Source Code: lib/fs.js
Source Code: lib/fs.js
The fs
module enables interacting with the file system in a way modeled on standard POSIX functions.
To use this module:
const fs = require('fs');
All file system operations have synchronous, callback, and promise-based forms.
The synchronous form blocks the Node.js event loop and further JavaScript execution until the operation is
complete. Exceptions are thrown immediately and can be handled using try…catch
, or can be allowed to
bubble up.
const fs = require('fs');
try {
fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
}
The callback form takes a completion callback function as its last argument and invokes the operation
asynchronously. The arguments passed to the completion callback depend on the method, but the first argument is
always reserved for an exception. If the operation is completed successfully, then the first argument is
null
or undefined
.
const fs = require('fs');
fs.unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
Promise-based operations return a Promise
that is resolved when the asynchronous operation is
complete.
const fs = require('fs/promises');
(async function(path) {
try {
await fs.unlink(path);
console.log(`successfully deleted ${path}`);
} catch (error) {
console.error('there was an error:', error.message);
}
})('/tmp/hello');
There is no guaranteed ordering when using either the callback or promise-based methods. For example, the following
is prone to error because the fs.stat()
operation might complete before the fs.rename()
operation:
fs.rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
To correctly order the operations, move the fs.stat()
call into the callback of the
fs.rename()
operation:
fs.rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
fs.stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
});
Or, use the promise-based API:
const fs = require('fs/promises');
(async function(from, to) {
try {
await fs.rename(from, to);
const stats = await fs.stat(to);
console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
console.error('there was an error:', error.message);
}
})('/tmp/hello', '/tmp/world');
Most fs
operations accept filepaths that may be specified in the form of a string, a Buffer
, or a
URL
object using the file:
protocol.
String form paths are interpreted as UTF-8 character sequences identifying the absolute or relative filename.
Relative paths will be resolved relative to the current working directory as determined by calling
process.cwd()
.
Example using an absolute path on POSIX:
const fs = require('fs');
fs.open('/open/some/file.txt', 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
Example using a relative path on POSIX (relative to process.cwd()
):
fs.open('file.txt', 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
Paths specified using a Buffer
are
useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems,
it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string
paths, Buffer
paths may be relative or absolute:
Example using an absolute path on POSIX:
fs.open(Buffer.from('/open/some/file.txt'), 'r', (err, fd) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
On Windows, Node.js follows the concept of per-drive working directory. This behavior can be observed when using a
drive path without a backslash. For example fs.readdirSync('C:\\')
can potentially return a different
result than fs.readdirSync('C:')
. For more information, see this
MSDN page.
Added in: v7.6.0
For most fs
module functions, the path
or filename
argument may be passed as
a WHATWG URL
object. Only URL
objects using the file:
protocol are supported.
const fs = require('fs');
const fileUrl = new URL('file:///tmp/hello');
fs.readFileSync(fileUrl);
file:
URLs are always absolute paths.
Using WHATWG URL
objects might introduce platform-specific behaviors.
On Windows, file:
URLs with a host name convert to UNC paths, while file:
URLs with drive
letters convert to local absolute paths. file:
URLs without a host name nor a drive letter will result
in a throw:
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
fs.readFileSync(new URL('file:///C:/tmp/hello'));
fs.readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
fs.readFileSync(new URL('file:///c/p/a/t/h/file'));
file:
URLs with drive letters must use :
as a separator just after the drive letter.
Using another separator will result in a throw.
On all other platforms, file:
URLs with a host name are unsupported and will result in a throw:
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
fs.readFileSync(new URL('file:///tmp/hello'));
A file:
URL having encoded slash characters will result in a throw on all platforms:
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
fs.readFileSync(new URL('file:///p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///p/a/t/h/%2f'));
On Windows, file:
URLs having encoded backslash will result in a throw:
fs.readFileSync(new URL('file:///C:/path/%5C'));
fs.readFileSync(new URL('file:///C:/path/%5c'));
On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the specific differences between operating systems and assigns all open files a numeric file descriptor.
The fs.open()
method is used to allocate a new file descriptor. Once allocated, the file descriptor
may be used to read data from, write data to, or request information about the file.
fs.open('/open/some/file.txt', 'r', (err, fd) => {
if (err) throw err;
fs.fstat(fd, (err, stat) => {
if (err) throw err;
fs.close(fd, (err) => {
if (err) throw err;
});
});
});
Most operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.
All file system APIs except fs.FSWatcher()
and those that are explicitly synchronous use libuv’s
threadpool, which can have surprising and negative performance implications for some applications. See the UV_THREADPOOL_SIZE
documentation for more information.
fs.Dir
#Added in: v12.12.0
A class representing a directory stream.
Created by fs.opendir()
, fs.opendirSync()
, or fsPromises.opendir()
.
const fs = require('fs');
async function print(path) {
const dir = await fs.promises.opendir(path);
for await (const dirent of dir) {
console.log(dirent.name);
}
}
print('./').catch(console.error);
dir.close()
#Added in: v12.12.0
Asynchronously close the directory’s underlying resource handle. Subsequent reads will result in errors.
A Promise
is returned that will be resolved after the resource has been closed.
dir.close(callback)
#Added in: v12.12.0
Asynchronously close the directory’s underlying resource handle. Subsequent reads will result in errors.
The callback
will be called after the resource handle has been closed.
dir.closeSync()
#Added in: v12.12.0
Synchronously close the directory’s underlying resource handle. Subsequent reads will result in errors.
dir.path
#Added in: v12.12.0
The read-only path of this directory as was provided to fs.opendir()
, fs.opendirSync()
, or fsPromises.opendir()
.
dir.read()
#Added in: v12.12.0
Asynchronously read the next directory entry via readdir(3)
as an fs.Dirent
.
After the read is completed, a Promise
is returned that will be resolved with an fs.Dirent
, or null
if there are no more directory entries
to read.
Directory entries returned by this function are in no particular order as provided by the operating system’s underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir.read(callback)
#Added in: v12.12.0
callback
err
dirent
<fs.Dirent>
|
Asynchronously read the next directory entry via readdir(3)
as an fs.Dirent
.
After the read is completed, the callback
will be called with an fs.Dirent
, or null
if there are no more directory entries
to read.
Directory entries returned by this function are in no particular order as provided by the operating system’s underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir.readSync()
#Added in: v12.12.0
Synchronously read the next directory entry via readdir(3)
as an fs.Dirent
.
If there are no more directory entries to read, null
will be returned.
Directory entries returned by this function are in no particular order as provided by the operating system’s underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir[Symbol.asyncIterator]()
#
Added in: v12.12.0
Asynchronously iterates over the directory via readdir(3)
until all entries have been
read.
Entries returned by the async iterator are always an fs.Dirent
. The
null
case from dir.read()
is handled internally.
See fs.Dir
for an example.
Directory entries returned by this iterator are in no particular order as provided by the operating system’s underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
fs.Dirent
#Added in: v10.10.0
A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by
reading from an fs.Dir
. The directory entry is a combination of the file
name and file type pairs.
Additionally, when fs.readdir()
or fs.readdirSync()
is called with the
withFileTypes
option set to true
, the resulting array is filled with
fs.Dirent
objects, rather than strings or Buffers
.
dirent.isBlockDevice()
#Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a block device.
dirent.isCharacterDevice()
#
Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a character device.
dirent.isDirectory()
#Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a file system directory.
dirent.isFIFO()
#Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a first-in-first-out (FIFO) pipe.
dirent.isFile()
#Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a regular file.
dirent.isSocket()
#Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a socket.
dirent.isSymbolicLink()
#Added in: v10.10.0
Returns true
if the fs.Dirent
object describes a symbolic link.
dirent.name
#Added in: v10.10.0
The file name that this fs.Dirent
object refers to. The type of this value is determined by the
options.encoding
passed to fs.readdir()
or fs.readdirSync()
.
fs.FSWatcher
#Added in: v0.5.8
A successful call to fs.watch()
method will
return a new fs.FSWatcher
object.
All fs.FSWatcher
objects emit a 'change'
event whenever a specific watched file is
modified.
'change'
#Added in: v0.5.8
eventType
filename
Emitted when something changes in a watched directory or file. See more details in fs.watch()
.
The filename
argument may not be provided depending on operating system support. If
filename
is provided, it will be provided as a Buffer
if fs.watch()
is called
with its encoding
option set to 'buffer'
, otherwise filename
will be a UTF-8
string.
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
}
});
'close'
#Added in: v10.0.0
Emitted when the watcher stops watching for changes. The closed fs.FSWatcher
object is no longer
usable in the event handler.
