excluding '.' and '..'.
See the POSIX readdir(3) documentation for more details.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes is set to true, the files array will contain <fs.Dirent> objects.
fs.readFile(path[, options], callback)
path <string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions <Object> | <string> encoding <string> | <null> Default: null
flag <string> See support of file system flags. Default: 'r'.signal <AbortSignal> allows aborting an in-progress readFilecallback <Function> err <Error> | <AggregateError>
data <string> | <Buffer>
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);
}); copy 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); copy 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';
// macOS, Linux, and Windows
readFile('<directory>', (err, data) => {
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
});
// FreeBSD
readFile('<directory>', (err, data) => {
// => null, <data>
}); copy 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) => {
// ...
});
// When you want to abort the request
controller.abort(); copy 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.
path, it will not be closed automatically.'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'.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)
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
callback <Function> Reads the contents of the symbolic link referred to by path. The callback gets two arguments (err, linkString).
See the POSIX readlink(2) documentation for more details.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a <Buffer> object.
fs.readv(fd, buffers[, position], callback)
fd <integer>
buffers <ArrayBufferView[]>
position <integer> | <null> Default: null
callback <Function> err <Error>
bytesRead <integer>
buffers <ArrayBufferView[]>
Read from a file specified by fd and write to an array of ArrayBufferViews using readv().
position is the offset from the beginning of the file from where data should be read. If typeof position !== 'number', the data will be read from the current position.
The callback will be given three arguments: err, bytesRead, and buffers. bytesRead is how many bytes were read from the file.
If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesRead and buffers properties.
fs.realpath(path[, options], callback)
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
callback <Function> 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:
No case conversion is performed on case-insensitive file systems.
The maximum number of symbolic links is platform-independent and generally (much) higher than what the native realpath(3) implementation supports.
The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths.
Only paths that can be converted to UTF8 strings are supported.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.
If path resolves to a socket or a pipe, the function will return a system dependent name for that object.
fs.realpath.native(path[, options], callback)
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
callback <Function> Asynchronous realpath(3).
The callback gets two arguments (err, resolvedPath).
Only paths that can be converted to UTF8 strings are supported.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.
fs.rename(oldPath, newPath, callback)
oldPath <string> | <Buffer> | <URL>
newPath <string> | <Buffer> | <URL>
callback <Function> err <Error>
Asynchronously rename file at oldPath to the pathname provided as newPath. In the case that newPath already exists, it will be overwritten. If there is a directory at newPath, an error will be raised instead. No arguments other than a possible exception are given to the completion callback.
See also: rename(2).
import { rename } from 'node:fs';
rename('oldFile.txt', 'newFile.txt', (err) => {
if (err) throw err;
console.log('Rename complete!');
}); copy fs.rmdir(path[, options], callback)
path <string> | <Buffer> | <URL>
options <Object> maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.callback <Function> err <Error>
Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.
Using fs.rmdir() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.
To get a behavior similar to the rm -rf Unix command, use fs.rm() with options { recursive: true, force: true }.
fs.rm(path[, options], callback)
path <string> | <Buffer> | <URL>
options <Object> force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.recursive <boolean> If true, perform a recursive removal. In recursive mode operations are retried on failure. Default: false.retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.callback <Function> err <Error>
Asynchronously removes files and directories (modeled on the standard POSIX rm utility). No arguments other than a possible exception are given to the completion callback.
fs.stat(path[, options], callback)
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.callback <Function> err <Error>
stats <fs.Stats>
Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is an <fs.Stats> object.
In case of an error, the err.code will be one of Common System Errors.
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 copy
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);
});
} copy 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
} copy fs.statfs(path[, options], callback)
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.StatFs> object should be bigint. Default: false.callback <Function> err <Error>
stats <fs.StatFs>
Asynchronous statfs(2). Returns information about the mounted file system which contains path. The callback gets two arguments (err, stats) where stats is an <fs.StatFs> object.
In case of an error, the err.code will be one of Common System Errors.
fs.symlink(target, path[, type], callback)
target <string> | <Buffer> | <URL>
path <string> | <Buffer> | <URL>
type <string> | <null> Default: null
callback <Function> err <Error>
Creates the link called path pointing to target. No arguments other than a possible exception are given to the completion callback.
See the POSIX symlink(2) documentation for more details.
