Node.js fs Module Flashcards for Interviews & Learning

Strengthen your Node.js expertise with this comprehensive flashcard set dedicated to the File System (fs) module. Designed for developers, technical interview preparation, certification study, and day-to-day backend development, this collection covers both fundamental concepts and advanced file system operations used in real-world Node.js applications. Topics include: • File and directory manipulation • Callback, Promise-based, and synchronous APIs • File descriptors and FileHandle objects • Reading and writing files efficiently • Streams (ReadStream and WriteStream) • Directory handling with fs.Dir and fs.Dirent • File watching with fs.watch() and fs.watchFile() • File system statistics and metadata • File permissions, ownership, and modes • Path handling using strings, Buffers, and file URLs • FS constants and flags • Performance considerations and thread pool behavior • Cross-platform caveats and operating system differences • Error handling and asynchronous execution patterns • Production best practices for scalable Node.js applications Whether you're preparing for a Node.js interview, building backend services, working with file uploads, implementing storage systems, or improving your understanding of operating system interactions, these flashcards help you learn faster and retain critical knowledge. The set progresses from core concepts to advanced APIs, making it suitable for intermediate, senior, and lead-level JavaScript developers. Each card focuses on practical understanding, common pitfalls, performance trade-offs, and real-world usage scenarios encountered in modern Node.js development. You may also be interested in: Master Node.js: From Basics to Production Node.js Stream Module Node.js Cluster Module Node.js Child Process Module Node.js Buffer API Interview Questions Node.js Crypto Module Node.js Operating System Module

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File system

The node:fs module enables interacting with the file system in away modeled on standard POSIX functions.

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Promise example

Promise-based operations return a promise that is fulfilled when theasynchronous operation is complete.

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Callback example

The callback form takes a completion callback function as its last argument and invokes the operation asynchronously. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception.

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Synchronous example

The synchronous APIs block the Node.js event loop and further JavaScriptexecution until the operation is complete. Exceptions are thrown immediatelyand can be handled using try…catch, or can be allowed to bubble up.

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Promises API

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

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Class: FileHandle

A <FileHandle> object is an object wrapper for a numeric file descriptor. Instances of the <FileHandle> object are created by the fsPromises.open()method. All <FileHandle> objects are <EventEmitter>s.

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Event: 'close'

The 'close' event is emitted when the <FileHandle> has been closed and can nolonger be used.

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filehandle.appendFile()

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().

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filehandle.chmod(mode)

Modifies the permissions on the file.

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filehandle.chown(uid, gid)

Changes the ownership of the file.

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filehandle.close()

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

Returns: <Promise> Fulfills with undefined upon success.

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filehandle .createReadStream([options])

options <Object>encoding <string> Default: null

autoClose <boolean> Default: true

emitClose <boolean> Default: true

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filehandle .createWriteStream([options])

options <Object>encoding <string> Default: 'utf8'

autoClose <boolean> Default: true

emitClose <boolean> Default: true

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filehandle.datasync()

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

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filehandle.fd

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

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

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

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

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filehandle.read([options])

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

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

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filehandle.read(buffer[, options])

buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with thefile data read. Reads data from the file and stores that in the given buffer. If the file is not modified concurrently, the end-of-file is reached when thenumber of bytes read is zero.

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filehandle .readableWebStream([options])

An error will be thrown if this method is called more than once or is calledafter the FileHandle is closed or closing.

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filehandle.readFile(options)

options <Object> | <string>encoding <string> | <null> Default: nullsignal <AbortSignal> allows aborting an in-progress readFile Asynchronously reads the entire contents of a file. If options is a string, then it specifies the encoding.

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filehandle.readLines([options])

options <Object>encoding <string> Default: nullautoClose <boolean> Default: trueemitClose <boolean> Default: truestart <integer>end <integer> Default: InfinityhighWaterMark <integer> Default: 64 * 1024 Returns: <readline.InterfaceConstructor>

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filehandle.readv(buffers[, position])

buffers <Buffer[]> | <TypedArray[]> | <DataView[]> position <integer> | <null> The offset from the beginning of the file wherethe data should be read from. If position is not a number, the data willbe read from the current position. Default: null

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filehandle.stat([options])

options <Object>bigint <boolean> Whether the numeric values in the returned<fs.Stats> object should be bigint. Default: false. Returns: <Promise> Fulfills with an <fs.Stats> for the file.

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filehandle.sync()

Returns: <Promise> Fulfills with undefined upon success. Request that all data for the open file descriptor is flushed to the storagedevice. The specific implementation is operating system and device specific.Refer to the POSIX fsync(2) documentation for more detail.

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filehandle.truncate(len)

len <integer> Default: 0 Returns: <Promise> Fulfills with undefined upon success. Truncates the file. If the file was larger than len bytes, only the first len bytes will beretained in the file. The following example retains only the first four bytes of the file:

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filehandle.utimes(atime, mtime)

atime <number> | <string> | <Date> mtime <number> | <string> | <Date> Returns: <Promise> Change the file system timestamps of the object referenced by the <FileHandle>then fulfills the promise with no arguments upon success.

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

buffer <Buffer> | <TypedArray> | <DataView> offset <integer> The start position from within buffer where the datato write begins. length <integer> The number of bytes from buffer to write. Default:buffer.byteLength - offset Returns: <Promise>

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filehandle.write(buffer[, options])

buffer <Buffer> | <TypedArray> | <DataView> options <Object>offset <integer> Default: 0length <integer> Default: buffer.byteLength - offsetposition <integer> Default: null Returns: <Promise> Write buffer to the file.