'error'
#Added in: v0.5.8
Emitted when an error occurs while watching the file. The errored fs.FSWatcher
object is no longer
usable in the event handler.
watcher.close()
#Added in: v0.5.8
Stop watching for changes on the given fs.FSWatcher
. Once stopped, the fs.FSWatcher
object is no longer usable.
watcher.ref()
#Added in: v14.3.0
When called, requests that the Node.js event loop not exit so long as the FSWatcher
is
active. Calling watcher.ref()
multiple times will have no effect.
By default, all FSWatcher
objects are "ref’ed", making it normally unnecessary to call
watcher.ref()
unless watcher.unref()
had been called previously.
watcher.unref()
#Added in: v14.3.0
When called, the active FSWatcher
object will not require the Node.js event loop to remain active. If
there is no other activity keeping the event loop running, the process may exit before the FSWatcher
object’s callback is invoked. Calling watcher.unref()
multiple times will have no effect.
fs.StatWatcher
#Added in: v14.3.0
A successful call to fs.watchFile()
method will return a new fs.StatWatcher
object.
watcher.ref()
#Added in: v14.3.0
When called, requests that the Node.js event loop not exit so long as the StatWatcher
is
active. Calling watcher.ref()
multiple times will have no effect.
By default, all StatWatcher
objects are "ref’ed", making it normally unnecessary to call
watcher.ref()
unless watcher.unref()
had been called previously.
watcher.unref()
#Added in: v14.3.0
When called, the active StatWatcher
object will not require the Node.js event loop to remain active.
If there is no other activity keeping the event loop running, the process may exit before the
StatWatcher
object’s callback is invoked. Calling watcher.unref()
multiple times will have
no effect.
fs.ReadStream
#Added in: v0.1.93
Instances of fs.ReadStream
are created and returned using the fs.createReadStream()
function.
'close'
#Added in: v0.1.93
Emitted when the fs.ReadStream
’s underlying file descriptor has been closed.
'open'
#Added in: v0.1.93
Emitted when the fs.ReadStream
’s file descriptor has been opened.
'ready'
#Added in: v9.11.0
Emitted when the fs.ReadStream
is ready to be used.
Fires immediately after 'open'
.
readStream.bytesRead
#Added in: v6.4.0
The number of bytes that have been read so far.
readStream.path
#Added in: v0.1.93
The path to the file the stream is reading from as specified in the first argument to
fs.createReadStream()
. If path
is passed as a string, then readStream.path
will be a string. If path
is passed as a Buffer
, then readStream.path
will be
a Buffer
.
readStream.pending
#Added in: v11.2.0, v10.16.0
This property is true
if the underlying file has not been opened yet, i.e. before the
'ready'
event is emitted.
fs.Stats
#A fs.Stats
object provides information about a file.
Objects returned from fs.stat()
, fs.lstat()
and fs.fstat()
and their synchronous counterparts are of this
type. If bigint
in the options
passed to those methods is true, the numeric values will be
bigint
instead of number
, and the object will contain additional nanosecond-precision
properties suffixed with Ns
.
Stats {
dev: 2114,
ino: 48064969,
mode: 33188,
nlink: 1,
uid: 85,
gid: 100,
rdev: 0,
size: 527,
blksize: 4096,
blocks: 8,
atimeMs:<PHONE_NUMBER>000.1,
mtimeMs:<PHONE_NUMBER>000.1,
ctimeMs:<PHONE_NUMBER>000.1,
birthtimeMs:<PHONE_NUMBER>000.1,
atime: Mon, 10 Oct 2011 23:24:11 GMT,
mtime: Mon, 10 Oct 2011 23:24:11 GMT,
ctime: Mon, 10 Oct 2011 23:24:11 GMT,
birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
bigint
version:
BigIntStats {
dev: 2114n,
ino: 48064969n,
mode: 33188n,
nlink: 1n,
uid: 85n,
gid: 100n,
rdev: 0n,
size: 527n,
blksize: 4096n,
blocks: 8n,
atimeMs:<PHONE_NUMBER>000n,
mtimeMs:<PHONE_NUMBER>000n,
ctimeMs:<PHONE_NUMBER>000n,
birthtimeMs:<PHONE_NUMBER>000n,
atimeNs:<PHONE_NUMBER>000000000n,
mtimeNs:<PHONE_NUMBER>000000000n,
ctimeNs:<PHONE_NUMBER>000000000n,
birthtimeNs:<PHONE_NUMBER>000000000n,
atime: Mon, 10 Oct 2011 23:24:11 GMT,
mtime: Mon, 10 Oct 2011 23:24:11 GMT,
ctime: Mon, 10 Oct 2011 23:24:11 GMT,
birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
stats.isBlockDevice()
#Added in: v0.1.10
Returns true
if the fs.Stats
object describes a block device.
stats.isCharacterDevice()
#
Added in: v0.1.10
Returns true
if the fs.Stats
object describes a character device.
stats.isDirectory()
#Added in: v0.1.10
Returns true
if the fs.Stats
object describes a file system directory.
If the fs.Stats
object was obtained from fs.lstat()
, this method will always return
false
. This is because fs.lstat()
returns
information about a symbolic link itself and not the path it resolves to.
stats.isFIFO()
#Added in: v0.1.10
Returns true
if the fs.Stats
object describes a first-in-first-out (FIFO) pipe.
stats.isFile()
#Added in: v0.1.10
Returns true
if the fs.Stats
object describes a regular file.
stats.isSocket()
#Added in: v0.1.10
Returns true
if the fs.Stats
object describes a socket.
stats.isSymbolicLink()
#Added in: v0.1.10
Returns true
if the fs.Stats
object describes a symbolic link.
This method is only valid when using fs.lstat()
.
stats.dev
#The numeric identifier of the device containing the file.
stats.ino
#The file system specific "Inode" number for the file.
stats.mode
#A bit-field describing the file type and mode.
stats.nlink
#The number of hard-links that exist for the file.
stats.uid
#The numeric user identifier of the user that owns the file (POSIX).
stats.gid
#The numeric group identifier of the group that owns the file (POSIX).
stats.rdev
#A numeric device identifier if the file represents a device.
stats.size
#The size of the file in bytes.
stats.blksize
#The file system block size for i/o operations.
stats.blocks
#The number of blocks allocated for this file.
stats.atimeMs
#Added in: v8.1.0
The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.
stats.mtimeMs
#Added in: v8.1.0
The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.
stats.ctimeMs
#Added in: v8.1.0
The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.
stats.birthtimeMs
#Added in: v8.1.0
The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.
stats.atimeNs
#Added in: v12.10.0
Only present when bigint: true
is passed into the method that generates the object. The timestamp
indicating the last time this file was accessed expressed in nanoseconds since the POSIX Epoch.
stats.mtimeNs
#Added in: v12.10.0
Only present when bigint: true
is passed into the method that generates the object. The timestamp
indicating the last time this file was modified expressed in nanoseconds since the POSIX Epoch.
stats.ctimeNs
#Added in: v12.10.0
Only present when bigint: true
is passed into the method that generates the object. The timestamp
indicating the last time the file status was changed expressed in nanoseconds since the POSIX Epoch.
stats.birthtimeNs
#Added in: v12.10.0
Only present when bigint: true
is passed into the method that generates the object. The timestamp
indicating the creation time of this file expressed in nanoseconds since the POSIX Epoch.
stats.atime
#Added in: v0.11.13
The timestamp indicating the last time this file was accessed.
stats.mtime
#Added in: v0.11.13
The timestamp indicating the last time this file was modified.
stats.ctime
#Added in: v0.11.13
The timestamp indicating the last time the file status was changed.
stats.birthtime
#Added in: v0.11.13
The timestamp indicating the creation time of this file.
The atimeMs
, mtimeMs
, ctimeMs
, birthtimeMs
properties are
numeric values that hold the corresponding times in milliseconds. Their precision is platform specific. When
bigint: true
is passed into the method that generates the object, the properties will be bigints, otherwise they will be numbers.
The atimeNs
, mtimeNs
, ctimeNs
, birthtimeNs
properties are bigints that hold the corresponding times in nanoseconds. They
are only present when bigint: true
is passed into the method that generates the object. Their precision
is platform specific.
atime
, mtime
, ctime
, and birthtime
are Date
object alternate representations of the various times. The Date
and number values are not connected.
Assigning a new number value, or mutating the Date
value, will not be reflected in the corresponding
alternate representation.
