All file system operations have synchronous, callback, and promise-based
forms, and are accessible using both CommonJS syntax and ES6 Modules (ESM).
Promises API#
The fs/promises API provides asynchronous file system methods that return
promises.
The promise APIs use the underlying Node.js threadpool to perform file
system operations off the event loop thread. These operations are not
synchronized or threadsafe. Care must be taken when performing multiple
concurrent modifications on the same file or data corruption may occur.
Class: FileHandle#
Added in: v10.0.0
A <FileHandle> object is an object wrapper for a numeric file descriptor.
Instances of the <FileHandle> object are created by the fsPromises.open()
method.
All <FileHandle> objects are <EventEmitter>s.
If a <FileHandle> is not closed using the filehandle.close() method, it will
try to automatically close the file descriptor and emit a process warning,
helping to prevent memory leaks. Please do not rely on this behavior because
it can be unreliable and the file may not be closed. Instead, always explicitly
close <FileHandle>s. Node.js may change this behavior in the future.
Event: 'close'#
Added in: v15.4.0
The 'close' event is emitted when the <FileHandle> has been closed and can no
longer be used.
filehandle.chmod(mode)#
Added in: v10.0.0
mode <integer> the file mode bit mask.
- Returns: <Promise> Fulfills with
undefined upon success.
Modifies the permissions on the file. See chmod(2).
filehandle.chown(uid, gid)#
Added in: v10.0.0
uid <integer> The file's new owner's user id.
gid <integer> The file's new group's group id.
- Returns: <Promise> Fulfills with
undefined upon success.
Changes the ownership of the file. A wrapper for chown(2).
filehandle.close()#
Added in: v10.0.0
- Returns: <Promise> Fulfills with
undefined upon success.
Closes the file handle after waiting for any pending operation on the handle to
complete.
import { open } from 'node:fs/promises';
let filehandle;
try {
filehandle = await open('thefile.txt', 'r');
} finally {
await filehandle?.close();
}
filehandle.createReadStream([options])#
Added in: v16.11.0
options can include start and end values to read a range of bytes from
the file instead of the entire file. Both start and end are inclusive and
start counting at 0, allowed values are in the
[0, Number.MAX_SAFE_INTEGER] range. If start is
omitted or undefined, filehandle.createReadStream() reads sequentially from
the current file position. The encoding can be any one of those accepted by
<Buffer>.
If the FileHandle points to a character device that only supports blocking
reads (such as keyboard or sound card), read operations do not finish until data
is available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close' event after it has been
destroyed. Set the emitClose option to false to change this behavior.
import { open } from 'node:fs/promises';
const fd = await open('/dev/input/event0');
const stream = fd.createReadStream();
setTimeout(() => {
stream.close();
stream.push(null);
stream.read(0);
}, 100);
If autoClose is false, then the file descriptor won't be closed, even if
there's an error. It is the application's responsibility to close it and make
sure there's no file descriptor leak. If autoClose is set to true (default
behavior), on 'error' or 'end' the file descriptor will be closed
automatically.
An example to read the last 10 bytes of a file which is 100 bytes long:
import { open } from 'node:fs/promises';
const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });
filehandle.createWriteStream([options])#
options may also include a start option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than
replacing it may require the flags open option to be set to r+ rather than
the default r. The encoding can be any one of those accepted by <Buffer>.
If autoClose is set to true (default behavior) on 'error' or 'finish'
the file descriptor will be closed automatically. If autoClose is false,
then the file descriptor won't be closed, even if there's an error.
It is the application's responsibility to close it and make sure there's no
file descriptor leak.
By default, the stream will emit a 'close' event after it has been
destroyed. Set the emitClose option to false to change this behavior.
filehandle.datasync()#
Added in: v10.0.0
- Returns: <Promise> Fulfills with
undefined upon success.
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2) documentation for details.
Unlike filehandle.sync this method does not flush modified metadata.
filehandle.fd#
Added in: v10.0.0
filehandle.read(buffer, offset, length, position)#
buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the
file data read.
offset <integer> The location in the buffer at which to start filling.
Default: 0
length <integer> The number of bytes to read. Default:
buffer.byteLength - offset
position <integer> | <bigint> | <null> The location where to begin reading data
from the file. If null or -1, data will be read from the current file
position, and the position will be updated. If position is a non-negative
integer, the current file position will remain unchanged.
Default: null
- Returns: <Promise> Fulfills upon success with an object with two properties:
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the
number of bytes read is zero.
filehandle.read([options])#
options <Object>
buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the
file data read. Default: Buffer.alloc(16384)
offset <integer> The location in the buffer at which to start filling.
Default: 0
length <integer> The number of bytes to read. Default:
buffer.byteLength - offset
position <integer> | <bigint> | <null> The location where to begin reading data
from the file. If null or -1, data will be read from the current file
position, and the position will be updated. If position is a non-negative
integer, the current file position will remain unchanged.
Default:: null
- Returns: <Promise> Fulfills upon success with an object with two properties:
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the
number of bytes read is zero.
filehandle.read(buffer[, options])#
buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the
file data read.
options <Object>
offset <integer> The location in the buffer at which to start filling.
Default: 0
length <integer> The number of bytes to read. Default:
buffer.byteLength - offset
position <integer> | <bigint> | <null> The location where to begin reading data
from the file. If null or -1, data will be read from the current file
position, and the position will be updated. If position is a non-negative
integer, the current file position will remain unchanged.
Default:: null
- Returns: <Promise> Fulfills upon success with an object with two properties:
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the
number of bytes read is zero.
filehandle.readableWebStream([options])#
Returns a byte-oriented ReadableStream that may be used to read the file's
contents.
An error will be thrown if this method is called more than once or is called
after the FileHandle is closed or closing.
import {
open,
} from 'node:fs/promises';
const file = await open('./some/file/to/read');
for await (const chunk of file.readableWebStream())
console.log(chunk);
await file.close();const {
open,
} = require('node:fs/promises');
(async () => {
const file = await open('./some/file/to/read');
for await (const chunk of file.readableWebStream())
console.log(chunk);
await file.close();
})();
While the ReadableStream will read the file to completion, it will not
close the FileHandle automatically. User code must still call the
fileHandle.close() method.
filehandle.readFile(options)#
Added in: v10.0.0
options <Object> | <string>
- Returns: <Promise> Fulfills upon a successful read with the contents of the
file. If no encoding is specified (using
options.encoding), the data is
returned as a <Buffer> object. Otherwise, the data will be a string.
Asynchronously reads the entire contents of a file.
If options is a string, then it specifies the encoding.
The <FileHandle> has to support reading.
If one or more filehandle.read() calls are made on a file handle and then a
filehandle.readFile() call is made, the data will be read from the current
position till the end of the file. It doesn't always read from the beginning
of the file.
filehandle.readv(buffers[, position])#
Added in: v13.13.0, v12.17.0
buffers <Buffer[]> | <TypedArray[]> | <DataView[]>
position <integer> | <null> The offset from the beginning of the file where
the data should be read from. If position is not a number, the data will
be read from the current position. Default: null
- Returns: <Promise> Fulfills upon success an object containing two properties:
Read from a file and write to an array of <ArrayBufferView>s
filehandle.stat([options])#
filehandle.sync()#
Added in: v10.0.0
- Returns: <Promise> Fulfills with
undefined upon success.
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2) documentation for more detail.
filehandle.truncate(len)#
Added in: v10.0.0
Truncates the file.
If the file was larger than len bytes, only the first len bytes will be
retained in the file.
The following example retains only the first four bytes of the file:
import { open } from 'node:fs/promises';
let filehandle = null;
try {
filehandle = await open('temp.txt', 'r+');
await filehandle.truncate(4);
} finally {
await filehandle?.close();
}
If the file previously was shorter than len bytes, it is extended, and the
extended part is filled with null bytes ('\0'):
If len is negative then 0 will be used.
filehandle.utimes(atime, mtime)#
Added in: v10.0.0
Change the file system timestamps of the object referenced by the <FileHandle>
then fulfills the promise with no arguments upon success.
filehandle.write(buffer, offset[, length[, position]])#
buffer <Buffer> | <TypedArray> | <DataView>
offset <integer> The start position from within buffer where the data
to write begins.
length <integer> The number of bytes from buffer to write. Default:
buffer.byteLength - offset
position <integer> | <null> The offset from the beginning of the file where the
data from buffer should be written. If position is not a number,
the data will be written at the current position. See the POSIX pwrite(2)
documentation for more detail. Default: null
- Returns: <Promise>
Write buffer to the file.
