npm i fs-ext


Extras missing from node's fs module

by Matt Sergeant

2.0.0 (see all)TypeScript:DefinitelyTyped
npm i fs-ext


Build Status Coverage Status Windows Status

Extras not included in Node's fs module.


  • From v2.0.0 onwards, module doesn't override fs and constants Node.js core modules. Instead import functions and constants directly:

    const {flock, constants} = require('fs-ext');
    // or
    const fsExt = require('fs-ext');
    // fsExt.flock
    // fsExt.constants
  • From v1.0.0 onwards, fs.utime and fs.utimeSync have been removed. Use fs.utimes and fs.utimesSync instead.


Install via npm:

npm install fs-ext


fs-ext imports all of the methods from the core 'fs' module, so you don't need two objects.

const fs = require('fs');
const {flock} = require('fs-ext');

const fd = fs.openSync('foo.txt', 'r');
flock(fd, 'ex', (err) => {
    if (err) {
        return console.error("Couldn't lock file");
    // file is locked

For an advanced example checkout example.js.


flock(fd, flags, [callback])

Asynchronous flock(2). No arguments other than a possible error are passed to the callback. Flags can be 'sh', 'ex', 'shnb', 'exnb', 'un' and correspond to the various LOCK_SH, LOCK_EX, LOCK_SH|LOCK_NB, etc.

NOTE (from flock() man page): flock() does not lock files over NFS. Use fcntl(2) instead: that does work over NFS, given a sufficiently recent version of Linux and a server which supports locking.

flockSync(fd, flags)

Synchronous flock(2). Throws an exception on error.

fcntl(fd, cmd, [arg], [callback])

Asynchronous fcntl(2).

callback will be given two arguments (err, result).

The supported commands are:

  • 'getfd' ( F_GETFD )
  • 'setfd' ( F_SETFD )
  • 'setlk' ( F_SETLK )
  • 'getlk' ( F_GETLK )
  • 'setlkw' ( F_SETLKW )

Requiring this module adds FD_CLOEXEC to the constants module, for use with F_SETFD, and also F_RDLCK, F_WRLCK and F_UNLCK for use with F_SETLK (etc).

File locking can be used like so:

const {fnctl, constants} = require('fs-ext');

fcntl(fd, 'setlkw', constants.F_WRLCK, (err) => {
    if (!err) {
        // Lock succeeded

fcntlSync(fd, flags)

Synchronous fcntl(2). Throws an exception on error.

seek(fd, offset, whence, [callback])

Asynchronous lseek(2).

callback will be given two arguments (err, currFilePos).

whence can be 0 (SEEK_SET) to set the new position in bytes to offset, 1 (SEEK_CUR) to set the new position to the current position plus offset bytes (can be negative), or 2 (SEEK_END) to set to the end of the file plus offset bytes (usually negative or zero to seek to the end of the file).

seekSync(fd, offset, whence)

Synchronous lseek(2). Throws an exception on error. Returns current file position.

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