The type argument is only available on Windows and ignored on other platforms. It can be set to 'dir', 'file', or 'junction'. If the type argument is not a string, 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); copy The above example creates a symbolic link mewtwo which points to mew in the same directory:
$ tree . . ├── mew └── mewtwo -> ./mew copy
fs.truncate(path[, len], callback)
path <string> | <Buffer> | <URL>
len <integer> Default: 0
callback <Function> err <Error> | <AggregateError>
Truncates the file. No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called.
MJS modules
import { truncate } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});
CJS modules
const { truncate } = require('node:fs');
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
}); Passing a file descriptor is deprecated and may result in an error being thrown in the future.
See the POSIX truncate(2) documentation for more details.
fs.unlink(path, callback)
path <string> | <Buffer> | <URL>
callback <Function> err <Error>
Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.
import { unlink } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
unlink('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was deleted');
}); copy fs.unlink() will not work on a directory, empty or otherwise. To remove a directory, use fs.rmdir().
See the POSIX unlink(2) documentation for more details.
fs.unwatchFile(filename[, listener])
filename <string> | <Buffer> | <URL>
listener <Function> Optional, a listener previously attached using fs.watchFile()
Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed, effectively stopping watching of filename.
Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error.
Using fs.watch() is more efficient than fs.watchFile() and fs.unwatchFile(). fs.watch() should be used instead of fs.watchFile() and fs.unwatchFile() when possible.
fs.utimes(path, atime, mtime, callback)
path <string> | <Buffer> | <URL>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
callback <Function> err <Error>
Change the file system timestamps of the object referenced by path.
The atime and mtime arguments follow these rules:
Dates, or a numeric string like '123456789.0'.NaN, Infinity, or -Infinity, an Error will be thrown.fs.watch(filename[, options][, listener])
filename <string> | <Buffer> | <URL>
options <string> | <Object> persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. Default: true.recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default: false.encoding <string> Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8'.signal <AbortSignal> allows closing the watcher with an AbortSignal.listener <Function> | <undefined> Default: undefined 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>.
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.
This feature depends on the underlying operating system providing a way to be notified of file system changes.
inotify(7).kqueue(2).kqueue(2) for files and FSEvents for directories.event ports.ReadDirectoryChangesW.AHAFS, which must be enabled.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.
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).
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');
}
}); copy fs.watchFile(filename[, options], listener)
filename <string> | <Buffer> | <URL>
options <Object> listener <Function> current <fs.Stats>
previous <fs.Stats>
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}`);
}); copy 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)
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView>
offset <integer> Default: 0
length <integer> Default: buffer.byteLength - offset
position <integer> | <null> Default: null
callback <Function> err <Error>
bytesWritten <integer>
buffer <Buffer> | <TypedArray> | <DataView>
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, buffer[, options], callback)
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView>
options <Object> callback <Function> err <Error>
bytesWritten <integer>
buffer <Buffer> | <TypedArray> | <DataView>
Write buffer to the file specified by fd.
Similar to the above fs.write function, this version takes an optional options object. If no options object is specified, it will default with the above values.
fs.write(fd, string[, position[, encoding]], callback)
fd <integer>
string <string>
position <integer> | <null> Default: null
encoding <string> Default: 'utf8'
callback <Function> 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)
file <string> | <Buffer> | <URL> | <integer> filename or file descriptordata <string> | <Buffer> | <TypedArray> | <DataView>
options <Object> | <string> encoding <string> | <null> Default: 'utf8'
mode <integer> Default: 0o666
flag <string> See support of file system flags. Default: 'w'.signal <AbortSignal> allows aborting an in-progress writeFilecallback <Function> err <Error> | <AggregateError>
When file is a filename, asynchronously writes data to the file, replacing the file if it already exists. data can be a string or a buffer.
When file is a file descriptor, the behavior is similar to calling fs.write() directly (which is recommended). See the notes below on using a file descriptor.
The encoding option is ignored if data is a buffer.
The mode option only affects the newly created file. See fs.open() for more details.
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!');
}); copy If options is a string, then it specifies the encoding:
import { writeFile } from 'node:fs';
writeFile('message.txt', 'Hello Node.js', 'utf8', callback); copy 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) => {
// When a request is aborted - the callback is called with an AbortError
});
// When the request should be aborted
controller.abort(); copy Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.writeFile performs.