The promise is fulfilled with an object containing two properties:
It is unsafe to use filehandle.write() multiple times on the same file
without waiting for the promise to be fulfilled (or rejected). For this
scenario, use filehandle.createWriteStream().
On Linux, positional writes do not work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
filehandle.write(buffer[, options])#
Added in: v18.3.0, v16.17.0
Write buffer to the file.
Similar to the above filehandle.write function, this version takes an
optional options object. If no options object is specified, it will
default with the above values.
filehandle.write(string[, position[, encoding]])#
string <string>
position <integer> | <null> The offset from the beginning of the file where the
data from string should be written. If position is not a number the
data will be written at the current position. See the POSIX pwrite(2)
documentation for more detail. Default: null
encoding <string> The expected string encoding. Default: 'utf8'
- Returns: <Promise>
Write string to the file. If string is not a string, the promise is
rejected with an error.
The promise is fulfilled with an object containing two properties:
bytesWritten <integer> the number of bytes written
buffer <string> a reference to the string written.
It is unsafe to use filehandle.write() multiple times on the same file
without waiting for the promise to be fulfilled (or rejected). For this
scenario, use filehandle.createWriteStream().
On Linux, positional writes do not work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
filehandle.writeFile(data, options)#
Asynchronously writes data to a file, replacing the file if it already exists.
data can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.
The promise is fulfilled with no arguments upon success.
If options is a string, then it specifies the encoding.
The <FileHandle> has to support writing.
It is unsafe to use filehandle.writeFile() multiple times on the same file
without waiting for the promise to be fulfilled (or rejected).
If one or more filehandle.write() calls are made on a file handle and then a
filehandle.writeFile() call is made, the data will be written from the
current position till the end of the file. It doesn't always write from the
beginning of the file.
filehandle.writev(buffers[, position])#
Added in: v12.9.0
Write an array of <ArrayBufferView>s to the file.
The promise is fulfilled with an object containing a two properties:
It is unsafe to call writev() multiple times on the same file without waiting
for the promise to be fulfilled (or rejected).
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
filehandle[Symbol.asyncDispose]()#
Added in: v20.4.0, v18.18.0
Calls filehandle.close() and returns a promise that fulfills when the
filehandle is closed.
fsPromises.access(path[, mode])#
Added in: v10.0.0
Tests a user's permissions for the file or directory specified by path.
The mode argument is an optional integer that specifies the accessibility
checks to be performed. mode should be either the value fs.constants.F_OK
or a mask consisting of the bitwise OR of any of fs.constants.R_OK,
fs.constants.W_OK, and fs.constants.X_OK (e.g.
fs.constants.W_OK | fs.constants.R_OK). Check File access constants for
possible values of mode.
If the accessibility check is successful, the promise is fulfilled with no
value. If any of the accessibility checks fail, the promise is rejected
with an <Error> object. The following example checks if the file
/etc/passwd can be read and written by the current process.
import { access, constants } from 'node:fs/promises';
try {
await access('/etc/passwd', constants.R_OK | constants.W_OK);
console.log('can access');
} catch {
console.error('cannot access');
}
Using fsPromises.access() to check for the accessibility of a file before
calling fsPromises.open() is not recommended. Doing so introduces a race
condition, since other processes may change the file's state between the two
calls. Instead, user code should open/read/write the file directly and handle
the error raised if the file is not accessible.
fsPromises.appendFile(path, data[, options])#
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)#
Added in: v10.0.0
Changes the permissions of a file.
fsPromises.chown(path, uid, gid)#
Added in: v10.0.0
Changes the ownership of a file.
fsPromises.copyFile(src, dest[, mode])#
src <string> | <Buffer> | <URL> source filename to copy
dest <string> | <Buffer> | <URL> destination filename of the copy operation
mode <integer> Optional modifiers that specify the behavior of the copy
operation. It is possible to create a mask consisting of the bitwise OR of
two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE)
Default: 0.
fs.constants.COPYFILE_EXCL: The copy operation will fail if dest
already exists.
fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create
a copy-on-write reflink. If the platform does not support copy-on-write,
then a fallback copy mechanism is used.
fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to
create a copy-on-write reflink. If the platform does not support
copy-on-write, then the operation will fail.
- Returns: <Promise> Fulfills with
undefined upon success.
Asynchronously copies src to dest. By default, dest is overwritten if it
already exists.
No guarantees are made about the atomicity of the copy operation. If an
error occurs after the destination file has been opened for writing, an attempt
will be made to remove the destination.
import { copyFile, constants } from 'node:fs/promises';
try {
await copyFile('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
} catch {
console.error('The file could not be copied');
}
try {
await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
console.log('source.txt was copied to destination.txt');
} catch {
console.error('The file could not be copied');
}
fsPromises.cp(src, dest[, options])#
src <string> | <URL> source path to copy.
dest <string> | <URL> destination path to copy to.
options <Object>
dereference <boolean> dereference symlinks. Default: false.
errorOnExist <boolean> when force is false, and the destination
exists, throw an error. Default: false.
filter <Function> Function to filter copied files/directories. Return
true to copy the item, false to ignore it. When ignoring a directory,
all of its contents will be skipped as well. Can also return a Promise
that resolves to true or false Default: undefined.
src <string> source path to copy.
dest <string> destination path to copy to.
- Returns: <boolean> | <Promise> A value that is coercible to
boolean or
a Promise that fulfils with such value.
force <boolean> overwrite existing file or directory. The copy
operation will ignore errors if you set this to false and the destination
exists. Use the errorOnExist option to change this behavior.
Default: true.
mode <integer> modifiers for copy operation. Default: 0.
See mode flag of fsPromises.copyFile().
preserveTimestamps <boolean> When true timestamps from src will
be preserved. Default: false.
recursive <boolean> copy directories recursively Default: false
verbatimSymlinks <boolean> When true, path resolution for symlinks will
be skipped. Default: false
- Returns: <Promise> Fulfills with
undefined upon success.
Asynchronously copies the entire directory structure from src to dest,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/.
fsPromises.lchmod(path, mode)#
Deprecated since: v10.0.0
Changes the permissions on a symbolic link.
This method is only implemented on macOS.
fsPromises.lchown(path, uid, gid)#
Changes the ownership on a symbolic link.
fsPromises.lutimes(path, atime, mtime)#
Added in: v14.5.0, v12.19.0
Changes the access and modification times of a file in the same way as
fsPromises.utimes(), with the difference that if the path refers to a
symbolic link, then the link is not dereferenced: instead, the timestamps of
the symbolic link itself are changed.
fsPromises.link(existingPath, newPath)#
Added in: v10.0.0
Creates a new link from the existingPath to the newPath. See the POSIX
link(2) documentation for more detail.
fsPromises.lstat(path[, options])#
Equivalent to fsPromises.stat() unless path refers to a symbolic link,
in which case the link itself is stat-ed, not the file that it refers to.
Refer to the POSIX lstat(2) document for more detail.
fsPromises.mkdir(path[, options])#
Added in: v10.0.0
Asynchronously creates a directory.
The optional options argument can be an integer specifying mode (permission
and sticky bits), or an object with a mode property and a recursive
property indicating whether parent directories should be created. Calling
fsPromises.mkdir() when path is a directory that exists results in a
rejection only when recursive is false.
import { mkdir } from 'node:fs/promises';
try {
const projectFolder = new URL('./test/project/', import.meta.url);
const createDir = await mkdir(projectFolder, { recursive: true });
console.log(`created ${createDir}`);
} catch (err) {
console.error(err.message);
}const { mkdir } = require('node:fs/promises');
const { join } = require('node:path');
async function makeDirectory() {
const projectFolder = join(__dirname, 'test', 'project');
const dirCreation = await mkdir(projectFolder, { recursive: true });
console.log(dirCreation);
return dirCreation;
}
makeDirectory().catch(console.error);
fsPromises.mkdtemp(prefix[, options])#
Creates a unique temporary directory. A unique directory name is generated by
appending six random characters to the end of the provided prefix. Due to
platform inconsistencies, avoid trailing X characters in prefix. Some
platforms, notably the BSDs, can return more than six random characters, and
replace trailing X characters in prefix with random characters.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use.
import { mkdtemp } from 'node:fs/promises';
import { join } from 'node:path';
import { tmpdir } from 'node:os';
try {
await mkdtemp(join(tmpdir(), 'foo-'));
} catch (err) {
console.error(err);
}
The fsPromises.mkdtemp() method will append the six randomly selected
characters directly to the prefix string. For instance, given a directory
/tmp, if the intention is to create a temporary directory within /tmp, the
prefix must end with a trailing platform-specific path separator
(require('node:path').sep).
fsPromises.open(path, flags[, mode])#
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])#
Asynchronously open a directory for iterative scanning. See the POSIX
opendir(3) documentation for more detail.
