ZenFS

ZenFS is a file system that emulates the NodeJS filesystem API.

It works using a system of backends, which are used by ZenFS to store and retrieve data. ZenFS can also integrate with other tools.

Backends

ZenFS is modular and extensible. The core includes some built-in backends:

• InMemory: Stores files in-memory. This is cleared when the runtime ends (e.g. a user navigating away from a web page or a Node process exiting)
• Overlay: Use a read-only file system as read-write by overlaying a writable file system on top of it. (copy-on-write)
• Fetch: Downloads files over HTTP with the fetch API (readonly)
• Port: Interacts with a remote over a MessagePort-like interface (e.g. a worker)

ZenFS supports a number of other backends. Many are provided as separate packages under @zenfs. More backends can be defined by separate libraries by extending the FileSystem class and providing a Backend object.

You can find all of the packages available over on NPM.

As an added bonus, all ZenFS backends support synchronous operations. All of the backends included with the core are cross-platform.

For more information, see the docs.

Installing

npm install @zenfs/core

Usage

import { fs } from '@zenfs/core'; // You can also use the default export

fs.writeFileSync('/test.txt', 'You can do this anywhere, including browsers!');

const contents = fs.readFileSync('/test.txt', 'utf-8');
console.log(contents);

Using different and/or multiple backends

A single InMemory backend is created by default, mounted on /.

You can configure ZenFS to use a different backend and mount multiple backends. It is strongly recommended to do so using the configure function.

You can use multiple backends by passing an object to configure which maps paths to file systems.

The following example mounts a zip file to /zip, in-memory storage to /tmp, and IndexedDB to /home. Note that / has the default in-memory backend.

import { configure, InMemory } from '@zenfs/core';
import { IndexedDB } from '@zenfs/dom';
import { Zip } from '@zenfs/archives';

const res = await fetch('mydata.zip');

await configure({
	mounts: {
		'/mnt/zip': { backend: Zip, data: await res.arrayBuffer() },
		'/tmp': InMemory,
		'/home': IndexedDB,
	}
};

Note that while you aren't required to use absolute paths for the keys of mounts, it is a good practice to do so.

Tip

When configuring a mount point, you can pass in

1. A Backend object, if the backend has no required options
2. An object that has the options accepted by the backend and a backend property which is a Backend object
3. A FileSystem instance

Here is an example that mounts the WebStorage backend from @zenfs/dom on /:

import { configureSingle, fs } from '@zenfs/core';
import { WebStorage } from '@zenfs/dom';

await configureSingle({ backend: WebStorage });

if (!fs.existsSync('/test.txt')) {
	fs.writeFileSync('/test.txt', 'This will persist across reloads!');
}

const contents = fs.readFileSync('/test.txt', 'utf-8');
console.log(contents);

FS Promises

The FS promises API is exposed as promises.

import { configureSingle } from '@zenfs/core';
import { exists, writeFile } from '@zenfs/core/promises';
import { IndexedDB } from '@zenfs/dom';

await configureSingle({ backend: IndexedDB });

const exists = await exists('/myfile.txt');
if (!exists) {
	await writeFile('/myfile.txt', 'Lots of persistent data');
}

Note

You can import the promises API using:

1. Exports from @zenfs/core/promises
2. The promises export from @zenfs/core
3. fs.promises on the exported fs from @zenfs/core.

Mounting and unmounting, creating backends

If you would like to create backends without configure (e.g. to do something dynamic at runtime), you may do so by importing the backend and calling resolveMountConfig with it.

You can then mount and unmount the backend instance by using mount and umount.

import { configure, resolveMountConfig, InMemory } from '@zenfs/core';
import { IndexedDB } from '@zenfs/dom';
import { Zip } from '@zenfs/archives';

await configure({
	mounts: {
		'/tmp': InMemory,
		'/home': IndexedDB,
	},
});

fs.mkdirSync('/mnt');

const res = await fetch('mydata.zip');
const zipfs = await resolveMountConfig({ backend: Zip, data: await res.arrayBuffer() });
fs.mount('/mnt/zip', zipfs);

// do stuff with the mounted zip

fs.umount('/mnt/zip'); // finished using the zip

Caution

Instances of backends follow the internal API. You should never use a backend's methods unless you are extending a backend.

Devices and device files

Warning

This is an experimental feature. Breaking changes may occur during non-major releases. Using this feature is the fastest way to make it stable.

ZenFS includes experimental support for device files. These are designed to follow Linux's device file behavior, for consistency and ease of use. You can automatically add some normal devices with the addDevices configuration option:

await configure({
	mounts: {
		/* ... */
	},
	addDevices: true,
});

fs.writeFileSync('/dev/null', 'Some data to be discarded');

const randomData = new Unit8Array(100);

const random = fs.openSync('/dev/random', 'r');
fs.readSync(random, randomData);
fs.closeSync(random);

You can create your own devices by implementing a DeviceDriver. For example, the null device looks similar to this:

const customNullDevice = {
	name: 'custom_null',
	isBuffered: false,
	read() {
		return 0;
	},
	write() {},
};

Note the actual implementation's write is slightly more complicated since it adds to the file position. You can find more information on the docs.

Finally, if you'd like to use your custom device with the file system, you can use so through the aptly named DeviceFS.

const devfs = fs.mounts.get('/dev') as DeviceFS;
devfs.createDevice('/custom', customNullDevice);

fs.writeFileSync('/dev/custom', 'This gets discarded.');

In the above example, createDevice works relative to the DeviceFS mount point.

Additionally, a type assertion ( as ...) is used since fs.mounts does not keep track of which file system type is mapped to which mount point. Doing so would create significant maintenance costs due to the complexity of implementing it.

If you would like to see a more intuitive way adding custom devices (e.g. fs.mknod), please feel free to open an issue for a feature request.

Using with bundlers

ZenFS exports a drop-in for Node's fs module (up to the version of @types/node in package.json), so you can use it for your bundler of preference using the default export.

Sponsors

A huge thank you to Deco.cx for sponsoring ZenFS and helping to make this possible.

Building

• Make sure you have Node and NPM installed. You must have Node v18 or newer.
• Install dependencies with npm install
• Build using npm run build
• You can find the built code in dist.

Testing

Run unit tests with npm test.

BrowserFS Fork

ZenFS is a fork of BrowserFS. If you are using ZenFS in a research paper, you may want to cite BrowserFS.