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); copy 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)
fd <integer>
buffers <ArrayBufferView[]>
position <integer> | <null> Default: null
callback <Function> err <Error>
bytesWritten <integer>
buffers <ArrayBufferView[]>
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.
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!');
} copy fs.appendFileSync(path, data[, options])
path <string> | <Buffer> | <URL> | <number> filename or file descriptordata <string> | <Buffer>
options <Object> | <string> encoding <string> | <null> Default: 'utf8'
mode <integer> Default: 0o666
flag <string> See support of file system flags. Default: 'a'.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) {
/* Handle the error */
} copy If options is a string, then it specifies the encoding:
import { appendFileSync } from 'node:fs';
appendFileSync('message.txt', 'data to append', 'utf8'); copy 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) {
/* Handle the error */
} finally {
if (fd !== undefined)
closeSync(fd);
} copy fs.chmodSync(path, mode)
For detailed information, see the documentation of the asynchronous version of this API: fs.chmod().
See the POSIX chmod(2) documentation for more detail.
fs.chownSync(path, uid, gid)
Synchronously changes owner and group of a file. Returns undefined. This is the synchronous version of fs.chown().
See the POSIX chown(2) documentation for more detail.
fs.closeSync(fd)
fd <integer>
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])
src <string> | <Buffer> | <URL> source filename to copydest <string> | <Buffer> | <URL> destination filename of the copy operationmode <integer> modifiers for copy operation. Default: 0.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).
fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.import { copyFileSync, constants } from 'node:fs';
// destination.txt will be created or overwritten by default.
copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL); copy fs.cpSync(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. Default: undefined force <boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.mode <integer> modifiers for copy operation. Default: 0. See mode flag of fs.copyFileSync().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
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.'); copy fs.fchmodSync(fd, mode)
Sets the permissions on the file. Returns undefined.
See the POSIX fchmod(2) documentation for more detail.
fs.fchownSync(fd, uid, gid)
fd <integer>
uid <integer> The file's new owner's user id.gid <integer> The file's new group's group id.Sets the owner of the file. Returns undefined.
See the POSIX fchown(2) documentation for more detail.
fs.fdatasyncSync(fd)
fd <integer>
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.fstatSync(fd[, options])
fd <integer>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.Retrieves the <fs.Stats> for the file descriptor.
See the POSIX fstat(2) documentation for more detail.
fs.fsyncSync(fd)
fd <integer>
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])
Truncates the file descriptor. Returns undefined.
For detailed information, see the documentation of the asynchronous version of this API: fs.ftruncate().
fs.futimesSync(fd, atime, mtime)
Synchronous version of fs.futimes(). Returns undefined.
fs.lchmodSync(path, mode)
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)
path <string> | <Buffer> | <URL>
uid <integer> The file's new owner's user id.gid <integer> The file's new group's group id.Set the owner for the path. Returns undefined.
See the POSIX lchown(2) documentation for more details.
fs.lutimesSync(path, atime, mtime)
path <string> | <Buffer> | <URL>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
Change the file system timestamps of the symbolic link referenced by path. Returns undefined, or throws an exception when parameters are incorrect or the operation fails. This is the synchronous version of fs.lutimes().
fs.linkSync(existingPath, newPath)
Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail. Returns undefined.
fs.lstatSync(path[, options])
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.throwIfNoEntry <boolean> Whether an exception will be thrown if no file system entry exists, rather than returning undefined. Default: true.Retrieves the <fs.Stats> for the symbolic link referred to by path.
See the POSIX lstat(2) documentation for more details.
fs.mkdirSync(path[, options])
Synchronously creates a directory. Returns undefined, or if recursive is true, the first directory path created. This is the synchronous version of fs.mkdir().
See the POSIX mkdir(2) documentation for more details.
fs.mkdtempSync(prefix[, options])
prefix <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
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.openSync(path[, flags[, mode]])
path <string> | <Buffer> | <URL>
flags <string> | <number> Default: 'r'. See support of file system flags.mode <string> | <integer> Default: 0o666
Returns an integer representing the file descriptor.
