@zenfs/core/readme.md

A filesystem, anywhere

# ZenFS

ZenFS is a file system that emulates the [NodeJS filesystem API](http://nodejs.org/api/fs.html).

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 read-only file system as read-write by overlaying a writable file system on top of it. ([copy-on-write](https://en.wikipedia.org/wiki/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 at [zenfs.dev](https://zenfs.dev).

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](https://zenfs.dev/core).

## Installing

```sh
npm install @zenfs/core
```

## Usage

```js
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.

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

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

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

> [!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 `/`:

```js
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`.

```js
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`.

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

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:

```ts
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:

```ts
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`.

```ts
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.

## 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](https://github.com/jvilk/BrowserFS). If you are using ZenFS in a research paper, you may want to [cite BrowserFS](https://github.com/jvilk/BrowserFS#citing).