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filehandle.write(string[, position[, encoding]])

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

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filehandle.writeFile(data, options)

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filehandle.writev(buffers[, position])

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

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filehandle[Symbol .asyncDispose]()

An alias for filehandle.close().

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fsPromises.access(path[, mode])

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

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

path <string> | <Buffer> | <URL> | <FileHandle> filename or <FileHandle> data <string> | <Buffer> Returns: <Promise> Fulfills with undefined upon success. If options is a string, then it specifies the encoding.

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fsPromises.chmod(path, mode)

path <string> | <Buffer> | <URL> mode <string> | <integer> Returns: <Promise> Fulfills with undefined upon success. Changes the permissions of a file.

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fsPromises.chown(path, uid, gid)

path <string> | <Buffer> | <URL> uid <integer> gid <integer> Returns: <Promise> Fulfills with undefined upon success. Changes the ownership of a file.

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

src <string> | <Buffer> | <URL> source filename to copy dest <string> | <Buffer> | <URL> destination filename of the copy operation Returns: <Promise> Fulfills with undefined upon success.

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

src <string> | <URL> source path to copy. dest <string> | <URL> destination path to copy to. Returns: <Promise> Fulfills with undefined upon success. Asynchronously copies the entire directory structure from src to dest,including subdirectories and files.

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fsPromises.glob(pattern[, options])

pattern <string> | <string[]> Returns: <AsyncIterator> An AsyncIterator that yields the paths of filesthat match the pattern.

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fsPromises.lchmod(path, mode)

Deprecated since: v10.0.0 path <string> | <Buffer> | <URL> mode <integer> Returns: <Promise> Fulfills with undefined upon success. Changes the permissions on a symbolic link. This method is only implemented on macOS.

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fsPromises.lchown(path, uid, gid)

path <string> | <Buffer> | <URL> uid <integer> gid <integer> Returns: <Promise> Fulfills with undefined upon success. Changes the ownership on a symbolic link.

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fsPromises.lutimes(path, atime, mtime)

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fsPromises.link(existingPath, newPath)

existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Returns: <Promise> Fulfills with undefined upon success. Creates a new link from the existingPath to the newPath. See the POSIXlink(2) documentation for more detail.

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fsPromises.lstat(path[, options])

path <string> | <Buffer> | <URL> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.Stats> object should be bigint. Default: false. Returns: <Promise> Fulfills with the <fs.Stats> object for the givensymbolic link path.

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fsPromises.mkdir(path[, options])

path <string> | <Buffer> | <URL> options <Object> | <integer>recursive <boolean> Default: falsemode <string> | <integer> Not supported on Windows. Default: 0o777. Asynchronously creates a directory.

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fsPromises.mkdtemp(prefix[, options])

prefix <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <Promise> Fulfills with a string containing the file system pathof the newly created temporary directory.

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fsPromises.open(path, flags[, mode])

path <string> | <Buffer> | <URL> flags <string> | <number> See support of file system flags.Default: 'r'. mode <string> | <integer> Sets the file mode (permission and sticky bits)if the file is created. Default: 0o666 (readable and writable)

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fsPromises.opendir(path[, options])

path <string> | <Buffer> | <URL> Returns: <Promise> Fulfills with an <fs.Dir>. Asynchronously open a directory for iterative scanning. See the POSIXopendir(3) documentation for more detail. Example using async iteration:

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fsPromises.readdir(path[, options])

path <string> | <Buffer> | <URL> Returns: <Promise> Fulfills with an array of the names of the files inthe directory excluding '.' and '..'. Reads the contents of a directory. If options.withFileTypes is set to true, the returned array will contain<fs.Dirent> objects.

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fsPromises.readFile(path[, options])

path <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle Returns: <Promise> Fulfills with the contents of the file. Asynchronously reads the entire contents of a file. If options is a string, then it specifies the encoding.

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fsPromises.readlink(path[, options])

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <Promise> Fulfills with the linkString upon success.

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fsPromises.realpath(path[, options])

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <Promise> Fulfills with the resolved path upon success. Only paths that can be converted to UTF8 strings are supported.

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fsPromises.rename(oldPath, newPath)

oldPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Returns: <Promise> Fulfills with undefined upon success. Renames oldPath to newPath.

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fsPromises.rmdir(path[, options])

path <string> | <Buffer> | <URL> Returns: <Promise> Fulfills with undefined upon success. Removes the directory identified by path. To get a behavior similar to the rm -rf Unix command, usefsPromises.rm() with options { recursive: true, force: true }.

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fsPromises.rm(path[, options])

path <string> | <Buffer> | <URL> Returns: <Promise> Fulfills with undefined upon success. Removes files and directories (modeled on the standard POSIX rm utility).

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fsPromises.stat(path[, options])

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fsPromises.statfs(path[, options])

path <string> | <Buffer> | <URL> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.StatFs> object should be bigint. Default: false. Returns: <Promise> Fulfills with the <fs.StatFs> object for thegiven path.

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fsPromises.symlink(target, path[, type])

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fsPromises.truncate(path[, len])

path <string> | <Buffer> | <URL> len <integer> Default: 0 Returns: <Promise> Fulfills with undefined upon success. Truncates (shortens or extends the length) of the content at path to lenbytes.

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fsPromises.unlink(path)

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

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fsPromises.utimes(path, atime, mtime)

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fsPromises.watch(filename[, options])

filename <string> | <Buffer> | <URL> Returns: <AsyncIterator> of objects with the properties:eventType <string> The type of changefilename <string> | <Buffer> | <null> The name of the file changed.