For detailed information, see the documentation of the asynchronous version of this API: fs.open().
fs.readdirSync(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> 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])
path <string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions <Object> | <string> encoding <string> | <null> Default: null
flag <string> See support of file system flags. Default: 'r'.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';
// macOS, Linux, and Windows
readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
// FreeBSD
readFileSync('<directory>'); // => <data> copy fs.readlinkSync(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
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, offset, length[, position])
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView>
offset <integer>
length <integer>
position <integer> | <bigint> | <null> Default: null
Returns the number of bytesRead.
For detailed information, see the documentation of the asynchronous version of this API: fs.read().
fs.readSync(fd, buffer[, options])
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView>
options <Object> 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.readvSync(fd, buffers[, position])
fd <integer>
buffers <ArrayBufferView[]>
position <integer> | <null> Default: null
For detailed information, see the documentation of the asynchronous version of this API: fs.readv().
fs.realpathSync(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
Returns the resolved pathname.
For detailed information, see the documentation of the asynchronous version of this API: fs.realpath().
fs.realpathSync.native(path[, options])
path <string> | <Buffer> | <URL>
options <string> | <Object> encoding <string> Default: 'utf8'
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.renameSync(oldPath, newPath)
Renames the file from oldPath to newPath. Returns undefined.
See the POSIX rename(2) documentation for more details.
fs.rmdirSync(path[, options])
path <string> | <Buffer> | <URL>
options <Object> maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.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])
path <string> | <Buffer> | <URL>
options <Object> force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.recursive <boolean> If true, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: false.retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.Synchronously removes files and directories (modeled on the standard POSIX rm utility). Returns undefined.
fs.statSync(path[, options])
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.Stats> object should be bigint. Default: false.throwIfNoEntry <boolean> Whether an exception will be thrown if no file system entry exists, rather than returning undefined. Default: true.Retrieves the <fs.Stats> for the path.
fs.statfsSync(path[, options])
path <string> | <Buffer> | <URL>
options <Object> bigint <boolean> Whether the numeric values in the returned <fs.StatFs> object should be bigint. Default: false.Synchronous statfs(2). Returns information about the mounted file system which contains path.
In case of an error, the err.code will be one of Common System Errors.
fs.symlinkSync(target, path[, type])
target <string> | <Buffer> | <URL>
path <string> | <Buffer> | <URL>
type <string> | <null> Default: null
Returns undefined.
For detailed information, see the documentation of the asynchronous version of this API: fs.symlink().
fs.truncateSync(path[, len])
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.
fs.unlinkSync(path)
Synchronous unlink(2). Returns undefined.
fs.utimesSync(path, atime, mtime)
path <string> | <Buffer> | <URL>
atime <number> | <string> | <Date>
mtime <number> | <string> | <Date>
Returns undefined.
For detailed information, see the documentation of the asynchronous version of this API: fs.utimes().
fs.writeFileSync(file, data[, options])
file <string> | <Buffer> | <URL> | <integer> filename or file descriptordata <string> | <Buffer> | <TypedArray> | <DataView>
options <Object> | <string> encoding <string> | <null> Default: 'utf8'
mode <integer> Default: 0o666
flag <string> See support of file system flags. Default: 'w'.Returns undefined.
The mode option only affects the newly created file. See fs.open() for more details.
For detailed information, see the documentation of the asynchronous version of this API: fs.writeFile().
fs.writeSync(fd, buffer, offset[, length[, position]])
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView>
offset <integer> Default: 0
length <integer> Default: buffer.byteLength - offset
position <integer> | <null> Default: null
For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, buffer...).
fs.writeSync(fd, buffer[, options])
fd <integer>
buffer <Buffer> | <TypedArray> | <DataView>
options <Object> For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, buffer...).
fs.writeSync(fd, string[, position[, encoding]])
fd <integer>
string <string>
position <integer> | <null> Default: null
encoding <string> Default: 'utf8'
For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, string...).
fs.writevSync(fd, buffers[, position])
fd <integer>
buffers <ArrayBufferView[]>
position <integer> | <null> Default: null
For detailed information, see the documentation of the asynchronous version of this API: fs.writev().
The common objects are shared by all of the file system API variants (promise, callback, and synchronous).
fs.Dir
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);
} copy When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.
dir.close()
Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
A promise is returned that will be resolved after the resource has been closed.
dir.close(callback)
callback <Function> err <Error>
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()
Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
dir.path
The read-only path of this directory as was provided to fs.opendir(), fs.opendirSync(), or fsPromises.opendir().
dir.read()
Asynchronously read the next directory entry via readdir(3) as an <fs.Dirent>.