Creates an <fs.Dir>, which contains all further functions for reading from
and cleaning up the directory.
The encoding option sets the encoding for the path while opening the
directory and subsequent read operations.
Example using async iteration:
import { opendir } from 'node:fs/promises';
try {
const dir = await opendir('./');
for await (const dirent of dir)
console.log(dirent.name);
} catch (err) {
console.error(err);
}
When using the async iterator, the <fs.Dir> object will be automatically
closed after the iterator exits.
fsPromises.readdir(path[, options])#
path <string> | <Buffer> | <URL>
options <string> | <Object>
encoding <string> Default: 'utf8'
withFileTypes <boolean> Default: false
recursive <boolean> If true, reads the contents of a directory
recursively. In recursive mode, it will list all files, sub files, and
directories. Default: false.
- Returns: <Promise> Fulfills with an array of the names of the files in
the directory excluding
'.' and '..'.
Reads the contents of a directory.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the filenames. If the encoding is set to 'buffer', the filenames returned
will be passed as <Buffer> objects.
If options.withFileTypes is set to true, the returned array will contain
<fs.Dirent> objects.
import { readdir } from 'node:fs/promises';
try {
const files = await readdir(path);
for (const file of files)
console.log(file);
} catch (err) {
console.error(err);
}
fsPromises.readFile(path[, options])#
Asynchronously reads the entire contents of a file.
If no encoding is specified (using options.encoding), the data is returned
as a <Buffer> object. Otherwise, the data will be a string.
If options is a string, then it specifies the encoding.
When the path is a directory, the behavior of fsPromises.readFile() is
platform-specific. On macOS, Linux, and Windows, the promise will be rejected
with an error. On FreeBSD, a representation of the directory's contents will be
returned.
An example of reading a package.json file located in the same directory of the
running code:
import { readFile } from 'node:fs/promises';
try {
const filePath = new URL('./package.json', import.meta.url);
const contents = await readFile(filePath, { encoding: 'utf8' });
console.log(contents);
} catch (err) {
console.error(err.message);
}const { readFile } = require('node:fs/promises');
const { resolve } = require('node:path');
async function logFile() {
try {
const filePath = resolve('./package.json');
const contents = await readFile(filePath, { encoding: 'utf8' });
console.log(contents);
} catch (err) {
console.error(err.message);
}
}
logFile();
It is possible to abort an ongoing readFile using an <AbortSignal>. If a
request is aborted the promise returned is rejected with an AbortError:
import { readFile } from 'node:fs/promises';
try {
const controller = new AbortController();
const { signal } = controller;
const promise = readFile(fileName, { signal });
controller.abort();
await promise;
} catch (err) {
console.error(err);
}
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.readFile performs.
Any specified <FileHandle> has to support reading.
fsPromises.readlink(path[, options])#
Added in: v10.0.0
Reads the contents of the symbolic link referred to by path. See the POSIX
readlink(2) documentation for more detail. The promise is fulfilled with the
linkString upon success.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the link path returned. If the encoding is set to 'buffer', the link path
returned will be passed as a <Buffer> object.
fsPromises.realpath(path[, options])#
Added in: v10.0.0
Determines the actual location of path using the same semantics as the
fs.realpath.native() function.
Only paths that can be converted to UTF8 strings are supported.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the path. If the encoding is set to 'buffer', the path returned will be
passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc in order for this function to work. Glibc does not have
this restriction.
fsPromises.rename(oldPath, newPath)#
Added in: v10.0.0
Renames oldPath to newPath.
fsPromises.rmdir(path[, options])#
path <string> | <Buffer> | <URL>
options <Object>
maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or
EPERM error is encountered, Node.js retries the operation with a linear
backoff wait of retryDelay milliseconds longer on each try. This option
represents the number of retries. This option is ignored if the recursive
option is not true. Default: 0.
recursive <boolean> If true, perform a recursive directory removal. In
recursive mode, operations are retried on failure. Default: false.
Deprecated.
retryDelay <integer> The amount of time in milliseconds to wait between
retries. This option is ignored if the recursive option is not true.
Default: 100.
- Returns: <Promise> Fulfills with
undefined upon success.
Removes the directory identified by path.
Using fsPromises.rmdir() on a file (not a directory) results in the
promise being rejected with an ENOENT error on Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf Unix command, use
fsPromises.rm() with options { recursive: true, force: true }.
fsPromises.rm(path[, options])#
Added in: v14.14.0
path <string> | <Buffer> | <URL>
options <Object>
force <boolean> When true, exceptions will be ignored if path does
not exist. Default: false.
maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or
EPERM error is encountered, Node.js will retry the operation with a linear
backoff wait of retryDelay milliseconds longer on each try. This option
represents the number of retries. This option is ignored if the recursive
option is not true. Default: 0.
recursive <boolean> If true, perform a recursive directory removal. In
recursive mode operations are retried on failure. Default: false.
retryDelay <integer> The amount of time in milliseconds to wait between
retries. This option is ignored if the recursive option is not true.
Default: 100.
- Returns: <Promise> Fulfills with
undefined upon success.
Removes files and directories (modeled on the standard POSIX rm utility).
fsPromises.stat(path[, options])#
fsPromises.statfs(path[, options])#
Added in: v19.6.0, v18.15.0
fsPromises.symlink(target, path[, type])#
Creates a symbolic link.
The type argument is only used on Windows platforms and can be one of 'dir',
'file', or 'junction'. If the type argument is 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])#
Added in: v10.0.0
Truncates (shortens or extends the length) of the content at path to len
bytes.
fsPromises.unlink(path)#
Added in: v10.0.0
If path refers to a symbolic link, then the link is removed without affecting
the file or directory to which that link refers. If the path refers to a file
path that is not a symbolic link, the file is deleted. See the POSIX unlink(2)
documentation for more detail.
fsPromises.utimes(path, atime, mtime)#
Added in: v10.0.0
Change the file system timestamps of the object referenced by path.
The atime and mtime arguments follow these rules:
- Values can be either numbers representing Unix epoch time,
Dates, or a
numeric string like '123456789.0'.
- If the value can not be converted to a number, or is
NaN, Infinity, or
-Infinity, an Error will be thrown.
fsPromises.watch(filename[, options])#
Added in: v15.9.0, v14.18.0
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.
maxQueue <number> Specifies the number of events to queue between iterations
of the <AsyncIterator> returned. Default: 2048.
overflow <string> Either 'ignore' or 'throw' when there are more events to be
queued than maxQueue allows. 'ignore' means overflow events are dropped and a
warning is emitted, while 'throw' means to throw an exception. Default: 'ignore'.
- Returns: <AsyncIterator> of objects with the properties:
Returns an async iterator that watches for changes on filename, where filename
is either a file or a directory.
const { watch } = require('node:fs/promises');
const ac = new AbortController();
const { signal } = ac;
setTimeout(() => ac.abort(), 10000);
(async () => {
try {
const watcher = watch(__filename, { signal });
for await (const event of watcher)
console.log(event);
} catch (err) {
if (err.name === 'AbortError')
return;
throw err;
}
})();
On most platforms, 'rename' is emitted whenever a filename appears or
disappears in the directory.
All the caveats for fs.watch() also apply to fsPromises.watch().
fsPromises.writeFile(file, data[, options])#
Asynchronously writes data to a file, replacing the file if it already exists.
data can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.
The encoding option is ignored if data is a buffer.
If options is a string, then it specifies the encoding.
The mode option only affects the newly created file. See fs.open()
for more details.
Any specified <FileHandle> has to support writing.
It is unsafe to use fsPromises.writeFile() multiple times on the same file
without waiting for the promise to be settled.
Similarly to fsPromises.readFile - fsPromises.writeFile is a convenience
method that performs multiple write calls internally to write the buffer
passed to it. For performance sensitive code consider using
fs.createWriteStream() or filehandle.createWriteStream().