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fsPromises.writeFile(file, data[, options])

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fsPromises.constants

<Object> Returns an object containing commonly used constants for file systemoperations. The object is the same as fs.constants. See FS constantsfor more details.

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Callback API

The callback APIs perform all operations asynchronously, without blocking theevent loop, then invoke a callback function upon completion or error.

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fs.access(path[, mode], callback)

path <string> | <Buffer> | <URL> mode <integer> Default: fs.constants.F_OK callback <Function>err <Error>

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fs.appendFile(path, data[, options], callback)

path <string> | <Buffer> | <URL> | <number> filename or file descriptor data <string> | <Buffer> callback <Function>err <Error> Asynchronously append data to a file, creating the file if it does not yetexist. data can be a string or a <Buffer>.

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fs.chmod(path, mode, callback)

path <string> | <Buffer> | <URL> mode <string> | <integer> callback <Function>err <Error> Asynchronously changes the permissions of a file. No arguments other than apossible exception are given to the completion callback.

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File modes

The mode argument used in both the fs.chmod() and fs.chmodSync()methods is a numeric bitmask created using a logical OR of the followingconstants: Constant Octal Description

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fs.chown(path, uid, gid, callback)

path <string> | <Buffer> | <URL> uid <integer> gid <integer> callback <Function>err <Error> Asynchronously changes owner and group of a file. No arguments other than apossible exception are given to the completion callback.

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fs.close(fd[, callback])

fd <integer> callback <Function>err <Error> Closes the file descriptor. No arguments other than a possible exception aregiven to the completion callback. See the POSIX close(2) documentation for more detail.

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fs.copyFile(src, dest[, mode], callback)

src <string> | <Buffer> | <URL> source filename to copy dest <string> | <Buffer> | <URL> destination filename of the copy operation mode <integer> modifiers for copy operation. Default: 0. callback <Function>err <Error>

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fs.cp(src, dest[, options], callback)

src <string> | <URL> source path to copy. dest <string> | <URL> destination path to copy to. callback <Function>err <Error> Asynchronously copies the entire directory structure from src to dest,including subdirectories and files.

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fs.createReadStream(path[, options])

path <string> | <Buffer> | <URL> Returns: <fs.ReadStream> By default, the stream will emit a 'close' event after it has beendestroyed. Set the emitClose option to false to change this behavior.

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fs.createWriteStream(path[, options])

path <string> | <Buffer> | <URL> Returns: <fs.WriteStream> By default, the stream will emit a 'close' event after it has beendestroyed. Set the emitClose option to false to change this behavior. If options is a string, then it specifies the encoding.

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fs.exists(path, callback)

path <string> | <Buffer> | <URL> callback <Function>exists <boolean> Test whether or not the element at the given path exists by checking with the file system.Then call the callback argument with either true or false:

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fs.fchmod(fd, mode, callback)

fd <integer> mode <string> | <integer> callback <Function>err <Error> Sets the permissions on the file. No arguments other than a possible exceptionare given to the completion callback. See the POSIX fchmod(2) documentation for more detail.

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fs.fchown(fd, uid, gid, callback)

fd <integer> uid <integer> gid <integer> callback <Function>err <Error> Sets the owner of the file. No arguments other than a possible exception aregiven to the completion callback. See the POSIX fchown(2) documentation for more detail.

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fs.fdatasync(fd, callback)

fd <integer> callback <Function>err <Error>

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fs.fstat(fd[, options], callback)

fd <integer> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.Stats> object should be bigint. Default: false. callback <Function>err <Error>stats <fs.Stats> Invokes the callback with the <fs.Stats> for the file descriptor.

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fs.fsync(fd, callback)

fd <integer> callback <Function>err <Error>

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fs.ftruncate(fd[, len], callback)

fd <integer> len <integer> Default: 0 callback <Function>err <Error> Truncates the file descriptor. No arguments other than a possible exception aregiven to the completion callback. See the POSIX ftruncate(2) documentation for more detail.

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fs.futimes(fd, atime, mtime, callback)

fd <integer> atime <number> | <string> | <Date> mtime <number> | <string> | <Date> callback <Function>err <Error> Change the file system timestamps of the object referenced by the supplied filedescriptor. See fs.utimes().

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fs.glob(pattern[, options], callback)

pattern <string> | <string[]> callback <Function>err <Error> Retrieves the files matching the specified pattern.

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fs.lchmod(path, mode, callback)

path <string> | <Buffer> | <URL> mode <integer> callback <Function>err <Error> | <AggregateError> Changes the permissions on a symbolic link. No arguments other than a possibleexception are given to the completion callback.

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fs.lchown(path, uid, gid, callback)

path <string> | <Buffer> | <URL> uid <integer> gid <integer> callback <Function>err <Error> Set the owner of the symbolic link. No arguments other than a possibleexception are given to the completion callback.

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fs.lutimes(path, atime, mtime, callback)

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fs.link(existingPath, newPath, callback)

existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> callback <Function>err <Error>

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fs.lstat(path[, options], callback)

path <string> | <Buffer> | <URL> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.Stats> object should be bigint. Default: false. callback <Function>err <Error>stats <fs.Stats>

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fs.mkdir(path[, options], callback)

path <string> | <Buffer> | <URL> options <Object> | <integer>recursive <boolean> Default: falsemode <string> | <integer> Not supported on Windows. Default: 0o777. Asynchronously creates a directory.

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fs.mkdtemp(prefix[, options], callback)

prefix <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' callback <Function>err <Error>directory <string> Creates a unique temporary directory.

