easywasi

A working, zero-dependency implementation of WASI preview1, with lots of filesystem options. It will work in places other than a browser, but that is the primary target. It's easy, light, simple, effortless to modify/extend, and should work with any WASI-enabled wasm and any modern js runtime.

install

Install into your bundled project with npm i easywasi. You can also use it on the web, directly from cdn:

<script type="module">
import WasiPreview1 from 'https://esm.sh/easywasi'
</script>

And I like to use sourcemaps:

<script type="importmap">
  {
    "imports": {
      "easywasi": "https://esm.sh/easywasi",
      "@zenfs/core": "https://esm.sh/@zenfs/core",
      "@zenfs/dom": "https://esm.sh/@zenfs/dom",
      "@zenfs/zip": "https://esm.sh/@zenfs/zip"
    }
  }
</script>
<script type="module">
import { WasiPreview1 } from 'easywasi'
import { configure, InMemory, fs } from '@zenfs/core'
import { IndexedDB } from '@zenfs/dom'
import { Zip } from '@zenfs/zip'
</script>

usage

You can use it without a filesystem, like this:

import WasiPreview1 from 'easywasi'

const wasi_snapshot_preview1 = new WasiPreview1()

const {instance: { exports }} = await WebAssembly.instantiateStreaming(fetch('example.wasm'), {
  wasi_snapshot_preview1,
  // your imports here
})

wasi_snapshot_preview1.start(exports)

To really unlock it's power, though, give it an fs instance, like from zen-fs. Here is an example that will mount a zip file to /zip, in-memory storage to /tmp, and IndexedDB to /home.

Things to note:

• / has the default in-memory backend.
• /mnt is a bit special in zenfs, and not traversed by a file-list, so if you want that, put it somewhere else

import WasiPreview1 from 'easywasi'
import { configure, InMemory } from '@zenfs/core'
import { IndexedDB } from '@zenfs/dom'
import { Zip } from '@zenfs/zip'

await configure({
  mounts: {
    '/zip': { backend: Zip, data: await await fetch('mydata.zip').then(r => r.arrayBuffer()) },
    '/tmp': InMemory,
    '/home': IndexedDB
  }
})

// here, you could use fs to modify filesystem however you need (write files, make directories, etc)

const wasi_snapshot_preview1 = new WasiPreview1({fs})

const {instance: { exports }} = await WebAssembly.instantiateStreaming(fetch('example.wasm'), {
  wasi_snapshot_preview1,
  // your imports here
})

wasi_snapshot_preview1.start(exports)

Have a look in example to see how I fit it all together.

Keep in mind, you can eaasily override every function yourself, too, like if you want to implement the socket-API, which is the only thing I left out:

import {defs, WasiPreview1} from 'easywasi'

class WasiPreview1WithSockets = {
  constructor(options={}) {
    super(options)
    // do something with options to setup socket
  }

  // obviously implement these however
  sock_accept (fd, flags) {
    return defs.ERRNO_NOSYS
  }
  sock_recv (fd, riData, riFlags) {
    return defs.ERRNO_NOSYS
  }
  sock_send (fd, siData, riFlags) {
    return defs.ERRNO_NOSYS
  }
  sock_shutdown (fd, how) {
    return defs.ERRNO_NOSYS
  }
}

// usage
const wasi_snapshot_preview1 = new WasiPreview1WithSockets({fs})

const {instance: { exports }} = await WebAssembly.instantiateStreaming(fetch('example.wasm'), {
  wasi_snapshot_preview1
})
wasi_snapshot_preview1.start(exports)

Have a look at WasiPreview1 to figure out how to implement it, if you want things to work differently.

inspiration

• this article has some nice initial ideas
• this article has some good WASI imeplentations
• browser-wasi-shim has a very nice interface, and this is basically the same in JS, but using more extensible filesystem, and I improved a few little things.