A promise is returned that will be resolved 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)
callback <Function> err <Error>
dirent <fs.Dirent> | <null>
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()
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]()
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.
fs.Dirent
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()
Returns true if the <fs.Dirent> object describes a block device.
dirent.isCharacterDevice()
Returns true if the <fs.Dirent> object describes a character device.
dirent.isDirectory()
Returns true if the <fs.Dirent> object describes a file system directory.
dirent.isFIFO()
Returns true if the <fs.Dirent> object describes a first-in-first-out (FIFO) pipe.
dirent.isFile()
Returns true if the <fs.Dirent> object describes a regular file.
dirent.isSocket()
Returns true if the <fs.Dirent> object describes a socket.
dirent.isSymbolicLink()
Returns true if the <fs.Dirent> object describes a symbolic link.
dirent.name
The file name that this <fs.Dirent> object refers to. The type of this value is determined by the options.encoding passed to fs.readdir() or fs.readdirSync().
dirent.path
The base path that this <fs.Dirent> object refers to.
fs.FSWatcher
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.
'change'
eventType <string> The type of change event that has occurredfilename <string> | <Buffer> The filename that changed (if relevant/available)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';
// Example when handled through fs.watch() listener
watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
// Prints: <Buffer ...>
}
}); copy 'close'
Emitted when the watcher stops watching for changes. The closed <fs.FSWatcher> object is no longer usable in the event handler.
'error'
error <Error>
Emitted when an error occurs while watching the file. The errored <fs.FSWatcher> object is no longer usable in the event handler.
watcher.close()
Stop watching for changes on the given <fs.FSWatcher>. Once stopped, the <fs.FSWatcher> object is no longer usable.
watcher.ref()
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()
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.
fs.StatWatcher
A successful call to fs.watchFile() method will return a new <fs.StatWatcher> object.
watcher.ref()
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()
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.
fs.ReadStream
Instances of <fs.ReadStream> are created and returned using the fs.createReadStream() function.
'close'
Emitted when the <fs.ReadStream>'s underlying file descriptor has been closed.
'open'
fd <integer> Integer file descriptor used by the <fs.ReadStream>.Emitted when the <fs.ReadStream>'s file descriptor has been opened.
'ready'
Emitted when the <fs.ReadStream> is ready to be used.
Fires immediately after 'open'.
readStream.bytesRead
The number of bytes that have been read so far.
readStream.path
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
This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted.
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.
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 } copy 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 } copy stats.isBlockDevice()
Returns true if the <fs.Stats> object describes a block device.
stats.isCharacterDevice()
Returns true if the <fs.Stats> object describes a character device.
stats.isDirectory()
Returns true if the <fs.Stats> object describes a file system directory.
If the <fs.Stats> object was obtained from fs.lstat(), this method will always return false. This is because fs.lstat() returns information about a symbolic link itself and not the path it resolves to.
stats.isFIFO()
Returns true if the <fs.Stats> object describes a first-in-first-out (FIFO) pipe.
stats.isFile()
Returns true if the <fs.Stats> object describes a regular file.
stats.isSocket()
Returns true if the <fs.Stats> object describes a socket.
stats.isSymbolicLink()
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.ino
The file system specific "Inode" number for the file.
stats.mode
A bit-field describing the file type and mode.
stats.nlink
The number of hard-links that exist for 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.blksize
The file system block size for i/o operations.
stats.blocks
The number of blocks allocated for this file.
stats.atimeMs
The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.
stats.mtimeMs
The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.
stats.ctimeMs
The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.
stats.birthtimeMs
The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.
stats.atimeNs
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
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
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
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
The timestamp indicating the last time this file was accessed.
stats.mtime
The timestamp indicating the last time this file was modified.
stats.ctime
The timestamp indicating the last time the file status was changed.
stats.birthtime
The timestamp indicating the creation time of this file.