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fs.open(path[, flags[, mode]], callback)

path <string> | <Buffer> | <URL> flags <string> | <number> See support of file system flags.Default: 'r'. mode <string> | <integer> Default: 0o666 (readable and writable) callback <Function>err <Error>fd <integer>

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fs.openAsBlob(path[, options])

path <string> | <Buffer> | <URL> options <Object>type <string> An optional mime type for the blob. Returns: <Promise> Fulfills with a <Blob> upon success. Returns a <Blob> whose data is backed by the given file.

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fs.opendir(path[, options], callback)

path <string> | <Buffer> | <URL> callback <Function>err <Error>dir <fs.Dir> Asynchronously open a directory. See the POSIX opendir(3) documentation formore details.

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fs.read(fd, buffer, offset, length, position, callback)

fd <integer> buffer <Buffer> | <TypedArray> | <DataView> The buffer that the data will bewritten to. offset <integer> The position in buffer to write the data to. length <integer> The number of bytes to read. Read data from the file specified by fd. For example:

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fs.read(fd[, options], callback)

fd <integer> options <Object>buffer <Buffer> | <TypedArray> | <DataView> Default: Buffer.alloc(16384)offset <integer> Default: 0length <integer> Default: buffer.byteLength - offsetposition <integer> | <bigint> | <null> Default: null

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fs.read(fd, buffer[, options], callback)

fd <integer> buffer <Buffer> | <TypedArray> | <DataView> The buffer that the data will bewritten to. options <Object>offset <integer> Default: 0length <integer> Default: buffer.byteLength - offsetposition <integer> | <bigint> Default: null

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fs.readdir(path[, options], callback)

path <string> | <Buffer> | <URL> callback <Function>err <Error>files <string[]> | <Buffer[]> | <fs.Dirent[]> See the POSIX readdir(3) documentation for more details.

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fs.readFile(path[, options], callback)

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File descriptors

Any specified file descriptor has to support reading. If a file descriptor is specified as the path, it will not be closedautomatically.

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Performance Considerations

The fs.readFile() method asynchronously reads the contents of a file intomemory 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 maybe using the underlying libuv thread pool but means that it will take longerto read a complete file into memory.

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fs.readlink(path[, options], callback)

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' callback <Function>err <Error>linkString <string> | <Buffer> See the POSIX readlink(2) documentation for more details.

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fs.readv(fd, buffers[, position], callback)

fd <integer> buffers <ArrayBufferView[]> position <integer> | <null> Default: null callback <Function>err <Error>bytesRead <integer>buffers <ArrayBufferView[]> Read from a file specified by fd and write to an array of ArrayBufferViewsusing readv().

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fs.realpath(path[, options], callback)

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' callback <Function>err <Error>resolvedPath <string> | <Buffer> This function behaves like realpath(3), with some exceptions:

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fs.realpath.native(path[, options], callback)

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' callback <Function>err <Error>resolvedPath <string> | <Buffer> Asynchronous realpath(3). The callback gets two arguments (err, resolvedPath).

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fs.rename(oldPath, newPath, callback)

oldPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> callback <Function>err <Error> See also: rename(2).

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fs.rmdir(path[, options], callback)

path <string> | <Buffer> | <URL> callback <Function>err <Error> Asynchronous rmdir(2). No arguments other than a possible exception are givento the completion callback.

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fs.rm(path[, options], callback)

path <string> | <Buffer> | <URL> callback <Function>err <Error> Asynchronously removes files and directories (modeled on the standard POSIX rmutility). No arguments other than a possible exception are given to thecompletion callback.

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fs.stat(path[, options], callback)

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fs.statfs(path[, options], callback)

path <string> | <Buffer> | <URL> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.StatFs> object should be bigint. Default: false. callback <Function>err <Error>stats <fs.StatFs>

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fs.symlink(target, path[, type], callback)

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fs.truncate(path[, len], callback)

path <string> | <Buffer> | <URL> len <integer> Default: 0 callback <Function>err <Error> | <AggregateError>

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fs.unlink(path, callback)

path <string> | <Buffer> | <URL> callback <Function>err <Error> Asynchronously removes a file or symbolic link. No arguments other than apossible exception are given to the completion callback.

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fs.unwatchFile(filename[, listener])

filename <string> | <Buffer> | <URL> listener <Function> Optional, a listener previously attached usingfs.watchFile() Calling fs.unwatchFile() with a filename that is not being watched is ano-op, not an error.

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fs.utimes(path, atime, mtime, callback)

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fs.watch(filename[, options][, listener])

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Caveats

The fs.watch API is not 100% consistent across platforms, and isunavailable in some situations. On Windows, no events will be emitted if the watched directory is moved orrenamed. An EPERM error is reported when the watched directory is deleted. Availability # On Linux systems, this uses inotify(7).

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fs.watchFile(filename[, options], listener)

filename <string> | <Buffer> | <URL> options <Object>bigint <boolean> Default: falsepersistent <boolean> Default: trueinterval <integer> Default: 5007 listener <Function>current <fs.Stats>previous <fs.Stats> Returns: <fs.StatWatcher>

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fs.write(fd, buffer, offset[, length[, position]], callback)

fd <integer> buffer <Buffer> | <TypedArray> | <DataView> offset <integer> Default: 0 length <integer> Default: buffer.byteLength - offset position <integer> | <null> Default: null Write buffer to the file specified by fd.

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fs.write(fd, buffer[, options], callback)

fd <integer> buffer <Buffer> | <TypedArray> | <DataView> options <Object>offset <integer> Default: 0length <integer> Default: buffer.byteLength - offsetposition <integer> Default: null Write buffer to the file specified by fd.

