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

• In-memory Node.js fs API
• In-memory browser File System (Access) API
• fs to File System (Access) API adapter
• File System (Access) API to fs adapter
• Directory snapshot utility
• print directory tree to terminal
• Code reference
• Test coverage

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

• 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

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