In-memory file-system with Node's fs API.

Usage no npm install needed!

<script type="module">
  import memfs from '';



In-memory file-system with Node's fs API.

  • Node's fs API implemented, see API Status
  • Stores files in memory, in Buffers
  • Throws sameish* errors as Node.js
  • Has concept of i-nodes
  • Implements hard links
  • Implements soft links (aka symlinks, symbolic links)
  • Permissions may* be implemented in the future
  • Can be used in browser, see memfs-webpack


npm install --save memfs


import { fs } from 'memfs';

fs.writeFileSync('/hello.txt', 'World!');
fs.readFileSync('/hello.txt', 'utf8'); // World!

Create a file system from a plain JSON:

import { fs, vol } from 'memfs';

const json = {
  './': '1',
  './src/index.js': '2',
  './node_modules/debug/index.js': '3',
vol.fromJSON(json, '/app');

fs.readFileSync('/app/', 'utf8'); // 1
vol.readFileSync('/app/src/index.js', 'utf8'); // 2

Export to JSON:

vol.writeFileSync('/', 'sudo rm -rf *');
vol.toJSON(); // {"/": "sudo rm -rf *"}

Use it for testing:

vol.writeFileSync('/foo', 'bar');
expect(vol.toJSON()).toEqual({ '/foo': 'bar' });

Create as many filesystem volumes as you need:

import { Volume } from 'memfs';

const vol = Volume.fromJSON({ '/foo': 'bar' });
vol.readFileSync('/foo'); // bar

const vol2 = Volume.fromJSON({ '/foo': 'bar 2' });
vol2.readFileSync('/foo'); // bar 2

Use memfs together with unionfs to create one filesystem from your in-memory volumes and the real disk filesystem:

import * as fs from 'fs';
import { ufs } from 'unionfs';


ufs.readFileSync('/foo'); // bar

Use fs-monkey to monkey-patch Node's require function:

import { patchRequire } from 'fs-monkey';

vol.writeFileSync('/index.js', 'console.log("hi world")');
require('/index'); // hi world


See also

  • spyfs - spies on filesystem actions
  • unionfs - creates a union of multiple filesystem volumes
  • linkfs - redirects filesystem paths
  • fs-monkey - monkey-patches Node's fs module and require function
  • libfs - real filesystem (that executes UNIX system calls) implemented in JavaScript


Unlicense - public domain.