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

JavaScript file system utilities for Node.js and browser.

Install

npm i memfs

Resources

• Documentation
  • In-memory Node.js fs API
  • experimental fs to File System Access API adapter
  • experimental File System Access API to fs adapter
  • experimental crudfs a CRUD-like file system abstraction
  • experimental casfs Content Addressable Storage file system abstraction
  • Directory snapshot utility
  • print directory tree to terminal
• Code reference
• Test coverage

Demos

• Git in browser, which writes to a real folder
• Git in browser, which writes to OPFS file system
• Git on in-memory file system
• fs in browser, creates a .tar file in real folder
• fs in browser, synchronous API, writes to real folder
• crudfs and casfs in browser and Node.js interoperability

See also

• unionfs - creates a union of multiple filesystem volumes
• fs-monkey - monkey-patches Node's fs module and require function
• linkfs - redirects filesystem paths
• spyfs - spies on filesystem actions

License

Apache 2.0

Changelog

4.17.2 (2025-05-15)

Performance Improvements

• use promises rather than setImmediate so that I/O is executed within the current task stack (#1100) (786072f)