It is possible to use an <AbortSignal> to cancel an fsPromises.writeFile().
Cancelation is "best effort", and some amount of data is likely still
to be written.
import { writeFile } from 'node:fs/promises';
import { Buffer } from 'node:buffer';
try {
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
const promise = writeFile('message.txt', data, { signal });
controller.abort();
await promise;
} catch (err) {
console.error(err);
}
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.writeFile performs.
fsPromises.constants#
Added in: v18.4.0, v16.17.0
Returns an object containing commonly used constants for file system
operations. The object is the same as fs.constants. See FS constants
for more details.
Callback API#
The callback APIs perform all operations asynchronously, without blocking the
event loop, then invoke a callback function upon completion or error.
The callback APIs use the underlying Node.js threadpool to perform file
system operations off the event loop thread. These operations are not
synchronized or threadsafe. Care must be taken when performing multiple
concurrent modifications on the same file or data corruption may occur.
fs.access(path[, mode], callback)#
Tests a user's permissions for the file or directory specified by path.
The mode argument is an optional integer that specifies the accessibility
checks to be performed. mode should be either the value fs.constants.F_OK
or a mask consisting of the bitwise OR of any of fs.constants.R_OK,
fs.constants.W_OK, and fs.constants.X_OK (e.g.
fs.constants.W_OK | fs.constants.R_OK). Check File access constants for
possible values of mode.
The final argument, callback, is a callback function that is invoked with
a possible error argument. If any of the accessibility checks fail, the error
argument will be an Error object. The following examples check if
package.json exists, and if it is readable or writable.
import { access, constants } from 'node:fs';
const file = 'package.json';
access(file, constants.F_OK, (err) => {
console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});
access(file, constants.R_OK, (err) => {
console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});
access(file, constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});
access(file, constants.R_OK | constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not' : 'is'} readable and writable`);
});
Do not use fs.access() to check for the accessibility of a file before calling
fs.open(), fs.readFile(), or fs.writeFile(). Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file is not accessible.
write (NOT RECOMMENDED)
import { access, open, close } from 'node:fs';
access('myfile', (err) => {
if (!err) {
console.error('myfile already exists');
return;
}
open('myfile', 'wx', (err, fd) => {
if (err) throw err;
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
});
write (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
read (NOT RECOMMENDED)
import { access, open, close } from 'node:fs';
access('myfile', (err) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
open('myfile', 'r', (err, fd) => {
if (err) throw err;
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
});
read (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
The "not recommended" examples above check for accessibility and then use the
file; the "recommended" examples are better because they use the file directly
and handle the error, if any.
In general, check for the accessibility of a file only if the file will not be
used directly, for example when its accessibility is a signal from another
process.
On Windows, access-control policies (ACLs) on a directory may limit access to
a file or directory. The fs.access() function, however, does not check the
ACL and therefore may report that a path is accessible even if the ACL restricts
the user from reading or writing to it.
fs.appendFile(path, data[, options], callback)#
Asynchronously append data to a file, creating the file if it does not yet
exist. data can be a string or a <Buffer>.
The mode option only affects the newly created file. See fs.open()
for more details.
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', (err) => {
if (err) throw err;
console.log('The "data to append" was appended to file!');
});
If options is a string, then it specifies the encoding:
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', 'utf8', callback);
The path may be specified as a numeric file descriptor that has been opened
for appending (using fs.open() or fs.openSync()). The file descriptor will
not be closed automatically.
import { open, close, appendFile } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('message.txt', 'a', (err, fd) => {
if (err) throw err;
try {
appendFile(fd, 'data to append', 'utf8', (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
throw err;
}
});
fs.chmod(path, mode, callback)#
Asynchronously changes the permissions of a file. No arguments other than a
possible exception are given to the completion callback.
See the POSIX chmod(2) documentation for more detail.
import { chmod } from 'node:fs';
chmod('my_file.txt', 0o775, (err) => {
if (err) throw err;
console.log('The permissions for file "my_file.txt" have been changed!');
});
File modes#
The mode argument used in both the fs.chmod() and fs.chmodSync()
methods is a numeric bitmask created using a logical OR of the following
constants:
| Constant | Octal | Description |
|---|
fs.constants.S_IRUSR | 0o400 | read by owner |
fs.constants.S_IWUSR | 0o200 | write by owner |
fs.constants.S_IXUSR | 0o100 | execute/search by owner |
fs.constants.S_IRGRP | 0o40 | read by group |
fs.constants.S_IWGRP | 0o20 | write by group |
fs.constants.S_IXGRP | 0o10 | execute/search by group |
fs.constants.S_IROTH | 0o4 | read by others |
fs.constants.S_IWOTH | 0o2 | write by others |
fs.constants.S_IXOTH | 0o1 | execute/search by others |
An easier method of constructing the mode is to use a sequence of three
octal digits (e.g. 765). The left-most digit (7 in the example), specifies
the permissions for the file owner. The middle digit (6 in the example),
specifies permissions for the group. The right-most digit (5 in the example),
specifies the permissions for others.
| Number | Description |
|---|
7 | read, write, and execute |
6 | read and write |
5 | read and execute |
4 | read only |
3 | write and execute |
2 | write only |
1 | execute only |
0 | no permission |
For example, the octal value 0o765 means:
- The owner may read, write, and execute the file.
- The group may read and write the file.
- Others may read and execute the file.
When using raw numbers where file modes are expected, any value larger than
0o777 may result in platform-specific behaviors that are not supported to work
consistently. Therefore constants like S_ISVTX, S_ISGID, or S_ISUID are
not exposed in fs.constants.
Caveats: on Windows only the write permission can be changed, and the
distinction among the permissions of group, owner, or others is not
implemented.
fs.chown(path, uid, gid, callback)#
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])#
Closes the file descriptor. No arguments other than a possible exception are
given to the completion callback.
Calling fs.close() on any file descriptor (fd) that is currently in use
through any other fs operation may lead to undefined behavior.
See the POSIX close(2) documentation for more detail.
fs.copyFile(src, dest[, mode], callback)#
Asynchronously copies src to dest. By default, dest is overwritten if it
already exists. No arguments other than a possible exception are given to the
callback function. Node.js makes no guarantees about the atomicity of the copy
operation. If an error occurs after the destination file has been opened for
writing, Node.js will attempt to remove the destination.
mode is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).
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');
}
copyFile('source.txt', 'destination.txt', callback);
copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback);
fs.cp(src, dest[, options], callback)#
src <string> | <URL> source path to copy.
dest <string> | <URL> destination path to copy to.
options <Object>
dereference <boolean> dereference symlinks. Default: false.
errorOnExist <boolean> when force is false, and the destination
exists, throw an error. Default: false.
filter <Function> Function to filter copied files/directories. Return
true to copy the item, false to ignore it. When ignoring a directory,
all of its contents will be skipped as well. Can also return a Promise
that resolves to true or false Default: undefined.
src <string> source path to copy.
dest <string> destination path to copy to.
- Returns: <boolean> | <Promise> A value that is coercible to
boolean or
a Promise that fulfils with such value.
force <boolean> overwrite existing file or directory. The copy
operation will ignore errors if you set this to false and the destination
exists. Use the errorOnExist option to change this behavior.
Default: true.
mode <integer> modifiers for copy operation. Default: 0.
See mode flag of 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])#
options can include start and end values to read a range of bytes from
the file instead of the entire file. Both start and end are inclusive and
start counting at 0, allowed values are in the
[0, Number.MAX_SAFE_INTEGER] range. If fd is specified and start is
omitted or undefined, fs.createReadStream() reads sequentially from the
current file position. The encoding can be any one of those accepted by
<Buffer>.
If fd is specified, ReadStream will ignore the path argument and will use
the specified file descriptor. This means that no 'open' event will be
emitted. fd should be blocking; non-blocking fds should be passed to
<net.Socket>.
If fd points to a character device that only supports blocking reads
(such as keyboard or sound card), read operations do not finish until data is
available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close' event after it has been
destroyed. Set the emitClose option to false to change this behavior.
By providing the fs option, it is possible to override the corresponding fs
implementations for open, read, and close. When providing the fs option,
an override for read is required. If no fd is provided, an override for
open is also required. If autoClose is true, an override for close is
also required.
import { createReadStream } from 'node:fs';
const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
stream.close();
stream.push(null);
stream.read(0);
}, 100);
If autoClose is false, then the file descriptor won't be closed, even if
there's an error. It is the application's responsibility to close it and make
sure there's no file descriptor leak. If autoClose is set to true (default
behavior), on 'error' or 'end' the file descriptor will be closed
automatically.
mode sets the file mode (permission and sticky bits), but only if the
file was created.