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fs.write(fd, string[, position[, encoding]], callback)

fd <integer> string <string> position <integer> | <null> Default: null encoding <string> Default: 'utf8' callback <Function>err <Error>written <integer>string <string> encoding is the expected string encoding.

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fs.writeFile(file, data[, options], callback)

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Using fs.writeFile() with file descriptors

When file is a file descriptor, the behavior is almost identical to directlycalling fs.write() like:

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fs.writev(fd, buffers[, position], callback)

fd <integer> buffers <ArrayBufferView[]> position <integer> | <null> Default: null callback <Function>err <Error>bytesWritten <integer>buffers <ArrayBufferView[]> Write an array of ArrayBufferViews to the file specified by fd usingwritev().

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Synchronous API

The synchronous APIs perform all operations synchronously, blocking theevent loop until the operation completes or fails.

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fs.accessSync(path[, mode])

path <string> | <Buffer> | <URL> mode <integer> Default: fs.constants.F_OK If any of the accessibility checks fail, an Error will be thrown. Otherwise,the method will return undefined.

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fs.appendFileSync(path, data[, options])

path <string> | <Buffer> | <URL> | <number> filename or file descriptor data <string> | <Buffer> Synchronously append data to a file, creating the file if it does not yetexist. data can be a string or a <Buffer>.

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fs.chmodSync(path, mode)

path <string> | <Buffer> | <URL> mode <string> | <integer> For detailed information, see the documentation of the asynchronous version ofthis API: fs.chmod(). See the POSIX chmod(2) documentation for more detail.

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fs.chownSync(path, uid, gid)

path <string> | <Buffer> | <URL> uid <integer> gid <integer> Synchronously changes owner and group of a file. Returns undefined.This is the synchronous version of fs.chown(). See the POSIX chown(2) documentation for more detail.

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fs.closeSync(fd)

fd <integer> Closes the file descriptor. Returns undefined. Calling fs.closeSync() on any file descriptor (fd) that is currently in usethrough any other fs operation may lead to undefined behavior. See the POSIX close(2) documentation for more detail.

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fs.copyFileSync(src, dest[, mode])

src <string> | <Buffer> | <URL> source filename to copy dest <string> | <Buffer> | <URL> destination filename of the copy operation mode <integer> modifiers for copy operation. Default: 0.

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fs.cpSync(src, dest[, options])

src <string> | <URL> source path to copy. dest <string> | <URL> destination path to copy to. Synchronously copies the entire directory structure from src to dest,including subdirectories and files.

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fs.existsSync(path)

path <string> | <Buffer> | <URL> Returns: <boolean> Returns true if the path exists, false otherwise. For detailed information, see the documentation of the asynchronous version ofthis API: fs.exists().

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fs.fchmodSync(fd, mode)

fd <integer> mode <string> | <integer> Sets the permissions on the file. Returns undefined. See the POSIX fchmod(2) documentation for more detail.

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fs.fchownSync(fd, uid, gid)

fd <integer> uid <integer> The file's new owner's user id. gid <integer> The file's new group's group id. Sets the owner of the file. Returns undefined. See the POSIX fchown(2) documentation for more detail.

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fs.fdatasyncSync(fd)

fd <integer> Forces all currently queued I/O operations associated with the file to theoperating system's synchronized I/O completion state. Refer to the POSIXfdatasync(2) documentation for details. Returns undefined.

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fs.fstatSync(fd[, options])

fd <integer> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.Stats> object should be bigint. Default: false. Returns: <fs.Stats> Retrieves the <fs.Stats> for the file descriptor. See the POSIX fstat(2) documentation for more detail.

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fs.fsyncSync(fd)

fd <integer> Request that all data for the open file descriptor is flushed to the storagedevice. The specific implementation is operating system and device specific.Refer to the POSIX fsync(2) documentation for more detail. Returns undefined.

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fs.ftruncateSync(fd[, len])

fd <integer> len <integer> Default: 0 Truncates the file descriptor. Returns undefined. For detailed information, see the documentation of the asynchronous version ofthis API: fs.ftruncate().

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fs.futimesSync(fd, atime, mtime)

fd <integer> atime <number> | <string> | <Date> mtime <number> | <string> | <Date> Synchronous version of fs.futimes(). Returns undefined.

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fs.globSync(pattern[, options])

pattern <string> | <string[]> Returns: <string[]> paths of files that match the pattern.

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fs.lchmodSync(path, mode)

Deprecated since: v0.4.7 path <string> | <Buffer> | <URL> mode <integer> Changes the permissions on a symbolic link. Returns undefined. This method is only implemented on macOS. See the POSIX lchmod(2) documentation for more detail.

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fs.lchownSync(path, uid, gid)

path <string> | <Buffer> | <URL> uid <integer> The file's new owner's user id. gid <integer> The file's new group's group id. Set the owner for the path. Returns undefined. See the POSIX lchown(2) documentation for more details.

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fs.lutimesSync(path, atime, mtime)

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fs.linkSync(existingPath, newPath)

existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Creates a new link from the existingPath to the newPath. See the POSIXlink(2) documentation for more detail. Returns undefined.

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fs.lstatSync(path[, options])

path <string> | <Buffer> | <URL> Returns: <fs.Stats> Retrieves the <fs.Stats> for the symbolic link referred to by path. See the POSIX lstat(2) documentation for more details.

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fs.mkdirSync(path[, options])

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fs.mkdtempSync(prefix[, options])

prefix <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <string> Returns the created directory path. For detailed information, see the documentation of the asynchronous version ofthis API: fs.mkdtemp().

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fs.opendirSync(path[, options])

path <string> | <Buffer> | <URL> Returns: <fs.Dir> Synchronously open a directory. See opendir(3). Creates an <fs.Dir>, which contains all further functions for reading fromand cleaning up the directory.