The times in the stat object have the following semantics:
atime
"Access Time": Time when file data last accessed. Changed by the mknod(2)
, utimes(2)
, and read(2)
system calls.mtime
"Modified Time": Time when file data last modified. Changed by the mknod(2)
, utimes(2)
, and write(2)
system calls.ctime
"Change Time": Time when file status was last changed (inode data modification). Changed by
the chmod(2)
, chown(2)
, link(2)
, mknod(2)
, rename(2)
, unlink(2)
, utimes(2)
, read(2)
, and write(2)
system calls.birthtime
"Birth Time": Time of file creation. Set once when the file is created. On filesystems
where birthtime is not available, this field may instead hold either the ctime
or
1970-01-01T00:00Z
(ie, Unix epoch timestamp 0
). This value may be greater than
atime
or mtime
in this case. On Darwin and other FreeBSD variants, also set if the
atime
is explicitly set to an earlier value than the current birthtime
using the utimes(2)
system call.
Prior to Node.js 0.12, the ctime
held the birthtime
on Windows systems. As of 0.12,
ctime
is not "creation time", and on Unix systems, it never was.
fs.WriteStream
#Added in: v0.1.93
Instances of fs.WriteStream
are created and returned using the fs.createWriteStream()
function.
'close'
#Added in: v0.1.93
Emitted when the WriteStream
’s underlying file descriptor has been closed.
'open'
#Added in: v0.1.93
Emitted when the WriteStream
’s file is opened.
'ready'
#Added in: v9.11.0
Emitted when the fs.WriteStream
is ready to be used.
Fires immediately after 'open'
.
writeStream.bytesWritten
#
Added in: v0.4.7
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.path
#Added in: v0.1.93
The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream()
. If path
is
passed as a string, then writeStream.path
will be a string. If path
is passed as a
Buffer
, then writeStream.path
will be a Buffer
.
writeStream.pending
#Added in: v11.2.0
This property is true
if the underlying file has not been opened yet, i.e. before the
'ready'
event is emitted.
fs.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. Check File access constants for possible values of mode
. It is
possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK |
fs.constants.R_OK
).
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.
const file = 'package.json';
fs.access(file, fs.constants.F_OK, (err) => {
console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});
fs.access(file, fs.constants.R_OK, (err) => {
console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});
fs.access(file, fs.constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});
fs.access(file, fs.constants.F_OK | fs.constants.W_OK, (err) => {
if (err) {
console.error(
`${file} ${err.code === 'ENOENT' ? 'does not exist' : 'is read-only'}`);
} else {
console.log(`${file} exists, and it is 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)
fs.access('myfile', (err) => {
if (!err) {
console.error('myfile already exists');
return;
}
fs.open('myfile', 'wx', (err, fd) => {
if (err) throw err;
writeMyData(fd);
});
});
write (RECOMMENDED)
fs.open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
writeMyData(fd);
});
read (NOT RECOMMENDED)
fs.access('myfile', (err) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
fs.open('myfile', 'r', (err, fd) => {
if (err) throw err;
readMyData(fd);
});
});
read (RECOMMENDED)
fs.open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
readMyData(fd);
});
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.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. Check File access constants for possible values of mode
. It is
possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK |
fs.constants.R_OK
).
If any of the accessibility checks fail, an Error
will be thrown. Otherwise, the method will return
undefined
.
try {
fs.accessSync('etc/passwd', fs.constants.R_OK | fs.constants.W_OK);
console.log('can read/write');
} catch (err) {
console.error('no access!');
}
fs.appendFile(path, data[, options], callback)
#path
data
options
|
encoding
'utf8'
mode
0o666
flag
flags
.
Default: 'a'
.
callback
Asynchronously append data to a file, creating the file if it does not yet exist. data
can be a string
or a Buffer
.
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:
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.
fs.open('message.txt', 'a', (err, fd) => {
if (err) throw err;
fs.appendFile(fd, 'data to append', 'utf8', (err) => {
fs.close(fd, (err) => {
if (err) throw err;
});
if (err) throw err;
});
});
fs.appendFileSync(path, data[, options])
#path
data
options
|
encoding
'utf8'
mode
0o666
flag
flags
.
Default: 'a'
.
Synchronously append data to a file, creating the file if it does not yet exist. data
can be a string
or a Buffer
.
try {
fs.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:
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.
let fd;
try {
fd = fs.openSync('message.txt', 'a');
fs.appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
} finally {
if (fd !== undefined)
fs.closeSync(fd);
}
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 also: chmod(2)
.
fs.chmod('my_file.txt', 0o775, (err) => {
if (err) throw err;
console.log('The permissions for file "my_file.txt" have been changed!');
});
The mode
argument used in both the fs.chmod()
and fs.chmodSync()
methods is
a numeric bitmask created using a logical OR of the following constants:
| Constant | Octal | Description |
|---|---|---|
fs.constants.S_IRUSR |
0o400 |
read by owner |
fs.constants.S_IWUSR |
0o200 |
write by owner |
fs.constants.S_IXUSR |
0o100 |
execute/search by owner |
fs.constants.S_IRGRP |
0o40 |
read by group |
fs.constants.S_IWGRP |
0o20 |
write by group |
fs.constants.S_IXGRP |
0o10 |
execute/search by group |
fs.constants.S_IROTH |
0o4 |
read by others |
fs.constants.S_IWOTH |
0o2 |
write by others |
fs.constants.S_IXOTH |
0o1 |
execute/search by others |
An easier method of constructing the mode
is to use a sequence of three octal digits
(e.g. 765
). The left-most digit (7
in the example), specifies the permissions for the file
owner. The middle digit (6
in the example), specifies permissions for the group. The right-most digit
(5
in the example), specifies the permissions for others.
| Number | Description |
|---|---|
7 |
read, write, and execute |
6 |
read and write |
5 |
read and execute |
4 |
read only |
3 |
write and execute |
2 |
write only |
1 |
execute only |
0 |
no permission |
For example, the octal value 0o765
means:
When using raw numbers where file modes are expected, any value larger than 0o777
may result in
platform-specific behaviors that are not supported to work consistently. Therefore constants like
S_ISVTX
, S_ISGID
or S_ISUID
are not exposed in fs.constants
.
Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner or others is not implemented.
fs.chmodSync(path, mode)
#For detailed information, see the documentation of the asynchronous version of this API: fs.chmod()
.
See also: chmod(2)
.
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 also: chown(2)
.
fs.chownSync(path, uid, gid)
#Synchronously changes owner and group of a file. Returns undefined
. This is the synchronous version of
fs.chown()
.
See also: chown(2)
.
fs.close(fd, callback)
#Asynchronous close(2)
. 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.
fs.closeSync(fd)
#Added in: v0.1.21
Synchronous close(2)
. 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.
fs.constants
#Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS constants.
fs.copyFile(src, dest[, mode], callback)
#src
dest
mode
0
.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.
const fs = require(‘fs’); const { COPYFILE_EXCL } = fs.constants;
function callback(err) { if (err) throw err; console.log(‘source.txt was copied to destination.txt’); }
fs.copyFile(‘source.txt’, ‘destination.txt’, callback);
fs.copyFile(‘source.txt’, ‘destination.txt’, COPYFILE_EXCL, callback);
fs.copyFileSync(src, dest[, mode])
#src
dest
mode
0
.Synchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. Returns undefined
. Node.js makes no guarantees about the atomicity of the copy
operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove
the destination.
mode
is an optional integer that specifies the behavior of the copy operation. It is possible to
create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL |
fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
: The copy operation will fail if dest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink.
If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write
reflink. If the platform does not support copy-on-write, then the operation will fail.
const fs = require(‘fs’); const { COPYFILE_EXCL } = fs.constants;
fs.copyFileSync(‘source.txt’, ‘destination.txt’); console.log(‘source.txt was copied to destination.txt’);
fs.copyFileSync(‘source.txt’, ‘destination.txt’, COPYFILE_EXCL);
fs.createReadStream(path[, options])
#path
options
flags
flags
.
Default: 'r'
.
encoding
null
fd
null
mode
0o666
autoClose
true
emitClose
true
start
end
Infinity
highWaterMark
64 * 1024
fs
|
null
Unlike the 16 kb default highWaterMark
for a readable stream, the stream returned by this method has a
default highWaterMark
of 64 kb.
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 fd
s 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, like most
Readable
streams. 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, overrides for open
, read
, and close
are required.
const fs = require('fs');
const stream = fs.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:
fs.createReadStream('sample.txt', { start: 90, end: 99 });
If options
is a string, then it specifies the encoding.
fs.createWriteStream(path[, options])
#path
options
flags
flags
.