An example to read the last 10 bytes of a file which is 100 bytes long:
import { createReadStream } from 'node:fs';
createReadStream('sample.txt', { start: 90, end: 99 });
If options is a string, then it specifies the encoding.
fs.createWriteStream(path[, options])#
options may also include a start option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than
replacing it may require the flags option to be set to r+ rather than the
default w. The encoding can be any one of those accepted by <Buffer>.
If autoClose is set to true (default behavior) on 'error' or 'finish'
the file descriptor will be closed automatically. If autoClose is false,
then the file descriptor won't be closed, even if there's an error.
It is the application's responsibility to close it and make sure there's no
file descriptor leak.
By default, the stream will emit a 'close' event after it has been
destroyed. Set the emitClose option to false to change this behavior.
By providing the fs option it is possible to override the corresponding fs
implementations for open, write, writev, and close. Overriding write()
without writev() can reduce performance as some optimizations (_writev())
will be disabled. When providing the fs option, overrides for at least one of
write and writev are required. If no fd option is supplied, an override
for open is also required. If autoClose is true, an override for close
is also required.
Like <fs.ReadStream>, if fd is specified, <fs.WriteStream> will ignore the
path argument and will use the specified file descriptor. This means that no
'open' event will be emitted. fd should be blocking; non-blocking fds
should be passed to <net.Socket>.
If options is a string, then it specifies the encoding.
fs.exists(path, callback)#
Test whether or not the element at the given path exists by checking with the file system.
Then call the callback argument with either true or false:
import { exists } from 'node:fs';
exists('/etc/passwd', (e) => {
console.log(e ? 'it exists' : 'no passwd!');
});
The parameters for this callback are not consistent with other Node.js
callbacks. Normally, the first parameter to a Node.js callback is an err
parameter, optionally followed by other parameters. The fs.exists() callback
has only one boolean parameter. This is one reason fs.access() is recommended
instead of fs.exists().
If path is a symbolic link, it is followed. Thus, if path exists but points
to a non-existent element, the callback will receive the value false.
Using fs.exists() to check for the existence of a file before calling
fs.open(), fs.readFile(), or fs.writeFile() is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file does not exist.
write (NOT RECOMMENDED)
import { exists, open, close } from 'node:fs';
exists('myfile', (e) => {
if (e) {
console.error('myfile already exists');
} else {
open('myfile', 'wx', (err, fd) => {
if (err) throw err;
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
}
});
write (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
read (NOT RECOMMENDED)
import { open, close, exists } from 'node:fs';
exists('myfile', (e) => {
if (e) {
open('myfile', 'r', (err, fd) => {
if (err) throw err;
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
} else {
console.error('myfile does not exist');
}
});
read (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
The "not recommended" examples above check for existence and then use the
file; the "recommended" examples are better because they use the file directly
and handle the error, if any.
In general, check for the existence of a file only if the file won't be
used directly, for example when its existence is a signal from another
process.
fs.fchmod(fd, mode, callback)#
Sets the permissions on the file. No arguments other than a possible exception
are given to the completion callback.
See the POSIX fchmod(2) documentation for more detail.
fs.fchown(fd, uid, gid, callback)#
Sets the owner of the file. No arguments other than a possible exception are
given to the completion callback.
See the POSIX fchown(2) documentation for more detail.
fs.fdatasync(fd, callback)#
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2) documentation for details. No arguments other than a possible
exception are given to the completion callback.
fs.fstat(fd[, options], callback)#
Invokes the callback with the <fs.Stats> for the file descriptor.
See the POSIX fstat(2) documentation for more detail.
fs.fsync(fd, callback)#
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2) documentation for more detail. No arguments other
than a possible exception are given to the completion callback.
fs.ftruncate(fd[, len], callback)#
Truncates the file descriptor. No arguments other than a possible exception are
given to the completion callback.
See the POSIX ftruncate(2) documentation for more detail.
If the file referred to by the file descriptor was larger than len bytes, only
the first len bytes will be retained in the file.
For example, the following program retains only the first four bytes of the
file:
import { open, close, ftruncate } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('temp.txt', 'r+', (err, fd) => {
if (err) throw err;
try {
ftruncate(fd, 4, (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
if (err) throw err;
}
});
If the file previously was shorter than len bytes, it is extended, and the
extended part is filled with null bytes ('\0'):
If len is negative then 0 will be used.
fs.futimes(fd, atime, mtime, callback)#
Change the file system timestamps of the object referenced by the supplied file
descriptor. See fs.utimes().
fs.lchmod(path, mode, callback)#
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)#
Changes the access and modification times of a file in the same way as
fs.utimes(), with the difference that if the path refers to a symbolic
link, then the link is not dereferenced: instead, the timestamps of the
symbolic link itself are changed.
No arguments other than a possible exception are given to the completion
callback.
fs.link(existingPath, newPath, callback)#
Creates a new link from the existingPath to the newPath. See the POSIX
link(2) documentation for more detail. No arguments other than a possible
exception are given to the completion callback.
fs.lstat(path[, options], callback)#
Retrieves the <fs.Stats> for the symbolic link referred to by the path.
The callback gets two arguments (err, stats) where stats is a <fs.Stats>
object. lstat() is identical to stat(), except that if path is a symbolic
link, then the link itself is stat-ed, not the file that it refers to.
See the POSIX lstat(2) documentation for more details.
fs.mkdir(path[, options], callback)#
Asynchronously creates a directory.
The callback is given a possible exception and, if recursive is true, the
first directory path created, (err[, path]).
path can still be undefined when recursive is true, if no directory was
created (for instance, if it was previously created).
The optional options argument can be an integer specifying mode (permission
and sticky bits), or an object with a mode property and a recursive
property indicating whether parent directories should be created. Calling
fs.mkdir() when path is a directory that exists results in an error only
when recursive is false. If recursive is false and the directory exists,
an EEXIST error occurs.
import { mkdir } from 'node:fs';
mkdir('./tmp/a/apple', { recursive: true }, (err) => {
if (err) throw err;
});
On Windows, using fs.mkdir() on the root directory even with recursion will
result in an error:
import { mkdir } from 'node:fs';
mkdir('/', { recursive: true }, (err) => {
});
See the POSIX mkdir(2) documentation for more details.
fs.mkdtemp(prefix[, options], callback)#
Creates a unique temporary directory.
Generates six random characters to be appended behind a required
prefix to create a unique temporary directory. Due to platform
inconsistencies, avoid trailing X characters in prefix. Some platforms,
notably the BSDs, can return more than six random characters, and replace
trailing X characters in prefix with random characters.
The created directory path is passed as a string to the callback's second
parameter.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use.
import { mkdtemp } from 'node:fs';
import { join } from 'node:path';
import { tmpdir } from 'node:os';
mkdtemp(join(tmpdir(), 'foo-'), (err, directory) => {
if (err) throw err;
console.log(directory);
});
The fs.mkdtemp() method will append the six randomly selected characters
directly to the prefix string. For instance, given a directory /tmp, if the
intention is to create a temporary directory within /tmp, the prefix
must end with a trailing platform-specific path separator
(require('node:path').sep).
import { tmpdir } from 'node:os';
import { mkdtemp } from 'node:fs';
const tmpDir = tmpdir();
mkdtemp(tmpDir, (err, directory) => {
if (err) throw err;
console.log(directory);
});
import { sep } from 'node:path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
if (err) throw err;
console.log(directory);
});
fs.open(path[, flags[, mode]], callback)#
Asynchronous file open. See the POSIX open(2) documentation for more details.
mode sets the file mode (permission and sticky bits), but only if the file was
created. On Windows, only the write permission can be manipulated; see
fs.chmod().
The callback gets two arguments (err, fd).
Some characters (< > : " / \ | ? *) are reserved under Windows as documented
by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains
a colon, Node.js will open a file system stream, as described by
this MSDN page.
Functions based on fs.open() exhibit this behavior as well:
fs.writeFile(), fs.readFile(), etc.
fs.opendir(path[, options], callback)#
Asynchronously open a directory. See the POSIX opendir(3) documentation for
more details.
Creates an <fs.Dir>, which contains all further functions for reading from
and cleaning up the directory.