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fs.openSync(path[, flags[, mode]])

path <string> | <Buffer> | <URL> flags <string> | <number> Default: 'r'.See support of file system flags. mode <string> | <integer> Default: 0o666 Returns: <number> Returns an integer representing the file descriptor.

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fs.readdirSync(path[, options])

path <string> | <Buffer> | <URL> Returns: <string[]> | <Buffer[]> | <fs.Dirent[]> Reads the contents of the directory. See the POSIX readdir(3) documentation for more details.

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fs.readFileSync(path[, options])

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fs.readlinkSync(path[, options])

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <string> | <Buffer> Returns the symbolic link's string value. See the POSIX readlink(2) documentation for more details.

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fs.readSync(fd, buffer, offset, length[, position])

fd <integer> buffer <Buffer> | <TypedArray> | <DataView> offset <integer> length <integer> position <integer> | <bigint> | <null> Default: null Returns: <number> Returns the number of bytesRead.

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fs.readSync(fd, buffer[, options])

fd <integer> buffer <Buffer> | <TypedArray> | <DataView> options <Object>offset <integer> Default: 0length <integer> Default: buffer.byteLength - offsetposition <integer> | <bigint> | <null> Default: null Returns: <number>

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fs.readvSync(fd, buffers[, position])

fd <integer> buffers <ArrayBufferView[]> position <integer> | <null> Default: null Returns: <number> The number of bytes read. For detailed information, see the documentation of the asynchronous version ofthis API: fs.readv().

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fs.realpathSync(path[, options])

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <string> | <Buffer> Returns the resolved pathname.

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fs.realpathSync.native(path[, options])

path <string> | <Buffer> | <URL> options <string> | <Object>encoding <string> Default: 'utf8' Returns: <string> | <Buffer> Synchronous realpath(3). Only paths that can be converted to UTF8 strings are supported.

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fs.renameSync(oldPath, newPath)

oldPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Renames the file from oldPath to newPath. Returns undefined. See the POSIX rename(2) documentation for more details.

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fs.rmdirSync(path[, options])

path <string> | <Buffer> | <URL> Synchronous rmdir(2). Returns undefined. Using fs.rmdirSync() on a file (not a directory) results in an ENOENT erroron Windows and an ENOTDIR error on POSIX.

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fs.rmSync(path[, options])

path <string> | <Buffer> | <URL> Synchronously removes files and directories (modeled on the standard POSIX rmutility). Returns undefined.

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fs.statSync(path[, options])

path <string> | <Buffer> | <URL> Returns: <fs.Stats> Retrieves the <fs.Stats> for the path.

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fs.statfsSync(path[, options])

path <string> | <Buffer> | <URL> options <Object>bigint <boolean> Whether the numeric values in the returned<fs.StatFs> object should be bigint. Default: false. Returns: <fs.StatFs> In case of an error, the err.code will be one of Common System Errors.

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fs.symlinkSync(target, path[, type])

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fs.truncateSync(path[, len])

path <string> | <Buffer> | <URL> len <integer> Default: 0 Truncates the file. Returns undefined. A file descriptor can also bepassed as the first argument. In this case, fs.ftruncateSync() is called.

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fs.unlinkSync(path)

path <string> | <Buffer> | <URL> Synchronous unlink(2). Returns undefined.

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fs.utimesSync(path, atime, mtime)

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fs.writeFileSync(file, data[, options])

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fs.writeSync(fd, buffer, offset[, length[, position]])

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fs.writeSync(fd, buffer[, options])

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fs.writeSync(fd, string[, position[, encoding]])

fd <integer> string <string> position <integer> | <null> Default: null encoding <string> Default: 'utf8' Returns: <number> The number of bytes written. For detailed information, see the documentation of the asynchronous version ofthis API: fs.write(fd, string...).

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fs.writevSync(fd, buffers[, position])

fd <integer> buffers <ArrayBufferView[]> position <integer> | <null> Default: null Returns: <number> The number of bytes written. For detailed information, see the documentation of the asynchronous version ofthis API: fs.writev().

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Common Objects

The common objects are shared by all of the file system API variants(promise, callback, and synchronous).

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Class: fs.Dir

A class representing a directory stream. Created by fs.opendir(), fs.opendirSync(), orfsPromises.opendir().

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dir.close()

Returns: <Promise> Asynchronously close the directory's underlying resource handle.Subsequent reads will result in errors. A promise is returned that will be fulfilled after the resource has beenclosed.

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dir.close(callback)

callback <Function>err <Error> Asynchronously close the directory's underlying resource handle.Subsequent reads will result in errors. The callback will be called after the resource handle has been closed.

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dir.closeSync()

Synchronously close the directory's underlying resource handle.Subsequent reads will result in errors.

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dir.path

<string> The read-only path of this directory as was provided to fs.opendir(),fs.opendirSync(), or fsPromises.opendir().

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dir.read()

Returns: <Promise> Fulfills with a <fs.Dirent> | <null> Asynchronously read the next directory entry via readdir(3) as an<fs.Dirent>. A promise is returned that will be fulfilled with an <fs.Dirent>, or nullif there are no more directory entries to read.

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dir.read(callback)

callback <Function>err <Error>dirent <fs.Dirent> | <null> Asynchronously read the next directory entry via readdir(3) as an<fs.Dirent>.

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dir.readSync()

Returns: <fs.Dirent> | <null> Synchronously read the next directory entry as an <fs.Dirent>. See thePOSIX readdir(3) documentation for more detail. If there are no more directory entries to read, null will be returned.

