streamich/memfs

Node.js "fs" and browser "File System API" adapters and in-memory implementations 👍

2,093

stars

2,152

commits

TypeScript

primary language

Sep 9, 2026

updated

jsonjoy.com/libs/memfs

browserfs

cas

crud

file

file-system

filesystem

fsa

in-memory

memfs

memory

node

opfs

webfs

Browse cluster: ESLint plugins and browser compatibility →

README

memfs

JavaScript file system utilities for Node.js and browser. Implementation of in-memory Node.js fs module API and in-memory browser File System API, as well as adapters from fs module to File Sytem API and from File Sytem API to fs module.

Install

npm i memfs

Resources

Watching

All file-watching APIs are implemented on both sides of the fs and File System API divide:

Feature	Package	Notes
`fs.watch` — `recursive`, `encoding`, `signal`, `throwIfNoEntry`, `ignore`	`@jsonjoy.com/fs-node`	`'error'`/`'close'` events, `ref()`/`unref()`
`fs.watchFile` / `fs.unwatchFile`	`@jsonjoy.com/fs-node`	polling, `bigint`, Node's reappearance semantics
`fs.promises.watch` async iterator	`@jsonjoy.com/fs-node`	`maxQueue`, `overflow`, `AbortError` on abort
`FileSystemObserver`	`@jsonjoy.com/fs-fsa`	deterministic OPFS profile: precise records, real `"moved"` records, microtask batching
`fs.watch`/`fs.watchFile` over a real FSA directory	`@jsonjoy.com/fs-fsa-to-node`	works over OPFS in Chrome 133+ via the native observer
`FileSystemObserver` over any `fs`	`@jsonjoy.com/fs-node-to-fsa`	best-effort profile: renames stat-classified, no `"moved"` records

Intentional divergences from real watchers (the in-memory backend is deterministic, so platform unreliability is not emulated):

Paths are watched semantically rather than by inode: after delete-and-recreate, memfs reports events for the recreated entry, while a real POSIX watcher keeps watching the dead inode.
Exactly one event per logical operation — no platform duplicate events.
filename is never null.
The Node-style FSWatcher delivers events synchronously inside the mutating operation, whereas real Node defers to the event loop; the FileSystemObserver batches per microtask, as the spec requires.

Demos

License

Apache 2.0

Contributors

(top 30 of 68)

989 commits

530 commits

152 commits

141 commits

Similar repos

zen-fs/core

A filesystem, anywhere.

422

shared topics

spf13/afero

The Universal Filesystem Abstraction for Go

6,696

shared topics

paulmillr/readdirp

Recursive version of fs.readdir with small RAM & CPU footprint.

420

shared topics

isomorphic-git/lightning-fs

A lean and fast 'fs' for the browser

610

mrmlnc/fast-glob

:rocket: It's a very fast and efficient glob library for Node.js

2,823

shared topics

thecodrr/fdir

⚡ The fastest directory crawler & globbing library for NodeJS. Crawls 1m files in < 1s

1,734

shared topics

streamich/git-cz

Semantic Git commits

1,876

shared contributors

stipsan/ioredis-mock

Emulates ioredis by performing all operations in-memory.

391

shared contributors

Languages

TypeScript

98.6%

streamich/memfs

Node.js "fs" and browser "File System API" adapters and in-memory implementations 👍

2,093

stars

2,152

commits

TypeScript

primary language

Sep 9, 2026

updated

jsonjoy.com/libs/memfs

browserfs

cas

crud

file

file-system

filesystem

fsa

in-memory

memfs

memory

node

opfs

webfs

Browse cluster: ESLint plugins and browser compatibility →

README

memfs

Install

npm i memfs

Resources

Watching

All file-watching APIs are implemented on both sides of the fs and File System API divide:

Feature	Package	Notes
`fs.watch` — `recursive`, `encoding`, `signal`, `throwIfNoEntry`, `ignore`	`@jsonjoy.com/fs-node`	`'error'`/`'close'` events, `ref()`/`unref()`
`fs.watchFile` / `fs.unwatchFile`	`@jsonjoy.com/fs-node`	polling, `bigint`, Node's reappearance semantics
`fs.promises.watch` async iterator	`@jsonjoy.com/fs-node`	`maxQueue`, `overflow`, `AbortError` on abort
`FileSystemObserver`	`@jsonjoy.com/fs-fsa`	deterministic OPFS profile: precise records, real `"moved"` records, microtask batching
`fs.watch`/`fs.watchFile` over a real FSA directory	`@jsonjoy.com/fs-fsa-to-node`	works over OPFS in Chrome 133+ via the native observer
`FileSystemObserver` over any `fs`	`@jsonjoy.com/fs-node-to-fsa`	best-effort profile: renames stat-classified, no `"moved"` records

Intentional divergences from real watchers (the in-memory backend is deterministic, so platform unreliability is not emulated):

Paths are watched semantically rather than by inode: after delete-and-recreate, memfs reports events for the recreated entry, while a real POSIX watcher keeps watching the dead inode.
Exactly one event per logical operation — no platform duplicate events.
filename is never null.
The Node-style FSWatcher delivers events synchronously inside the mutating operation, whereas real Node defers to the event loop; the FileSystemObserver batches per microtask, as the spec requires.