The encoding option sets the encoding for the path while opening the
directory and subsequent read operations.
fs.read(fd, buffer, offset, length, position, callback)#
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
a non-negative 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.
The fs.read() method reads data from the file specified
by the file descriptor (fd).
The length argument indicates the maximum number
of bytes that Node.js
will attempt to read from the kernel.
However, the actual number of bytes read (bytesRead) can be lower
than the specified length for various reasons.
For example:
- If the file is shorter than the specified
length, bytesRead
will be set to the actual number of bytes read.
- If the file encounters EOF (End of File) before the buffer could
be filled, Node.js will read all available bytes until EOF is encountered,
and the
bytesRead parameter in the callback will indicate
the actual number of bytes read, which may be less than the specified length.
- If the file is on a slow network
filesystem
or encounters any other issue during reading,
bytesRead can be lower than the specified length.
Therefore, when using fs.read(), it's important to
check the bytesRead value to
determine how many bytes were actually read from the file.
Depending on your application
logic, you may need to handle cases where bytesRead
is lower than the specified length,
such as by wrapping the read call in a loop if you require
a minimum amount of bytes.
This behavior is similar to the POSIX preadv2 function.
fs.read(fd[, options], callback)#
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)#
Added in: v18.2.0, v16.17.0
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)#
Reads the contents of a directory. The callback gets two arguments (err, files)
where files is an array of the names of the files in the directory excluding
'.' and '..'.
See the POSIX readdir(3) documentation for more details.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the filenames passed to the callback. If the encoding is set to 'buffer',
the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes is set to true, the files array will contain
<fs.Dirent> objects.
fs.readFile(path[, options], callback)#
Asynchronously reads the entire contents of a file.
import { readFile } from 'node:fs';
readFile('/etc/passwd', (err, data) => {
if (err) throw err;
console.log(data);
});
The callback is passed two arguments (err, data), where data is the
contents of the file.
If no encoding is specified, then the raw buffer is returned.
If options is a string, then it specifies the encoding:
import { readFile } from 'node:fs';
readFile('/etc/passwd', 'utf8', callback);
When the path is a directory, the behavior of fs.readFile() and
fs.readFileSync() is platform-specific. On macOS, Linux, and Windows, an
error will be returned. On FreeBSD, a representation of the directory's contents
will be returned.
import { readFile } from 'node:fs';
readFile('<directory>', (err, data) => {
});
readFile('<directory>', (err, data) => {
});
It is possible to abort an ongoing request using an AbortSignal. If a
request is aborted the callback is called with an AbortError:
import { readFile } from 'node:fs';
const controller = new AbortController();
const signal = controller.signal;
readFile(fileInfo[0].name, { signal }, (err, buf) => {
});
controller.abort();
The fs.readFile() function buffers the entire file. To minimize memory costs,
when possible prefer streaming via fs.createReadStream().
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.readFile performs.
File descriptors#
- Any specified file descriptor has to support reading.
- If a file descriptor is specified as the
path, it will not be closed
automatically.
- The reading will begin at the current position. For example, if the file
already had
'Hello World' and six bytes are read with the file descriptor,
the call to fs.readFile() with the same file descriptor, would give
'World', rather than 'Hello World'.
Performance Considerations#
The fs.readFile() method asynchronously reads the contents of a file into
memory one chunk at a time, allowing the event loop to turn between each chunk.
This allows the read operation to have less impact on other activity that may
be using the underlying libuv thread pool but means that it will take longer
to read a complete file into memory.
The additional read overhead can vary broadly on different systems and depends
on the type of file being read. If the file type is not a regular file (a pipe
for instance) and Node.js is unable to determine an actual file size, each read
operation will load on 64 KiB of data. For regular files, each read will process
512 KiB of data.
For applications that require as-fast-as-possible reading of file contents, it
is better to use fs.read() directly and for application code to manage
reading the full contents of the file itself.
The Node.js GitHub issue #25741 provides more information and a detailed
analysis on the performance of fs.readFile() for multiple file sizes in
different Node.js versions.
fs.readlink(path[, options], callback)#
Reads the contents of the symbolic link referred to by path. The callback gets
two arguments (err, linkString).
See the POSIX readlink(2) documentation for more details.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the link path passed to the callback. If the encoding is set to 'buffer',
the link path returned will be passed as a <Buffer> object.
fs.readv(fd, buffers[, position], callback)#
Read from a file specified by fd and write to an array of ArrayBufferViews
using readv().
position is the offset from the beginning of the file from where data
should be read. If typeof position !== 'number', the data will be read
from the current position.
The callback will be given three arguments: err, bytesRead, and
buffers. bytesRead is how many bytes were read from the file.
If this method is invoked as its util.promisify()ed version, it returns
a promise for an Object with bytesRead and buffers properties.
fs.realpath(path[, options], callback)#
Asynchronously computes the canonical pathname by resolving ., .., and
symbolic links.
A canonical pathname is not necessarily unique. Hard links and bind mounts can
expose a file system entity through many pathnames.
This function behaves like realpath(3), with some exceptions:
-
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.
A path that does not exist results in an ENOENT error.
error.path is the absolute file path.
fs.realpath.native(path[, options], callback)#
Asynchronous realpath(3).
The callback gets two arguments (err, resolvedPath).
Only paths that can be converted to UTF8 strings are supported.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the path passed to the callback. If the encoding is set to 'buffer',
the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc in order for this function to work. Glibc does not have
this restriction.
fs.rename(oldPath, newPath, callback)#
Asynchronously rename file at oldPath to the pathname provided
as newPath. In the case that newPath already exists, it will
be overwritten. If there is a directory at newPath, an error will
be raised instead. No arguments other than a possible exception are
given to the completion callback.
See also: rename(2).
import { rename } from 'node:fs';
rename('oldFile.txt', 'newFile.txt', (err) => {
if (err) throw err;
console.log('Rename complete!');
});
fs.rmdir(path[, options], callback)#
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>
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>
Asynchronously removes files and directories (modeled on the standard POSIX rm
utility). No arguments other than a possible exception are given to the
completion callback.
fs.stat(path[, options], callback)#
Asynchronous stat(2). The callback gets two arguments (err, stats) where
stats is an <fs.Stats> object.
In case of an error, the err.code will be one of Common System Errors.
fs.stat() follows symbolic links. Use fs.lstat() to look at the
links themselves.
Using fs.stat() to check for the existence of a file before calling
fs.open(), fs.readFile(), or fs.writeFile() is not recommended.
Instead, user code should open/read/write the file directly and handle the
error raised if the file is not available.
To check if a file exists without manipulating it afterwards, fs.access()
is recommended.
For example, given the following directory structure:
- txtDir
-- file.txt
- app.js
The next program will check for the stats of the given paths:
import { stat } from 'node:fs';
const pathsToCheck = ['./txtDir', './txtDir/file.txt'];
for (let i = 0; i < pathsToCheck.length; i++) {
stat(pathsToCheck[i], (err, stats) => {
console.log(stats.isDirectory());
console.log(stats);
});
}
The resulting output will resemble:
true
Stats {
dev: 16777220,
mode: 16877,
nlink: 3,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 14214262,
size: 96,
blocks: 0,
atimeMs: 1561174653071.963,
mtimeMs: 1561174614583.3518,
ctimeMs: 1561174626623.5366,
birthtimeMs: 1561174126937.2893,
atime: 2019-06-22T03:37:33.072Z,
mtime: 2019-06-22T03:36:54.583Z,
ctime: 2019-06-22T03:37:06.624Z,
birthtime: 2019-06-22T03:28:46.937Z
}
false
Stats {
dev: 16777220,
mode: 33188,
nlink: 1,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 14214074,
size: 8,
blocks: 8,
atimeMs: 1561174616618.8555,
mtimeMs: 1561174614584,
ctimeMs: 1561174614583.8145,
birthtimeMs: 1561174007710.7478,
atime: 2019-06-22T03:36:56.619Z,
mtime: 2019-06-22T03:36:54.584Z,
ctime: 2019-06-22T03:36:54.584Z,
birthtime: 2019-06-22T03:26:47.711Z
}
fs.statfs(path[, options], callback)#
Added in: v19.6.0, v18.15.0
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)#
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);
The above example creates a symbolic link mewtwo which points to mew in the
same directory:
$ tree .
.