Default: 'w'
.
encoding
'utf8'
fd
null
mode
0o666
autoClose
true
emitClose
true
start
fs
|
null
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, like most
Writable
streams. 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
open
, close
, and at least one of write
and writev
are required.
Like ReadStream
, if fd
is specified, 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 fd
s 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 given path exists by checking with the file system. Then call the callback
argument with either true or false:
fs.exists('/etc/passwd', (exists) => {
console.log(exists ? 'it\'s there' : '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()
.
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)
fs.exists('myfile', (exists) => {
if (exists) {
console.error('myfile already exists');
} else {
fs.open('myfile', 'wx', (err, fd) => {
if (err) throw err;
writeMyData(fd);
});
}
});
write (RECOMMENDED)
fs.open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
writeMyData(fd);
});
read (NOT RECOMMENDED)
fs.exists('myfile', (exists) => {
if (exists) {
fs.open('myfile', 'r', (err, fd) => {
if (err) throw err;
readMyData(fd);
});
} else {
console.error('myfile does not exist');
}
});
read (RECOMMENDED)
fs.open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
readMyData(fd);
});
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.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.
if (fs.existsSync('/etc/passwd')) {
console.log('The path exists.');
}
fs.fchmod(fd, mode, callback)
#Asynchronous fchmod(2)
. No arguments
other than a possible exception are given to the completion callback.
fs.fchmodSync(fd, mode)
#Added in: v0.4.7
Synchronous fchmod(2)
. Returns
undefined
.
fs.fchown(fd, uid, gid, callback)
#Asynchronous fchown(2)
. No arguments
other than a possible exception are given to the completion callback.
fs.fchownSync(fd, uid, gid)
#
Added in: v0.4.7
Synchronous fchown(2)
. Returns
undefined
.
fs.fdatasync(fd, callback)
#
Asynchronous fdatasync(2)
. No
arguments other than a possible exception are given to the completion callback.
fs.fdatasyncSync(fd)
#Added in: v0.1.96
Synchronous fdatasync(2)
. Returns
undefined
.
fs.fstat(fd[, options], callback)
#fd
options
bigint
fs.Stats
object
should be bigint
. Default: false
.callback
err
stats
<fs.Stats>
Asynchronous fstat(2)
. The callback
gets two arguments (err, stats)
where stats
is an fs.Stats
object. fstat()
is identical to stat()
,
except that the file to be stat-ed is specified by the file descriptor fd
.
fs.fstatSync(fd[, options])
#
fd
options
bigint
fs.Stats
object
should be bigint
. Default: false
.Synchronous fstat(2)
.
fs.fsync(fd, callback)
#Asynchronous fsync(2)
. No arguments
other than a possible exception are given to the completion callback.
fs.fsyncSync(fd)
#Added in: v0.1.96
Synchronous fsync(2)
. Returns
undefined
.
fs.ftruncate(fd[, len], callback)
#Asynchronous ftruncate(2)
. No
arguments other than a possible exception are given to the completion callback.
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:
console.log(fs.readFileSync('temp.txt', 'utf8'));
const fd = fs.openSync('temp.txt', 'r+');
fs.ftruncate(fd, 4, (err) => {
assert.ifError(err);
console.log(fs.readFileSync('temp.txt', 'utf8'));
});
If the file previously was shorter than len
bytes, it is extended, and the extended part is filled
with null bytes ('\0'
):
console.log(fs.readFileSync('temp.txt', 'utf8'));
const fd = fs.openSync('temp.txt', 'r+');
fs.ftruncate(fd, 10, (err) => {
assert.ifError(err);
console.log(fs.readFileSync('temp.txt'));
});
The last three bytes are null bytes ('\0'
), to compensate the over-truncation.
fs.ftruncateSync(fd[, len])
#
Added in: v0.8.6
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of this API: fs.ftruncate()
.
fs.futimes(fd, atime, mtime, callback)
#Change the file system timestamps of the object referenced by the supplied file descriptor. See fs.utimes()
.
This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS
.
fs.futimesSync(fd, atime, mtime)
#Synchronous version of fs.futimes()
. Returns
undefined
.
fs.lchmod(path, mode, callback)
#Asynchronous lchmod(2)
.
No arguments other than a possible exception are given to the completion callback.
Only available on macOS.
fs.lchmodSync(path, mode)
#Deprecated since: v0.4.7
Synchronous lchmod(2)
.
Returns undefined
.
fs.lchown(path, uid, gid, callback)
#Asynchronous lchown(2)
. No arguments
other than a possible exception are given to the completion callback.
fs.lchownSync(path, uid, gid)
#Synchronous lchown(2)
. Returns
undefined
.
fs.lutimes(path, atime, mtime, callback)
#Added in: v14.5.0, v12.19.0
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.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.link(existingPath, newPath, callback)
#Asynchronous link(2)
. No arguments
other than a possible exception are given to the completion callback.
fs.linkSync(existingPath, newPath)
#Synchronous link(2)
. Returns
undefined
.
fs.lstat(path[, options], callback)
#path
options
bigint
fs.Stats
object
should be bigint
. Default: false
.callback
err
stats
<fs.Stats>
Asynchronous lstat(2)
. 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.
fs.lstatSync(path[, options])
#path
options
bigint
fs.Stats
object
should be bigint
. Default: false
.throwIfNoEntry
undefined
. Default: true
.
Synchronous lstat(2)
.
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.
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.
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:
fs.mkdir('/', { recursive: true }, (err) => {
});
See also: mkdir(2)
.
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 also: mkdir(2)
.
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.
fs.mkdtemp(path.join(os.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('path').sep
).
const tmpDir = os.tmpdir();
fs.mkdtemp(tmpDir, (err, directory) => {
if (err) throw err;
console.log(directory);
});
const { sep } = require('path');
fs.mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
if (err) throw err;
console.log(directory);
});
fs.mkdtempSync(prefix[, options])
#Added in: v5.10.0
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.open(path[, flags[, mode]], callback)
#path
flags
flags
. Default:
'r'
.
mode
0o666
(readable and writable)callback
Asynchronous file open. See open(2)
.
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)
#path
options
callback
err
dir
<fs.Dir>Asynchronously 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.opendirSync(path[, options])
#path
options
Synchronously open a directory. See opendir(3)
.
Creates an fs.Dir
, which contains all further functions for reading
from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the directory and
subsequent read operations.
fs.openSync(path[, flags, mode])
#path
flags
'r'
. See support of file system
flags
.mode
0o666
Returns an integer representing the file descriptor.
For detailed information, see the documentation of the asynchronous version of this API: fs.open()
.
fs.read(fd, buffer, offset, length, position,
callback)
#Read data from the file specified by fd
.
buffer
is the buffer that the data (read from the fd) will be written to.
offset
is the offset in the buffer to start writing at.
length
is an integer specifying the number of bytes to read.
position
is an argument specifying where to begin reading from in the file. If position
is null
, data will be read from the current file position, and the file position will be updated. If
position
is an integer, the file position will remain unchanged.
The callback is given the three arguments, (err, bytesRead, buffer)
.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
If this method is invoked as its util.promisify()
ed
version, it returns a Promise
for an Object
with bytesRead
and
buffer
properties.
fs.read(fd, [options,] callback)
#Similar to the above fs.read
function, this version takes an optional options
object. If
no options
object is specified, it will default with the above values.
fs.readdir(path[, options], callback)
#path
options
callback
err
files
<string[]>
| <Buffer[]>
| <fs.Dirent[]>
Asynchronous readdir(3)
. 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 '..'
.
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.readdirSync(path[, options])
#path
options
Synchronous readdir(3)
.
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.readFile(path[, options], callback)
#path
options
|
encoding
null
flag
flags
.
Default: 'r'
.
signal
callback
Asynchronously reads the entire contents of a file.
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:
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.
fs.readFile('<directory>', (err, data) => {
});
fs.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
:
const controller = new AbortController();
const signal = controller.signal;
fs.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.
path
, it will not be closed automatically.'Hello World
’
and six bytes are read with the file descriptor, the call to fs.readFile()
with the same file
descriptor, would give 'World'
, rather than 'Hello World'
.The fs.readFile()
method asynchronously reads the contents of a file into memory one chunk at a time,
allowing the event loop to turn between each chunk. This allows the read operation to have less impact on other
activity that may be using the underlying libuv thread pool but means that it will take longer to read a complete
file into memory.
The additional read overhead can vary broadly on different systems and depends on the type of file being read. If the file type is not a regular file (a pipe for instance) and Node.js is unable to determine an actual file size, each read operation will load on 64kb of data. For regular files, each read will process 512kb 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.readFileSync(path[, options])
#path
options
|
encoding
null
flag
flags
.