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:
atime "Access Time": Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.mtime "Modified Time": Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.ctime "Change Time": Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.birthtime "Birth Time": Time of file creation. Set once when the file is created. On file systems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, Unix epoch timestamp 0). This value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call.Prior to Node.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.
fs.StatFs
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
} copy bigint version:
StatFs {
type: 1397114950n,
bsize: 4096n,
blocks: 121938943n,
bfree: 61058895n,
bavail: 61058895n,
files: 999n,
ffree: 1000000n
} copy statfs.bavail
Free blocks available to unprivileged users.
statfs.bfree
Free blocks in file system.
statfs.blocks
Total data blocks in file system.
statfs.bsize
Optimal transfer block size.
statfs.ffree
Free file nodes in file system.
statfs.files
Total file nodes in file system.
statfs.type
Type of file system.
fs.WriteStream
Instances of <fs.WriteStream> are created and returned using the fs.createWriteStream() function.
'close'
Emitted when the <fs.WriteStream>'s underlying file descriptor has been closed.
'open'
fd <integer> Integer file descriptor used by the <fs.WriteStream>.Emitted when the <fs.WriteStream>'s file is opened.
'ready'
Emitted when the <fs.WriteStream> is ready to be used.
Fires immediately after 'open'.
writeStream.bytesWritten
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.close([callback])
callback <Function> err <Error>
Closes writeStream. Optionally accepts a callback that will be executed once the writeStream is closed.
writeStream.path
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
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.
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) => {
// ...
}); copy 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.
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.
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.
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.
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.
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)}`);
}); copy It is important to correctly order the operations by awaiting the results of one before invoking the other:
MJS modules
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);
}
CJS modules
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:
MJS modules
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)}`);
});
});
CJS modules
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)}`);
});
}); 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 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');
// Do something with the file
} finally {
await fd?.close();
} copy 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');
// Do something with the file
} finally {
await fd?.close();
} copy 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')); copy file: URLs are always absolute paths.
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';
// On Windows :
// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
readFileSync(new URL('file://hostname/p/a/t/h/file'));
// - WHATWG file URLs with drive letters convert to absolute path
// file:///C:/tmp/hello => C:\tmp\hello
readFileSync(new URL('file:///C:/tmp/hello'));
// - WHATWG file URLs without hostname must have a drive letters
readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute copy 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';
// On other platforms:
// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute
// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
readFileSync(new URL('file:///tmp/hello')); copy A file: <URL> having encoded slash characters will result in an error on all platforms:
import { readFileSync } from 'node:fs';
// On Windows
readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
// On POSIX
readFileSync(new URL('file:///p/a/t/h/%2F'));
readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */ copy On Windows, file: <URL>s having encoded backslash will result in an error:
import { readFileSync } from 'node:fs';
// On Windows
readFileSync(new URL('file:///C:/path/%5C'));
readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */ copy 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');
// Do something with the file
} finally {
await fd?.close();
} copy 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.
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;
}
// use stat
closeFd(fd);
});
} catch (err) {
closeFd(fd);
throw err;
}
}); copy 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();
// use stat
} finally {
await file.close();
} copy 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.
The following flags are available wherever the flag option takes a string.
'a': Open file for appending. The file is created if it does not exist.
'ax': Like 'a' but fails if the path exists.
'a+': Open file for reading and appending. The file is created if it does not exist.
'ax+': Like 'a+' but fails if the path exists.
'as': Open file for appending in synchronous mode. The file is created if it does not exist.
'as+': Open file for reading and appending in synchronous mode. The file is created if it does not exist.
'r': Open file for reading. An exception occurs if the file does not exist.
'rs': Open file for reading in synchronous mode. An exception occurs if the file does not exist.
'r+': Open file for reading and writing. An exception occurs if the file does not exist.
'rs+': Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.
This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.
This doesn't turn fs.open() or fsPromises.open() into a synchronous blocking call. If synchronous operation is desired, something like fs.openSync() should be used.
'w': Open file for writing. The file is created (if it does not exist) or truncated (if it exists).
'wx': Like 'w' but fails if the path exists.
'w+': Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).
'wx+': Like 'w+' but fails if the path exists.
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.
// macOS and Linux
fs.open('<directory>', 'a+', (err, fd) => {
// => [Error: EISDIR: illegal operation on a directory, open <directory>]
});
// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
// => null, <fd>
}); copy 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.
© Joyent, Inc. and other Node contributors
Licensed under the MIT License.
Node.js is a trademark of Joyent, Inc. and is used with its permission.
We are not endorsed by or affiliated with Joyent.
https://nodejs.org/api/fs.html