├── mew
└── mewtwo -> ./mew
fs.truncate(path[, len], callback)#
Truncates the file. No arguments other than a possible exception are
given to the completion callback. A file descriptor can also be passed as the
first argument. In this case, fs.ftruncate() is called.
import { truncate } from 'node:fs';
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});const { truncate } = require('node:fs');
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});
Passing a file descriptor is deprecated and may result in an error being thrown
in the future.
See the POSIX truncate(2) documentation for more details.
fs.unlink(path, callback)#
Asynchronously removes a file or symbolic link. No arguments other than a
possible exception are given to the completion callback.
import { unlink } from 'node:fs';
unlink('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was deleted');
});
fs.unlink() will not work on a directory, empty or otherwise. To remove a
directory, use fs.rmdir().
See the POSIX unlink(2) documentation for more details.
fs.unwatchFile(filename[, listener])#
Added in: v0.1.31
Stop watching for changes on filename. If listener is specified, only that
particular listener is removed. Otherwise, all listeners are removed,
effectively stopping watching of filename.
Calling fs.unwatchFile() with a filename that is not being watched is a
no-op, not an error.
Using fs.watch() is more efficient than fs.watchFile() and
fs.unwatchFile(). fs.watch() should be used instead of fs.watchFile()
and fs.unwatchFile() when possible.
fs.utimes(path, atime, mtime, callback)#
Change the file system timestamps of the object referenced by path.
The atime and mtime arguments follow these rules:
- Values can be either numbers representing Unix epoch time in seconds,
Dates, or a numeric string like '123456789.0'.
- If the value can not be converted to a number, or is
NaN, Infinity, or
-Infinity, an Error will be thrown.
fs.watch(filename[, options][, listener])#
filename <string> | <Buffer> | <URL>
options <string> | <Object>
persistent <boolean> Indicates whether the process should continue to run
as long as files are being watched. Default: true.
recursive <boolean> Indicates whether all subdirectories should be
watched, or only the current directory. This applies when a directory is
specified, and only on supported platforms (See caveats). Default:
false.
encoding <string> Specifies the character encoding to be used for the
filename passed to the listener. Default: 'utf8'.
signal <AbortSignal> allows closing the watcher with an AbortSignal.
listener <Function> | <undefined> Default: undefined
- Returns: <fs.FSWatcher>
Watch for changes on filename, where filename is either a file or a
directory.
The second argument is optional. If options is provided as a string, it
specifies the encoding. Otherwise options should be passed as an object.
The listener callback gets two arguments (eventType, filename). eventType
is either 'rename' or 'change', and filename is the name of the file
which triggered the event.
On most platforms, 'rename' is emitted whenever a filename appears or
disappears in the directory.
The listener callback is attached to the 'change' event fired by
<fs.FSWatcher>, but it is not the same thing as the 'change' value of
eventType.
If a signal is passed, aborting the corresponding AbortController will close
the returned <fs.FSWatcher>.
Caveats#
The fs.watch API is not 100% consistent across platforms, and is
unavailable in some situations.
On Windows, no events will be emitted if the watched directory is moved or
renamed. An EPERM error is reported when the watched directory is deleted.
The fs.watch API does not provide any protection with respect
to malicious actions on the file system. For example, on Windows it is
implemented by monitoring changes in a directory versus specific files. This
allows substitution of a file and fs reporting changes on the new file
with the same filename.
Availability#
This feature depends on the underlying operating system providing a way
to be notified of file system changes.
- On Linux systems, this uses
inotify(7).
- On BSD systems, this uses
kqueue(2).
- On macOS, this uses
kqueue(2) for files and FSEvents for
directories.
- On SunOS systems (including Solaris and SmartOS), this uses
event ports.
- On Windows systems, this feature depends on
ReadDirectoryChangesW.
- On AIX systems, this feature depends on
AHAFS, which must be enabled.
- On IBM i systems, this feature is not supported.
If the underlying functionality is not available for some reason, then
fs.watch() will not be able to function and may throw an exception.
For example, watching files or directories can be unreliable, and in some
cases impossible, on network file systems (NFS, SMB, etc) or host file systems
when using virtualization software such as Vagrant or Docker.
It is still possible to use fs.watchFile(), which uses stat polling, but
this method is slower and less reliable.
Inodes#
On Linux and macOS systems, fs.watch() resolves the path to an inode and
watches the inode. If the watched path is deleted and recreated, it is assigned
a new inode. The watch will emit an event for the delete but will continue
watching the original inode. Events for the new inode will not be emitted.
This is expected behavior.
AIX files retain the same inode for the lifetime of a file. Saving and closing a
watched file on AIX will result in two notifications (one for adding new
content, and one for truncation).
Filename argument#
Providing filename argument in the callback is only supported on Linux,
macOS, Windows, and AIX. Even on supported platforms, filename is not always
guaranteed to be provided. Therefore, don't assume that filename argument is
always provided in the callback, and have some fallback logic if it is null.
import { watch } from 'node:fs';
watch('somedir', (eventType, filename) => {
console.log(`event type is: ${eventType}`);
if (filename) {
console.log(`filename provided: ${filename}`);
} else {
console.log('filename not provided');
}
});
fs.watchFile(filename[, options], listener)#
Watch for changes on filename. The callback listener will be called each
time the file is accessed.
The options argument may be omitted. If provided, it should be an object. The
options object may contain a boolean named persistent that indicates
whether the process should continue to run as long as files are being watched.
The options object may specify an interval property indicating how often the
target should be polled in milliseconds.
The listener gets two arguments the current stat object and the previous
stat object:
import { watchFile } from 'node:fs';
watchFile('message.text', (curr, prev) => {
console.log(`the current mtime is: ${curr.mtime}`);
console.log(`the previous mtime was: ${prev.mtime}`);
});
These stat objects are instances of fs.Stat. If the bigint option is true,
the numeric values in these objects are specified as BigInts.
To be notified when the file was modified, not just accessed, it is necessary
to compare curr.mtimeMs and prev.mtimeMs.
When an fs.watchFile operation results in an ENOENT error, it
will invoke the listener once, with all the fields zeroed (or, for dates, the
Unix Epoch). If the file is created later on, the listener will be called
again, with the latest stat objects. This is a change in functionality since
v0.10.
Using fs.watch() is more efficient than fs.watchFile and
fs.unwatchFile. fs.watch should be used instead of fs.watchFile and
fs.unwatchFile when possible.
When a file being watched by fs.watchFile() disappears and reappears,
then the contents of previous in the second callback event (the file's
reappearance) will be the same as the contents of previous in the first
callback event (its disappearance).
This happens when:
- the file is deleted, followed by a restore
- the file is renamed and then renamed a second time back to its original name
fs.write(fd, buffer, offset[, length[, position]], callback)#
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)#
Added in: v18.3.0, v16.17.0
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)#
Write string to the file specified by fd. If string is not a string,
an exception is thrown.
position refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number' the data will be written at
the current position. See pwrite(2).
encoding is the expected string encoding.
The callback will receive the arguments (err, written, string) where written
specifies how many bytes the passed string required to be written. Bytes
written is not necessarily the same as string characters written. See
Buffer.byteLength.
It is unsafe to use fs.write() multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream() is
recommended.
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
On Windows, if the file descriptor is connected to the console (e.g. fd == 1
or stdout) a string containing non-ASCII characters will not be rendered
properly by default, regardless of the encoding used.
It is possible to configure the console to render UTF-8 properly by changing the
active codepage with the chcp 65001 command. See the chcp docs for more
details.
fs.writeFile(file, data[, options], callback)#
When file is a filename, asynchronously writes data to the file, replacing the
file if it already exists. data can be a string or a buffer.
When file is a file descriptor, the behavior is similar to calling
fs.write() directly (which is recommended). See the notes below on using
a file descriptor.
The encoding option is ignored if data is a buffer.
The mode option only affects the newly created file. See fs.open()
for more details.
import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, (err) => {
if (err) throw err;
console.log('The file has been saved!');
});
If options is a string, then it specifies the encoding:
import { writeFile } from 'node:fs';
writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
It is unsafe to use fs.writeFile() multiple times on the same file without
waiting for the callback. For this scenario, fs.createWriteStream() is
recommended.
Similarly to fs.readFile - fs.writeFile is a convenience method that
performs multiple write calls internally to write the buffer passed to it.
For performance sensitive code consider using fs.createWriteStream().
It is possible to use an <AbortSignal> to cancel an fs.writeFile().