Default: 'r'
.
Returns the contents of the path
.
For detailed information, see the documentation of the asynchronous version of this API: fs.readFile()
.
If the encoding
option is specified then this function returns a string. Otherwise it returns a
buffer.
Similar to fs.readFile()
, when the path is a
directory, the behavior of fs.readFileSync()
is platform-specific.
fs.readFileSync('<directory>');
fs.readFileSync('<directory>');
fs.readlink(path[, options], callback)
#Asynchronous readlink(2)
. The
callback gets two arguments (err, linkString)
.
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.readlinkSync(path[, options])
#Synchronous readlink(2)
. Returns
the symbolic link’s string value.
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.readv(fd, buffers[, position], callback)
#Added in: v13.13.0, v12.17.0
fd
buffers
<ArrayBufferView[]>position
callback
err
bytesRead
buffers
<ArrayBufferView[]>Read from a file specified by fd
and write to an array of ArrayBufferView
s 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.readvSync(fd, buffers[, position])
#Added in: v13.13.0, v12.17.0
fd
buffers
<ArrayBufferView[]>position
For detailed information, see the documentation of the asynchronous version of this API: fs.readv()
.
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.
fs.realpath.native(path[, options], callback)
#Added in: v9.2.0
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.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.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)
.
fs.rename('oldFile.txt', 'newFile.txt', (err) => {
if (err) throw err;
console.log('Rename complete!');
});
fs.renameSync(oldPath, newPath)
#Synchronous rename(2)
. Returns
undefined
.
fs.rmdir(path[, options], callback)
#path
options
maxRetries
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
true
, perform a recursive directory removal. In recursive mode, errors are not reported
if path
does not exist, and operations are retried on failure. Default:
false
.
retryDelay
recursive
option is not true
. Default: 100
.
callback
Asynchronous rmdir(2)
. No arguments
other than a possible exception are given to the completion callback.
Using fs.rmdir()
on a file (not a directory) results in an ENOENT
error on Windows and an
ENOTDIR
error on POSIX.
Setting recursive
to true
results in behavior similar to the Unix command rm
-rf
: an error will not be raised for paths that do not exist, and paths that represent files will be
deleted. The permissive behavior of the recursive
option is deprecated, ENOTDIR
and
ENOENT
will be thrown in the future.
fs.rmdirSync(path[, options])
#path
options
maxRetries
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
true
, perform a recursive directory removal. In recursive mode, errors are not reported
if path
does not exist, and operations are retried on failure. Default:
false
.
retryDelay
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.
Setting recursive
to true
results in behavior similar to the Unix command rm
-rf
: an error will not be raised for paths that do not exist, and paths that represent files will be
deleted. The permissive behavior of the recursive
option is deprecated, ENOTDIR
and
ENOENT
will be thrown in the future.
fs.rm(path[, options], callback)
#Added in: v14.14.0
path
options
force
true
, exceptions will be ignored if path
does not exist.
Default: false
.
maxRetries
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
true
, perform a recursive removal. In recursive mode operations are retried on failure.
Default: false
.
retryDelay
recursive
option is not true
. Default: 100
.
callback
Asynchronously removes files and directories (modeled on the standard POSIX rm
utility). No arguments
other than a possible exception are given to the completion callback.
fs.rmSync(path[, options])
#Added in: v14.14.0
path
options
force
true
, exceptions will be ignored if path
does not exist.
Default: false
.
maxRetries
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
true
, perform a recursive directory removal. In recursive mode operations are retried on
failure. Default: false
.retryDelay
recursive
option is not true
. Default: 100
.
Synchronously removes files and directories (modeled on the standard POSIX rm
utility). Returns
undefined
.
fs.stat(path[, options], callback)
#path
options
bigint
fs.Stats
object
should be bigint
. Default: false
.callback
err
stats
<fs.Stats>
Asynchronous stat(2)
. The callback gets
two arguments (err, stats)
where stats
is an fs.Stats
object.
In case of an error, the err.code
will be one of Common System
Errors.
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:
const fs = require('fs');
const pathsToCheck = ['./txtDir', './txtDir/file.txt'];
for (let i = 0; i < pathsToCheck.length; i++) {
fs.stat(pathsToCheck[i], function(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:<PHONE_NUMBER>071.963,
mtimeMs:<PHONE_NUMBER>583.3518,
ctimeMs:<PHONE_NUMBER>623.5366,
birthtimeMs:<PHONE_NUMBER>937.2893,
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:<PHONE_NUMBER>618.8555,
mtimeMs:<PHONE_NUMBER>584,
ctimeMs:<PHONE_NUMBER>583.8145,
birthtimeMs:<PHONE_NUMBER>710.7478,
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.statSync(path[, options])
#
path
options
bigint
fs.Stats
object
should be bigint
. Default: false
.throwIfNoEntry
undefined
. Default: true
.
Synchronous stat(2)
.
fs.symlink(target, path[, type], callback)
#Asynchronous symlink(2)
which
creates the link called path
pointing to target
. No arguments other than a possible
exception are given to the completion callback.
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 set,
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.
Relative targets are relative to the link’s parent directory.
fs.symlink('./mew', './example/mewtwo', callback);
The above example creates a symbolic link mewtwo
in the example
which points to
mew
in the same directory:
$ tree example/
example/
├── mew
└── mewtwo -> ./mew
fs.symlinkSync(target, path[, type])
#Returns undefined
.
For detailed information, see the documentation of the asynchronous version of this API: fs.symlink()
.
fs.truncate(path[, len], callback)
#Asynchronous truncate(2)
. 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.
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
fs.truncateSync(path[, len])
#
Added in: v0.8.6
Synchronous truncate(2)
. 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.unlink(path, callback)
#Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.
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 also: unlink(2)
.
fs.unlinkSync(path)
#Synchronous unlink(2)
. Returns
undefined
.
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:
Date
s, or a numeric string
like '123456789.0'
.NaN
, Infinity
or
-Infinity
, an Error
will be thrown.
fs.utimesSync(path, atime, mtime)
#Returns undefined
.
For detailed information, see the documentation of the asynchronous version of this API: fs.utimes()
.
fs.watch(filename[, options][, listener])
#filename
options
persistent
true
.
recursive
false
.
encoding
'utf8'
.
listener
undefined
Watch for changes on filename
, where filename
is either a file or a directory.
The second argument is optional. If options
is provided as a string, it specifies the
encoding
. Otherwise options
should be passed as an object.
The listener callback gets two arguments (eventType, filename)
. eventType
is either
'rename'
or 'change'
, and filename
is the name of the file which triggered
the event.
On most platforms, 'rename'
is emitted whenever a filename appears or disappears in the directory.
The listener callback is attached to the 'change'
event fired by fs.FSWatcher
, but it is not the same thing as the
'change'
value of eventType
.
The fs.watch
API is not 100% consistent across platforms, and is unavailable in some situations.
The recursive option is only supported on macOS and Windows. An ERR_FEATURE_UNAVAILABLE_ON_PLATFORM
exception will be thrown when the option is used on a platform that does not support it.
On Windows, no events will be emitted if the watched directory is moved or renamed. An EPERM
error is
reported when the watched directory is deleted.
This feature depends on the underlying operating system providing a way to be notified of filesystem changes.
inotify(7)
.kqueue(2)
.kqueue(2)
for files and
FSEvents
for
directories.
event ports
.ReadDirectoryChangesW
.
AHAFS
, which
must be enabled.If the underlying functionality is not available for some reason, then fs.watch()
will not be able to
function and may thrown an exception. For example, watching files or directories can be unreliable, and in some
cases impossible, on network file systems (NFS, SMB, etc) or host file systems when using virtualization software
such as Vagrant or Docker.
It is still possible to use fs.watchFile()
, which uses stat polling, but this method is slower and
less reliable.
On Linux and macOS systems, fs.watch()
resolves the path to an inode and watches the inode. If the watched path is deleted and
recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the
original inode. Events for the new inode will not be emitted. This is expected behavior.
AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).
Providing filename
argument in the callback is only supported on Linux, macOS, Windows, and AIX. Even
on supported platforms, filename
is not always guaranteed to be provided. Therefore, don’t assume that
filename
argument is always provided in the callback, and have some fallback logic if it is
null
.
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)
#filename
options
listener
current
<fs.Stats>
previous
<fs.Stats>
Watch for changes on filename
. The callback listener
will be called each time the file is
accessed.
The options
argument may be omitted. If provided, it should be an object. The options
object may contain a boolean named persistent
that indicates whether the process should continue to run
as long as files are being watched. The options
object may specify an interval
property
indicating how often the target should be polled in milliseconds.
