memfs

What is memfs?

The memfs npm package is an in-memory filesystem that mimics the Node.js fs module. It allows you to create an ephemeral file system that resides entirely in memory, without touching the actual disk. This can be useful for testing, mocking, and various other scenarios where you don't want to perform I/O operations on the real file system.

What are memfs's main functionalities?

Creating and manipulating files

This feature allows you to create, read, and write files in memory as if you were using the native fs module.

const { Volume } = require('memfs');
const vol = new Volume();
vol.writeFileSync('/hello.txt', 'Hello, World!');
const content = vol.readFileSync('/hello.txt', 'utf8');
console.log(content); // Outputs: Hello, World!

Directory operations

This feature enables you to create directories, list their contents, and perform other directory-related operations, all in memory.

const { Volume } = require('memfs');
const vol = new Volume();
vol.mkdirSync('/mydir');
vol.writeFileSync('/mydir/file.txt', 'My file content');
const files = vol.readdirSync('/mydir');
console.log(files); // Outputs: ['file.txt']

Linking and symlinking

This feature allows you to create hard links and symbolic links, mimicking the behavior of links on a real file system.

const { Volume } = require('memfs');
const vol = new Volume();
vol.writeFileSync('/original.txt', 'Content of original');
vol.linkSync('/original.txt', '/link.txt');
vol.symlinkSync('/original.txt', '/symlink.txt');
const linkContent = vol.readFileSync('/link.txt', 'utf8');
const symlinkContent = vol.readlinkSync('/symlink.txt');
console.log(linkContent); // Outputs: Content of original
console.log(symlinkContent); // Outputs: /original.txt

File system watching

This feature provides the ability to watch for changes in the file system, similar to fs.watch in the native fs module.

const { Volume } = require('memfs');
const vol = new Volume();
const fs = vol.promises;

async function watchExample() {
  await fs.writeFile('/watched.txt', 'Initial content');
  fs.watch('/watched.txt', (eventType, filename) => {
    console.log(`Event type: ${eventType}; File: ${filename}`);
  });
  await fs.writeFile('/watched.txt', 'Updated content');
}

watchExample();

Other packages similar to memfs

mock-fs

mock-fs is a package that provides a mock file system for Node.js. It is similar to memfs in that it allows you to create an in-memory file system for testing purposes. However, mock-fs overrides the native fs module, while memfs provides a separate file system instance.

unionfs

unionfs is a package that can combine multiple file systems into a single cohesive file system interface. It can be used with memfs to overlay an in-memory file system on top of the real file system, providing a way to mix real and virtual file system operations.

fs-mock

fs-mock is another in-memory file system module for Node.js. It offers similar functionality to memfs, allowing you to mock the fs module for testing. It differs in API and implementation details, and the choice between fs-mock and memfs may come down to personal preference or specific use-case requirements.

README

memfs

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

• Node's fs API implemented, see old API Status, missing list, missing opendir
• 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

Install

npm install --save memfs

Usage

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

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

Export to JSON:

vol.writeFileSync('/script.sh', 'sudo rm -rf *');
vol.toJSON(); // {"/script.sh": "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.use(fs).use(vol);

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")');
patchRequire(vol);
require('/index'); // hi world

Docs

• Reference
• Relative paths
• API status
• Dependencies

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

License

Unlicense - public domain.

Changelog

3.4.7 (2022-06-24)

Bug Fixes

• dont patch getuid and getgid on process anymore (#847) (1c19e87)