Cancelation is "best effort", and some amount of data is likely still
to be written.
import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, { signal }, (err) => {
});
controller.abort();
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.writeFile performs.
Using fs.writeFile() with file descriptors#
When file is a file descriptor, the behavior is almost identical to directly
calling fs.write() like:
import { write } from 'node:fs';
import { Buffer } from 'node:buffer';
write(fd, Buffer.from(data, options.encoding), callback);
The difference from directly calling fs.write() is that under some unusual
conditions, fs.write() might write only part of the buffer and need to be
retried to write the remaining data, whereas fs.writeFile() retries until
the data is entirely written (or an error occurs).
The implications of this are a common source of confusion. In
the file descriptor case, the file is not replaced! The data is not necessarily
written to the beginning of the file, and the file's original data may remain
before and/or after the newly written data.
For example, if fs.writeFile() is called twice in a row, first to write the
string 'Hello', then to write the string ', World', the file would contain
'Hello, World', and might contain some of the file's original data (depending
on the size of the original file, and the position of the file descriptor). If
a file name had been used instead of a descriptor, the file would be guaranteed
to contain only ', World'.
fs.writev(fd, buffers[, position], callback)#
Write an array of ArrayBufferViews to the file specified by fd using
writev().
position is the offset from the beginning of the file where this data
should be written. If typeof position !== 'number', the data will be written
at the current position.
The callback will be given three arguments: err, bytesWritten, and
buffers. bytesWritten is how many bytes were written from buffers.
If this method is util.promisify()ed, it returns a promise for an
Object with bytesWritten and buffers properties.
It is unsafe to use fs.writev() multiple times on the same file without
waiting for the callback. For this scenario, use fs.createWriteStream().
On Linux, positional writes don't work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.
Synchronous API#
The synchronous APIs perform all operations synchronously, blocking the
event loop until the operation completes or fails.
fs.accessSync(path[, mode])#
Synchronously tests a user's permissions for the file or directory specified
by path. The mode argument is an optional integer that specifies the
accessibility checks to be performed. mode should be either the value
fs.constants.F_OK or a mask consisting of the bitwise OR of any of
fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g.
fs.constants.W_OK | fs.constants.R_OK). Check File access constants for
possible values of mode.
If any of the accessibility checks fail, an Error will be thrown. Otherwise,
the method will return undefined.
import { accessSync, constants } from 'node:fs';
try {
accessSync('etc/passwd', constants.R_OK | constants.W_OK);
console.log('can read/write');
} catch (err) {
console.error('no access!');
}
fs.appendFileSync(path, data[, options])#
Synchronously append data to a file, creating the file if it does not yet
exist. data can be a string or a <Buffer>.
The mode option only affects the newly created file. See fs.open()
for more details.
import { appendFileSync } from 'node:fs';
try {
appendFileSync('message.txt', 'data to append');
console.log('The "data to append" was appended to file!');
} catch (err) {
}
If options is a string, then it specifies the encoding:
import { appendFileSync } from 'node:fs';
appendFileSync('message.txt', 'data to append', 'utf8');
The path may be specified as a numeric file descriptor that has been opened
for appending (using fs.open() or fs.openSync()). The file descriptor will
not be closed automatically.
import { openSync, closeSync, appendFileSync } from 'node:fs';
let fd;
try {
fd = openSync('message.txt', 'a');
appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
} finally {
if (fd !== undefined)
closeSync(fd);
}
fs.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)#
Added in: v0.1.21
Closes the file descriptor. Returns undefined.
Calling fs.closeSync() on any file descriptor (fd) that is currently in use
through any other fs operation may lead to undefined behavior.
See the POSIX close(2) documentation for more detail.
fs.copyFileSync(src, dest[, mode])#
Synchronously copies src to dest. By default, dest is overwritten if it
already exists. Returns undefined. Node.js makes no guarantees about the
atomicity of the copy operation. If an error occurs after the destination file
has been opened for writing, Node.js will attempt to remove the destination.
mode is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).
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';
copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
fs.cpSync(src, dest[, options])#
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
src <string> source path to copy.
dest <string> destination path to copy to.
- Returns: <boolean> Any non-
Promise value that is coercible
to boolean.
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.');
fs.fchmodSync(fd, mode)#
Added in: v0.4.7
Sets the permissions on the file. Returns undefined.
See the POSIX fchmod(2) documentation for more detail.
fs.fchownSync(fd, uid, gid)#
Added in: v0.4.7
Sets the owner of the file. Returns undefined.
See the POSIX fchown(2) documentation for more detail.
fs.fdatasyncSync(fd)#
Added in: v0.1.96
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2) documentation for details. Returns undefined.
fs.fstatSync(fd[, options])#
Retrieves the <fs.Stats> for the file descriptor.
See the POSIX fstat(2) documentation for more detail.
fs.fsyncSync(fd)#
Added in: v0.1.96
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2) documentation for more detail. Returns undefined.
fs.ftruncateSync(fd[, len])#
Added in: v0.8.6
Truncates the file descriptor. Returns undefined.
For detailed information, see the documentation of the asynchronous version of
this API: fs.ftruncate().
fs.futimesSync(fd, atime, mtime)#
Synchronous version of fs.futimes(). Returns undefined.
fs.lchmodSync(path, mode)#
Deprecated since: v0.4.7
Changes the permissions on a symbolic link. Returns undefined.
This method is only implemented on macOS.
See the POSIX lchmod(2) documentation for more detail.
fs.lchownSync(path, uid, gid)#
Set the owner for the path. Returns undefined.
See the POSIX lchown(2) documentation for more details.
fs.lutimesSync(path, atime, mtime)#
Added in: v14.5.0, v12.19.0
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])#
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])#
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]])#
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])#
Reads the contents of the directory.
See the POSIX readdir(3) documentation for more details.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the filenames returned. If the encoding is set to 'buffer',
the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes is set to true, the result will contain
<fs.Dirent> objects.
fs.readFileSync(path[, options])#
Returns the contents of the path.
For detailed information, see the documentation of the asynchronous version of
this API: fs.readFile().
If the encoding option is specified then this function returns a
string. Otherwise it returns a buffer.
Similar to fs.readFile(), when the path is a directory, the behavior of
fs.readFileSync() is platform-specific.
import { readFileSync } from 'node:fs';
readFileSync('<directory>');
readFileSync('<directory>');
fs.readlinkSync(path[, options])#
Returns the symbolic link's string value.
See the POSIX readlink(2) documentation for more details.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the link path returned. If the encoding is set to 'buffer',
the link path returned will be passed as a <Buffer> object.
fs.readSync(fd, buffer, offset, length[, position])#
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])#
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])#
Added in: v13.13.0, v12.17.0
For detailed information, see the documentation of the asynchronous version of
this API: fs.readv().
fs.realpathSync(path[, options])#
Returns the resolved pathname.
For detailed information, see the documentation of the asynchronous version of
this API: fs.realpath().
fs.realpathSync.native(path[, options])#
Added in: v9.2.0
Synchronous realpath(3).
Only paths that can be converted to UTF8 strings are supported.
The optional options argument can be a string specifying an encoding, or an
object with an encoding property specifying the character encoding to use for
the path returned. If the encoding is set to 'buffer',
the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc in order for this function to work. Glibc does not have
this restriction.
fs.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])#
Retrieves the <fs.Stats> for the path.
fs.statfsSync(path[, options])#
Added in: v19.6.0, v18.15.0
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])#
For detailed information, see the documentation of the asynchronous version of
this API: fs.symlink().
fs.truncateSync(path[, len])#
Added in: v0.8.6
Truncates the file. Returns undefined. A file descriptor can also be
passed as the first argument. In this case, fs.ftruncateSync() is called.
Passing a file descriptor is deprecated and may result in an error being thrown
in the future.
fs.unlinkSync(path)#
Synchronous unlink(2). Returns undefined.
fs.utimesSync(path, atime, mtime)#
For detailed information, see the documentation of the asynchronous version of
this API: fs.utimes().
fs.writeFileSync(file, data[, options])#
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]])#
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, buffer...).
fs.writeSync(fd, buffer[, options])#
Added in: v18.3.0, v16.17.0
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, buffer...).
fs.writeSync(fd, string[, position[, encoding]])#
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, string...).
fs.writevSync(fd, buffers[, position])#
Added in: v12.9.0
For detailed information, see the documentation of the asynchronous version of
this API: fs.writev().