The listener
gets two arguments the current stat object and the previous stat object:
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 BigInt
s.
To be notified when the file was modified, not just accessed, it is necessary to compare curr.mtime
and prev.mtime
.
When an fs.watchFile
operation results in an ENOENT
error, it will invoke the listener
once, with all the fields zeroed (or, for dates, the Unix Epoch). If the file is created later on, the listener will
be called again, with the latest stat objects. This is a change in functionality since v0.10.
Using fs.watch()
is more efficient than
fs.watchFile
and fs.unwatchFile
. fs.watch
should be used instead of
fs.watchFile
and fs.unwatchFile
when possible.
When a file being watched by fs.watchFile()
disappears and reappears, then the contents of
previous
in the second callback event (the file’s reappearance) will be the same as the contents of
previous
in the first callback event (its disappearance).
This happens when:
fs.write(fd, buffer[, offset[, length[, position]]],
callback)
#Write buffer
to the file specified by fd
. If buffer
is a normal object, it
must have an own toString
function property.
offset
determines the part of the buffer to be written, and length
is an integer
specifying the number of bytes to write.
position
refers to the offset from the beginning of the file where this data should be written. If
typeof position !== 'number'
, the data will be written at the current position. See pwrite(2)
.
The callback will be given three arguments (err, bytesWritten, buffer)
where bytesWritten
specifies how many bytes were written from buffer
.
If this method is invoked as its util.promisify()
ed
version, it returns a Promise
for an Object
with bytesWritten
and
buffer
properties.
It is unsafe to use fs.write()
multiple times on the same file without waiting for the callback. For
this scenario, fs.createWriteStream()
is
recommended.
On Linux, positional writes don’t work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
fs.write(fd, string[, position[, encoding]],
callback)
#Write string
to the file specified by fd
. If string
is not a string, or an
object with an own toString
function property, then an exception is thrown.
position
refers to the offset from the beginning of the file where this data should be written. If
typeof position !== 'number'
the data will be written at the current position. See pwrite(2)
.
encoding
is the expected string encoding.
The callback will receive the arguments (err, written, string)
where written
specifies
how many bytes the passed string required to be written. Bytes written is not necessarily the same as
string characters written. See Buffer.byteLength
.
It is unsafe to use fs.write()
multiple times on the same file without waiting for the callback. For
this scenario, fs.createWriteStream()
is
recommended.
On Linux, positional writes don’t work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
On Windows, if the file descriptor is connected to the console (e.g. fd == 1
or stdout
) a
string containing non-ASCII characters will not be rendered properly by default, regardless of the encoding used. It
is possible to configure the console to render UTF-8 properly by changing the active codepage with the chcp
65001
command. See the chcp docs for more details.
fs.writeFile(file, data[, options], callback)
#file
data
options
|
encoding
'utf8'
mode
0o666
flag
flags
.
Default: 'w'
.
signal
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. If data
is a normal
object, it must have an own toString
function property.
const data = new Uint8Array(Buffer.from('Hello Node.js'));
fs.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:
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
fs.writeFile()
. Cancelation is "best effort", and some amount of data is likely still
to be written.
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
fs.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.
fs.writeFile()
with file descriptors#When file
is a file descriptor, the behavior is almost identical to directly calling
fs.write()
like:
fs.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.writeFileSync(file, data[, options])
#file
data
options
|
encoding
'utf8'
mode
0o666
flag
flags
.
Default: 'w'
.
Returns undefined
.
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, string[, position[, encoding]])
#For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, string...)
.
fs.writev(fd, buffers[, position], callback)
#Added in: v12.9.0
fd
buffers
<ArrayBufferView[]>position
callback
err
bytesWritten
buffers
<ArrayBufferView[]>Write an array of ArrayBufferView
s 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.
fs.writevSync(fd, buffers[, position])
#Added in: v12.9.0
fd
buffers
<ArrayBufferView[]>position
For detailed information, see the documentation of the asynchronous version of this API: fs.writev()
.
fs
Promises API#The fs.promises
API provides an alternative set of asynchronous file system methods that return
Promise
objects rather than using callbacks. The API is accessible via
require('fs').promises
or require('fs/promises')
.
FileHandle
#Added in: v10.0.0
A FileHandle
object is a wrapper for a numeric file descriptor. Instances of FileHandle
are distinct from numeric file descriptors in that they provide an object oriented API for working with files.
If a FileHandle
is not closed using the filehandle.close()
method, it might automatically
close the file descriptor and will emit a process warning, thereby helping to prevent memory leaks. Please do not
rely on this behavior because it is unreliable and the file may not be closed. Instead, always explicitly close
FileHandle
s. Node.js may change this behavior in the future.
Instances of the FileHandle
object are created internally by the fsPromises.open()
method.
Unlike the callback-based API (fs.fstat()
, fs.fchown()
, fs.fchmod()
, and so
on), a numeric file descriptor is not used by the promise-based API. Instead, the promise-based API uses the
FileHandle
class in order to help avoid accidental leaking of unclosed file descriptors after a
Promise
is resolved or rejected.
'close'
#Added in: v15.4.0
The 'close'
event is emitted when the FileHandle
and any of its underlying resources (a
file descriptor, for example) have been closed.
filehandle.appendFile(data, options)
#Added in: v10.0.0
Alias of filehandle.writeFile()
.
When operating on file handles, the mode cannot be changed from what it was set to with fsPromises.open()
. Therefore, this is equivalent to filehandle.writeFile()
.
filehandle.chmod(mode)
#Added in: v10.0.0
Modifies the permissions on the file. The Promise
is resolved with no arguments upon success.
filehandle.chown(uid, gid)
#
Added in: v10.0.0
Changes the ownership of the file then resolves the Promise
with no arguments upon success.
filehandle.close()
#Added in: v10.0.0
Promise
that will be resolved once the underlying file descriptor is closed, or will be
rejected if an error occurs while closing.Closes the file handle after waiting for any pending operation on the handle to complete.
const fsPromises = require('fs').promises;
async function openAndClose() {
let filehandle;
try {
filehandle = await fsPromises.open('thefile.txt', 'r');
} finally {
if (filehandle !== undefined)
await filehandle.close();
}
}
filehandle.datasync()
#Added in: v10.0.0
Asynchronous fdatasync(2)
. The
Promise
is resolved with no arguments upon success.
filehandle.fd
#Added in: v10.0.0
filehandle.read(buffer, offset, length, position)
#Added in: v10.0.0
Read data from the file.
buffer
is the buffer that the data will be written to.
offset
is the offset in the buffer to start writing at.
length
is an integer specifying the number of bytes to read.
position
is an argument specifying where to begin reading from in the file. If position
is null
, data will be read from the current file position, and the file position will be updated. If
position
is an integer, the file position will remain unchanged.
Following successful read, the Promise
is resolved with an object with a bytesRead
property specifying the number of bytes read, and a buffer
property that is a reference to the passed
in buffer
argument.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.read(options)
#Added in: v13.11.0, v12.17.0
filehandle.readFile(options)
#Added in: v10.0.0
Asynchronously reads the entire contents of a file.
The Promise
is resolved 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.
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
<ArrayBufferView[]>position
Read from a file and write to an array of ArrayBufferView
s
The Promise
is resolved with an object containing a bytesRead
property identifying the
number of bytes read, and a buffers
property containing a reference to the buffers
input.
position
is the offset from the beginning of the file where this data should be read from. If
typeof position !== 'number'
, the data will be read from the current position.
filehandle.stat([options])
#
options
bigint
fs.Stats
object
should be bigint
. Default: false
.Retrieves the fs.Stats
for the file.
filehandle.sync()
#Added in: v10.0.0
Asynchronous fsync(2)
. The
Promise
is resolved with no arguments upon success.
filehandle.truncate(len)
#Added in: v10.0.0
Truncates the file then resolves the Promise
with no arguments upon success.
If the file 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:
const fs = require('fs');
const fsPromises = fs.promises;
console.log(fs.readFileSync('temp.txt', 'utf8'));
async function doTruncate() {
let filehandle = null;
try {
filehandle = await fsPromises.open('temp.txt', 'r+');
await filehandle.truncate(4);
} finally {
if (filehandle) {
await filehandle.close();
}
}
console.log(fs.readFileSync('temp.txt', 'utf8'));
}
doTruncate().catch(console.error);
If the file previously was shorter than len
bytes, it is extended, and the extended part is filled
with null bytes ('\0'
):
const fs = require('fs');
const fsPromises = fs.promises;
console.log(fs.readFileSync('temp.txt', 'utf8'));
async function doTruncate() {
let filehandle = null;
try {
filehandle = await fsPromises.open('temp.txt', 'r+');
await filehandle.truncate(10);
} finally {
if (filehandle) {
await filehandle.close();
}
}
console.log(fs.readFileSync('temp.txt', 'utf8'));
}
doTruncate().catch(console.error);
The last three bytes are null bytes ('\0'
), to compensate the over-truncation.
filehandle.utimes(atime, mtime)
#Added in: v10.0.0
Change the file system timestamps of the object referenced by the FileHandle
then resolves the
Promise
with no arguments upon success.