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dir[Symbol.asyncIterator]()

Returns: <AsyncIterator> An AsyncIterator of <fs.Dirent> Asynchronously iterates over the directory until all entries havebeen read. Refer to the POSIX readdir(3) documentation for more detail. See <fs.Dir> for an example.

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Class: fs.Dirent

A representation of a directory entry, which can be a file or a subdirectorywithin the directory, as returned by reading from an <fs.Dir>. Thedirectory entry is a combination of the file name and file type pairs.

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dirent.isBlockDevice()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a block device.

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dirent.isCharacterDevice()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a character device.

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dirent.isDirectory()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a file systemdirectory.

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dirent.isFIFO()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a first-in-first-out(FIFO) pipe.

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dirent.isFile()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a regular file.

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dirent.isSocket()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a socket.

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dirent.isSymbolicLink()

Returns: <boolean> Returns true if the <fs.Dirent> object describes a symbolic link.

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dirent.name

<string> | <Buffer> The file name that this <fs.Dirent> object refers to. The type of thisvalue is determined by the options.encoding passed to fs.readdir() orfs.readdirSync().

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dirent.parentPath

<string> The path to the parent directory of the file this <fs.Dirent> object refers to.

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dirent.path

<string> Alias for dirent.parentPath.

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Class: fs.FSWatcher

Extends <EventEmitter> 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 watchedfile is modified.

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Event: 'change'

eventType <string> The type of change event that has occurred filename <string> | <Buffer> The filename that changed (if relevant/available) Emitted when something changes in a watched directory or file.See more details in fs.watch().

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Event: 'close'

Emitted when the watcher stops watching for changes. The closed<fs.FSWatcher> object is no longer usable in the event handler.

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Event: 'error'

error <Error> Emitted when an error occurs while watching the file. The errored<fs.FSWatcher> object is no longer usable in the event handler.

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watcher.close()

Stop watching for changes on the given <fs.FSWatcher>. Once stopped, the<fs.FSWatcher> object is no longer usable.

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watcher.ref()

Returns: <fs.FSWatcher> When called, requests that the Node.js event loop not exit so long as the<fs.FSWatcher> is active. Calling watcher.ref() multiple times will haveno effect.

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watcher.unref()

Returns: <fs.FSWatcher>

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Class: fs.StatWatcher

Extends <EventEmitter> A successful call to fs.watchFile() method will return a new <fs.StatWatcher>object.

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watcher.ref()

Returns: <fs.StatWatcher> When called, requests that the Node.js event loop not exit so long as the<fs.StatWatcher> is active. Calling watcher.ref() multiple times will haveno effect.

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watcher.unref()

Returns: <fs.StatWatcher>

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Class: fs.ReadStream

Extends: <stream.Readable> Instances of <fs.ReadStream> are created and returned using thefs.createReadStream() function.

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Event: 'close'

Emitted when the <fs.ReadStream>'s underlying file descriptor has been closed.

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Event: 'open'

fd <integer> Integer file descriptor used by the <fs.ReadStream>. Emitted when the <fs.ReadStream>'s file descriptor has been opened.

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Event: 'ready'

Emitted when the <fs.ReadStream> is ready to be used. Fires immediately after 'open'.

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readStream.bytesRead

<number> The number of bytes that have been read so far.

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readStream.path

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readStream.pending

<boolean> This property is true if the underlying file has not been opened yet,i.e. before the 'ready' event is emitted.

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Class: fs.Stats

A <fs.Stats> object provides information about a file.

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stats.isBlockDevice()

Returns: <boolean> Returns true if the <fs.Stats> object describes a block device.

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stats.isCharacterDevice()

Returns: <boolean> Returns true if the <fs.Stats> object describes a character device.

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stats.isDirectory()

Returns: <boolean> Returns true if the <fs.Stats> object describes a file system directory.

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stats.isFIFO()

Returns: <boolean> Returns true if the <fs.Stats> object describes a first-in-first-out (FIFO)pipe.

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stats.isFile()

Returns: <boolean> Returns true if the <fs.Stats> object describes a regular file.

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stats.isSocket()

Returns: <boolean> Returns true if the <fs.Stats> object describes a socket.

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stats.isSymbolicLink()

Returns: <boolean> Returns true if the <fs.Stats> object describes a symbolic link. This method is only valid when using fs.lstat().

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stats.dev

<number> | <bigint> The numeric identifier of the device containing the file.

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stats.ino

<number> | <bigint> The file system specific "Inode" number for the file.

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stats.mode

<number> | <bigint> A bit-field describing the file type and mode.

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stats.nlink

<number> | <bigint> The number of hard-links that exist for the file.

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stats.uid

<number> | <bigint> The numeric user identifier of the user that owns the file (POSIX).

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stats.gid

<number> | <bigint> The numeric group identifier of the group that owns the file (POSIX).

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stats.rdev

<number> | <bigint> A numeric device identifier if the file represents a device.

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stats.size

<number> | <bigint> The size of the file in bytes. If the underlying file system does not support getting the size of the file,this will be 0.

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stats.blksize

<number> | <bigint> The file system block size for i/o operations.

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stats.blocks

<number> | <bigint> The number of blocks allocated for this file.

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stats.atimeMs

<number> | <bigint> The timestamp indicating the last time this file was accessed expressed inmilliseconds since the POSIX Epoch.

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stats.mtimeMs

<number> | <bigint> The timestamp indicating the last time this file was modified expressed inmilliseconds since the POSIX Epoch.

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stats.ctimeMs

<number> | <bigint> The timestamp indicating the last time the file status was changed expressedin milliseconds since the POSIX Epoch.

