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

Readme

Source

memfs

A fs API to work with virtual in-memory files.

var memfs = require('memfs');

var mem = new memfs.Volume;
mem.mountSync('./', {
    "test.js": "console.log(123);",
    "dir/hello.js": "console.log('hello world');"
});

console.log(mem.readFileSync('./dir/hello.js').toString());

Use it together with unionfs:

var unionfs = require('unionfs');
var fs = require('fs');

// Create a union of two file systems:
unionfs
    .use(fs)
    .use(mem);
    
// Now `unionfs` has the `fs` API but on both file systems.
console.log(unionfs.readFileSync('./test.js').toString()); // console.log(123);
    
// Replace `fs` with the union of those file systems.
unionfs.replace(fs);

// Now you can do this.
console.log(fs.readFileSync('./test.js').toString()); // console.log(123);

// ... and this:
require('./test.js'); // 123

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FAQs

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Last updated on 21 Dec 2016

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