This function does not work on AIX versions before 7.1, it will resolve the Promise
with an error
using code UV_ENOSYS
.
filehandle.write(buffer[, offset[, length[,
position]]])
#Write buffer
to the file.
The Promise
is resolved with an object containing a bytesWritten
property identifying the
number of bytes written, and a buffer
property containing a reference to the buffer
written.
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)
.
It is unsafe to use filehandle.write()
multiple times on the same file without waiting for the
Promise
to be resolved (or rejected). For this scenario, use fs.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(string[, position[, encoding]])
#Write string
to the file. If string
is not a string, or an object with an own
toString
function property, then an exception is thrown.
The Promise
is resolved with an object containing a bytesWritten
property identifying the
number of bytes written, and a buffer
property containing a reference to the string
written.
position
refers to the offset from the beginning of the file where this data should be written. If the
type of position
is not a number
the data will be written at the current position. See pwrite(2)
.
encoding
is the expected string encoding.
It is unsafe to use filehandle.write()
multiple times on the same file without waiting for the
Promise
to be resolved (or rejected). For this scenario, use fs.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, or an object with an own toString
function property. The Promise
is resolved with
no arguments upon success.
The encoding
option is ignored if data
is a buffer.
If options
is a string, then it specifies the encoding.
The FileHandle
has to support writing.
It is unsafe to use filehandle.writeFile()
multiple times on the same file without waiting for the
Promise
to be resolved (or rejected).
If one or more filehandle.write()
calls are made on a file handle and then a
filehandle.writeFile()
call is made, the data will be written from the current position till the end of
the file. It doesn’t always write from the beginning of the file.
filehandle.writev(buffers[, position])
#Added in: v12.9.0
buffers
<ArrayBufferView[]>position
Write an array of ArrayBufferView
s to the file.
The Promise
is resolved with an object containing a bytesWritten
property identifying the
number of bytes written, and a buffers
property containing a reference to the buffers
input.
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.
It is unsafe to call writev()
multiple times on the same file without waiting for the previous
operation to complete.
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.
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. Check File access constants for possible values of mode
. It is
possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK |
fs.constants.R_OK
).
If the accessibility check is successful, the Promise
is resolved with no value. If any of the
accessibility checks fail, the Promise
is rejected with an Error
object. The following
example checks if the file /etc/passwd
can be read and written by the current process.
const fs = require('fs');
const fsPromises = fs.promises;
fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
.then(() => 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])
#Added in: v10.0.0
path
FileHandle
data
options
|
encoding
'utf8'
mode
0o666
flag
flags
.
Default: 'a'
.
Asynchronously append data to a file, creating the file if it does not yet exist. data
can be a string
or a Buffer
. The
Promise
will be resolved with no arguments upon success.
If options
is a string, then it specifies the encoding.
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 then resolves the Promise
with no arguments upon succces.
fsPromises.chown(path, uid, gid)
#Added in: v10.0.0
Changes the ownership of a file then resolves the Promise
with no arguments upon success.
fsPromises.copyFile(src, dest[, mode])
#src
dest
mode
0
.Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. The Promise
will be resolved with no arguments upon success.
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.
const { promises: fsPromises, constants: { COPYFILE_EXCL } } = require(‘fs’);
fsPromises.copyFile(‘source.txt’, ‘destination.txt’) .then(() => console.log(‘source.txt was copied to destination.txt’)) .catch(() => console.log(‘The file could not be copied’));
fsPromises.copyFile(‘source.txt’, ‘destination.txt’, COPYFILE_EXCL) .then(() => console.log(‘source.txt was copied to destination.txt’)) .catch(() => console.log(‘The file could not be copied’));
fsPromises.lchmod(path, mode)
#Deprecated since: v10.0.0
Changes the permissions on a symbolic link then resolves the Promise
with no arguments upon success.
This method is only implemented on macOS.
fsPromises.lchown(path, uid, gid)
#Changes the ownership on a symbolic link then resolves the Promise
with no arguments upon success.
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.
Upon success, the Promise
is resolved without arguments.
fsPromises.link(existingPath, newPath)
#Added in: v10.0.0
Asynchronous link(2)
. The
Promise
is resolved with no arguments upon success.
fsPromises.lstat(path[, options])
#path
options
bigint
fs.Stats
object
should be bigint
. Default: false
.Asynchronous lstat(2)
. The
Promise
is resolved with the fs.Stats
object for the
given symbolic link path
.
fsPromises.mkdir(path[, options])
#Added in: v10.0.0
Asynchronously creates a directory then resolves the Promise
with either no arguments, or the first
directory path created if recursive
is true
.
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.
fsPromises.mkdtemp(prefix[, options])
#Added in: v10.0.0
Creates a unique temporary directory and resolves the Promise
with the created directory path. 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.
fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
.catch(console.error);
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('path').sep
).
fsPromises.open(path, flags[, mode])
#path
flags
flags
. Default:
'r'
.
mode
0o666
(readable and writable)Asynchronous file open that returns a Promise
that, when resolved, yields a FileHandle
object. See open(2)
.
mode
sets the file mode (permission and sticky bits), but only if the file was created.
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented by Naming Files, Paths, and
Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described
by this MSDN page.
fsPromises.opendir(path[, options])
#path
options
Asynchronously 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.
Example using async iteration:
const fs = require('fs');
async function print(path) {
const dir = await fs.promises.opendir(path);
for await (const dirent of dir) {
console.log(dirent.name);
}
}
print('./').catch(console.error);
fsPromises.readdir(path[, options])
#Reads the contents of a directory then resolves the Promise
with an array of the names of the files in
the directory excluding '.'
and '..'
.
The optional options
argument can be a string specifying an encoding, or an object with an
encoding
property specifying the character encoding to use for the filenames. If the
encoding
is set to 'buffer'
, the filenames returned will be passed as Buffer
objects.
If options.withFileTypes
is set to true
, the resolved array will contain fs.Dirent
objects.
const fs = require('fs');
async function print(path) {
const files = await fs.promises.readdir(path);
for (const file of files) {
console.log(file);
}
}
print('./').catch(console.error);
fsPromises.readFile(path[, options])
#path
FileHandle
options
|
encoding
null
flag
flags
.
Default: 'r'
.
signal
Asynchronously reads the entire contents of a file.
The Promise
is resolved 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.
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.
It is possible to abort an ongoing readFile
using an AbortSignal
. If a request is aborted
the promise returned is rejected with an AbortError
:
const controller = new AbortController();
const signal = controller.signal;
readFile(fileName, { signal }).then((file) => { });
controller.abort();
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
Asynchronous readlink(2)
. The
Promise
is resolved with the linkString
upon success.
The optional options
argument can be a string specifying an encoding, or an object with an
encoding
property specifying the character encoding to use for the link path returned. If the
encoding
is set to 'buffer'
, the link path returned will be passed as a
Buffer
object.
fsPromises.realpath(path[, options])
#Added in: v10.0.0
Determines the actual location of path
using the same semantics as the
fs.realpath.native()
function then resolves the Promise
with the resolved path.
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
and resolves the Promise
with no arguments upon
success.
fsPromises.rmdir(path[, options])
#path
options
maxRetries
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
true
, perform a recursive directory removal. In recursive mode, errors are not reported
if path
does not exist, and operations are retried on failure. Default:
false
.
retryDelay
recursive
option is not true
. Default: 100
.
Removes the directory identified by path
then resolves the Promise
with no arguments upon
success.
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.
Setting recursive
to true
results in behavior similar to the Unix command rm
-rf
: an error will not be raised for paths that do not exist, and paths that represent files will be
deleted. The permissive behavior of the recursive
option is deprecated, ENOTDIR
and
ENOENT
will be thrown in the future.
fsPromises.rm(path[, options])
#Added in: v14.14.0
path
options
force
true
, exceptions will be ignored if path
does not exist.
Default: false
.
maxRetries
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
true
, perform a recursive directory removal. In recursive mode operations are retried on
failure. Default: false
.retryDelay
recursive
option is not true
. Default: 100
.