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stats.birthtimeMs

<number> | <bigint> The timestamp indicating the creation time of this file expressed inmilliseconds since the POSIX Epoch.

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stats.atimeNs

<bigint> Only present when bigint: true is passed into the method that generatesthe object.The timestamp indicating the last time this file was accessed expressed innanoseconds since the POSIX Epoch.

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stats.mtimeNs

<bigint> Only present when bigint: true is passed into the method that generatesthe object.The timestamp indicating the last time this file was modified expressed innanoseconds since the POSIX Epoch.

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stats.ctimeNs

<bigint> Only present when bigint: true is passed into the method that generatesthe object.The timestamp indicating the last time the file status was changed expressedin nanoseconds since the POSIX Epoch.

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stats.birthtimeNs

<bigint> Only present when bigint: true is passed into the method that generatesthe object.The timestamp indicating the creation time of this file expressed innanoseconds since the POSIX Epoch.

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stats.atime

<Date> The timestamp indicating the last time this file was accessed.

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stats.mtime

<Date> The timestamp indicating the last time this file was modified.

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stats.ctime

<Date> The timestamp indicating the last time the file status was changed.

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stats.birthtime

<Date> The timestamp indicating the creation time of this file.

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Stat time values

The atimeMs, mtimeMs, ctimeMs, birthtimeMs properties arenumeric values that hold the corresponding times in milliseconds. Theirprecision is platform specific. When bigint: true is passed into themethod that generates the object, the properties will be bigints,otherwise they will be numbers.

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Class: fs.StatFs

Provides information about a mounted file system. Objects returned from fs.statfs() and its synchronous counterpart are ofthis type. If bigint in the options passed to those methods is true, thenumeric values will be bigint instead of number.

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statfs.bavail

<number> | <bigint> Free blocks available to unprivileged users.

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statfs.bfree

<number> | <bigint> Free blocks in file system.

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statfs.blocks

<number> | <bigint> Total data blocks in file system.

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statfs.bsize

<number> | <bigint> Optimal transfer block size.

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statfs.ffree

<number> | <bigint> Free file nodes in file system.

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statfs.files

<number> | <bigint> Total file nodes in file system.

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statfs.type

<number> | <bigint> Type of file system.

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Class: fs.WriteStream

Extends <stream.Writable> Instances of <fs.WriteStream> are created and returned using thefs.createWriteStream() function.

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Event: 'close'

Emitted when the <fs.WriteStream>'s underlying file descriptor has been closed.

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Event: 'open'

fd <integer> Integer file descriptor used by the <fs.WriteStream>. Emitted when the <fs.WriteStream>'s file is opened.

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Event: 'ready'

Emitted when the <fs.WriteStream> is ready to be used. Fires immediately after 'open'.

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writeStream.bytesWritten

The number of bytes written so far. Does not include data that is still queuedfor writing.

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writeStream.close([callback])

callback <Function>err <Error> Closes writeStream. Optionally accepts acallback that will be executed once the writeStreamis closed.

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writeStream.path

The path to the file the stream is writing to as specified in the firstargument to fs.createWriteStream(). If path is passed as a string, thenwriteStream.path will be a string. If path is passed as a <Buffer>, thenwriteStream.path will be a <Buffer>.

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writeStream.pending

<boolean> This property is true if the underlying file has not been opened yet,i.e. before the 'ready' event is emitted.

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fs.constants

<Object> Returns an object containing commonly used constants for file systemoperations.

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FS constants

The following constants are exported by fs.constants and fsPromises.constants. To use more than one constant, use the bitwise OR | operator. Example:

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Ordering of callback and promise-based operations

Because they are executed asynchronously by the underlying thread pool,there is no guaranteed ordering when using either the callback orpromise-based methods. For example, the following is prone to error because the fs.stat()operation might complete before the fs.rename() operation:

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File paths

Most fs operations accept file paths that may be specified in the form ofa string, a <Buffer>, or a <URL> object using the file: protocol.

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String paths

String paths are interpreted as UTF-8 character sequences identifyingthe absolute or relative filename. Relative paths will be resolved relativeto the current working directory as determined by calling process.cwd(). Example using an absolute path on POSIX:

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File URL paths

For most node:fs module functions, the path or filename argument may bepassed as a <URL> object using the file: protocol.

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Buffer paths

Paths specified using a <Buffer> are useful primarily on certain POSIXoperating systems that treat file paths as opaque byte sequences. On suchsystems, it is possible for a single file path to contain sub-sequences thatuse multiple character encodings. As with string paths, <Buffer> paths maybe relative or absolute:

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Per-drive working directories on Windows

On Windows, Node.js follows the concept of per-drive working directory. Thisbehavior can be observed when using a drive path without a backslash. Forexample fs.readdirSync('C:\\') can potentially return a different result thanfs.readdirSync('C:'). For more information, seethis MSDN page.

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File descriptors

On POSIX systems, for every process, the kernel maintains a table of currentlyopen files and resources. Each open file is assigned a simple numericidentifier called a file descriptor. At the system-level, all file systemoperations use these file descriptors to identify and track each specificfile. Windows systems use a different but conceptually similar mechanism fortracking resources. To simplify things for users, Node.js abstracts away thedifferences between operating systems and assigns all open files a numeric filedescriptor.

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Threadpool usage

All callback and promise-based file system APIs (with the exception offs.FSWatcher()) use libuv's threadpool. This can have surprising and negativeperformance implications for some applications. See theUV_THREADPOOL_SIZE documentation for more information.

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File system flags

The following flags are available wherever the flag option takes astring. '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.

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