Removes files and directories (modeled on the standard POSIX rm
utility). Resolves the
Promise
with no arguments on success.
fsPromises.stat(path[, options])
#path
options
bigint
fs.Stats
object
should be bigint
. Default: false
.The Promise
is resolved with the fs.Stats
object for the
given path
.
fsPromises.symlink(target, path[, type])
#Added in: v10.0.0
Creates a symbolic link then resolves the Promise
with no arguments upon success.
The type
argument is only used on Windows platforms and can be one of 'dir'
,
'file'
, or 'junction'
. Windows junction points require the destination path to be
absolute. When using 'junction'
, the target
argument will automatically be normalized to
absolute path.
fsPromises.truncate(path[, len])
#Added in: v10.0.0
Truncates the path
then resolves the Promise
with no arguments upon success. The
path
must be a string or Buffer
.
fsPromises.unlink(path)
#Added in: v10.0.0
Asynchronous unlink(2)
. The
Promise
is resolved with no arguments upon success.
fsPromises.utimes(path, atime, mtime)
#Added in: v10.0.0
Change the file system timestamps of the object referenced by path
then resolves the
Promise
with no arguments upon success.
The atime
and mtime
arguments follow these rules:
Date
s, or a numeric string like
'123456789.0'
.
NaN
, Infinity
or
-Infinity
, an Error
will be thrown.
fsPromises.writeFile(file, data[, options])
#file
FileHandle
data
options
|
encoding
'utf8'
mode
0o666
flag
flags
.
Default: 'w'
.
signal
Asynchronously writes data to a file, replacing the file if it already exists. data
can be a string, a
buffer, or an object with an own toString
function property. The Promise
is resolved with
no arguments upon success.
The encoding
option is ignored if data
is a buffer.
If options
is a string, then it specifies the encoding.
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 fulfilled (or rejected).
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()
.
It is possible to use an
fsPromises.writeFile()
. Cancelation is "best effort", and some amount of data is
likely still to be written.
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
(async () => {
try {
await fs.writeFile('message.txt', data, { signal });
} catch (err) {
}
})();
controller.abort();
Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering
fs.writeFile
performs.
The following constants are exported by fs.constants
.
Not every constant will be available on every operating system.
To use more than one constant, use the bitwise OR |
operator.
Example:
const fs = require('fs');
const {
O_RDWR,
O_CREAT,
O_EXCL
} = fs.constants;
fs.open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
});
The following constants are meant for use with fs.access()
.
| Constant | Description |
|---|---|
F_OK |
Flag indicating that the file is visible to the calling process. This is useful for determining if a file
exists, but says nothing about rwx permissions. Default if no mode is specified. |
R_OK |
Flag indicating that the file can be read by the calling process. |
W_OK |
Flag indicating that the file can be written by the calling process. |
X_OK |
Flag indicating that the file can be executed by the calling process. This has no effect on Windows (will
behave like fs.constants.F_OK ). |
The following constants are meant for use with fs.copyFile()
.
| Constant | Description |
|---|---|
COPYFILE_EXCL |
If present, the copy operation will fail with an error if the destination path already exists. |
COPYFILE_FICLONE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used. |
COPYFILE_FICLONE_FORCE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error. |
The following constants are meant for use with fs.open()
.
| Constant | Description |
|---|---|
O_RDONLY |
Flag indicating to open a file for read-only access. |
O_WRONLY |
Flag indicating to open a file for write-only access. |
O_RDWR |
Flag indicating to open a file for read-write access. |
O_CREAT |
Flag indicating to create the file if it does not already exist. |
O_EXCL |
Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already
exists. |
O_NOCTTY |
Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one). |
O_TRUNC |
Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero. |
O_APPEND |
Flag indicating that data will be appended to the end of the file. |
O_DIRECTORY |
Flag indicating that the open should fail if the path is not a directory. |
O_NOATIME |
Flag indicating reading accesses to the file system will no longer result in an update to the
atime information associated with the file. This flag is available on Linux operating systems
only.
|
O_NOFOLLOW |
Flag indicating that the open should fail if the path is a symbolic link. |
O_SYNC |
Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity. |
O_DSYNC |
Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity. |
O_SYMLINK |
Flag indicating to open the symbolic link itself rather than the resource it is pointing to. |
O_DIRECT |
When set, an attempt will be made to minimize caching effects of file I/O. |
O_NONBLOCK |
Flag indicating to open the file in nonblocking mode when possible. |
UV_FS_O_FILEMAP |
When set, a memory file mapping is used to access the file. This flag is available on Windows operating systems only. On other operating systems, this flag is ignored. |
The following constants are meant for use with the fs.Stats
object’s
mode
property for determining a file’s type.
| Constant | Description |
|---|---|
S_IFMT |
Bit mask used to extract the file type code. |
S_IFREG |
File type constant for a regular file. |
S_IFDIR |
File type constant for a directory. |
S_IFCHR |
File type constant for a character-oriented device file. |
S_IFBLK |
File type constant for a block-oriented device file. |
S_IFIFO |
File type constant for a FIFO/pipe. |
S_IFLNK |
File type constant for a symbolic link. |
S_IFSOCK |
File type constant for a socket. |
The following constants are meant for use with the fs.Stats
object’s
mode
property for determining the access permissions for a file.
| Constant | Description |
|---|---|
S_IRWXU |
File mode indicating readable, writable, and executable by owner. |
S_IRUSR |
File mode indicating readable by owner. |
S_IWUSR |
File mode indicating writable by owner. |
S_IXUSR |
File mode indicating executable by owner. |
S_IRWXG |
File mode indicating readable, writable, and executable by group. |
S_IRGRP |
File mode indicating readable by group. |
S_IWGRP |
File mode indicating writable by group. |
S_IXGRP |
File mode indicating executable by group. |
S_IRWXO |
File mode indicating readable, writable, and executable by others. |
S_IROTH |
File mode indicating readable by others. |
S_IWOTH |
File mode indicating writable by others. |
S_IXOTH |
File mode indicating executable by others. |
The following flags are available wherever the flag
option takes a string.
'a'
: Open file for appending. The file is created if it does not exist.
'ax'
: Like 'a'
but fails if the path exists.
'a+'
: Open file for reading and appending. The file is created if it does not exist.
'ax+'
: Like 'a+'
but fails if the path exists.
'as'
: Open file for appending in synchronous mode. The file is created if it does not exist.
'as+'
: Open file for reading and appending in synchronous mode. The file is created if it does not
exist.
'r'
: Open file for reading. An exception occurs if the file does not exist.
'r+'
: Open file for reading and writing. An exception occurs if the file does not exist.
'rs+'
: Open file for reading and writing in synchronous mode. Instructs the operating system to
bypass the local file system cache.
This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.
This doesn’t turn fs.open()
or fsPromises.open()
into a synchronous blocking call. If
synchronous operation is desired, something like fs.openSync()
should be used.
'w'
: Open file for writing. The file is created (if it does not exist) or truncated (if it
exists).
'wx'
: Like 'w'
but fails if the path exists.
'w+'
: Open file for reading and writing. The file is created (if it does not exist) or truncated
(if it exists).
'wx+'
: Like 'w+'
but fails if the path exists.
flag
can also be a number as documented by open(2)
; commonly used constants are
available from fs.constants
. On Windows, flags are translated to their equivalent ones where
applicable, e.g. O_WRONLY
to FILE_GENERIC_WRITE
, or O_EXCL|O_CREAT
to
CREATE_NEW
, as accepted by CreateFileW
.
The exclusive flag 'x'
(O_EXCL
flag in open(2)
) causes the operation to return
an error if the path already exists. On POSIX, if the path is a symbolic link, using O_EXCL
returns an
error even if the link is to a path that does not exist. The exclusive flag might not work with network file
systems.
On Linux, positional writes don’t work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
Modifying a file rather than replacing it may require the flag
option to be set to 'r+'
rather than the default 'w'
.
The behavior of some flags are platform-specific. As such, opening a directory on macOS and Linux with the
'a+'
flag, as in the example below, will return an error. In contrast, on Windows and FreeBSD, a file
descriptor or a FileHandle
will be returned.
fs.open('<directory>', 'a+', (err, fd) => {
});
fs.open('<directory>', 'a+', (err, fd) => {
});
On Windows, opening an existing hidden file using the 'w'
flag (either through fs.open()
or fs.writeFile()
or fsPromises.open()
) will fail with EPERM
. Existing hidden
files can be opened for writing with the 'r+'
flag.
A call to fs.ftruncate()
or filehandle.truncate()
can be used to reset the file contents.