μWASI

This library provides a WASI implementation for Node.js and browsers in a tree-shaking friendly way. The system calls provided by this library are configurable.

With minimal configuration, it provides WASI system calls which just return WASI_ENOSYS.

Features

• No dependencies
• Tree-shaking friendly
  • 3 KB when minimal configuration
  • 6 KB when all features enabled
• Almost compatible interface with Node.js WASI implementation
• Well tested, thanks to wasi-test-suite by Casper Beyer

Installation

npm install uwasi

Example

With all system calls enabled

import { WASI, useAll } from "uwasi";
import fs from "node:fs/promises";

async function main() {
    const wasi = new WASI({
        args: process.argv.slice(2),
        features: [useAll()],
    });
    const bytes = await fs.readFile(process.argv[2]);
    const { instance } = await WebAssembly.instantiate(bytes, {
        wasi_snapshot_preview1: wasi.wasiImport,
    });
    const exitCode = wasi.start(instance);
    console.log("exit code:", exitCode);

/* With Reactor model
    wasi.initialize(instance);
*/
}

main()

With no system calls enabled

import { WASI, useAll } from "uwasi";

const wasi = new WASI({
    features: [],
});

With environ, args, clock, proc, and random enabled

import { WASI, useArgs, useClock } from "uwasi";

const wasi = new WASI({
    args: ["./a.out", "hello", "world"],
    features: [useEnviron, useArgs, useClock, useProc, useRandom()],
});

With fd (file descriptor) enabled only for stdio

By default, stdin behaves like /dev/null, stdout and stderr print to the console.

import { WASI, useStdio } from "uwasi";

const wasi = new WASI({
    features: [useStdio()],
});

You can use custom backends for stdio by passing handlers to useStdio.

import { WASI, useStdio } from "uwasi";

const inputs = ["Y", "N", "Y", "Y"];
const wasi = new WASI({
    features: [useStdio({
        stdin: () => inputs.shift() || "",
        stdout: (str) => document.body.innerHTML += str,
        stderr: (str) => document.body.innerHTML += str,
    })],
});

By default, the stdout and stderr handlers are passed strings. You can pass outputBuffers: true to get Uint8Array buffers instead. Along with that, you can also pass Uint8Array buffers to stdin.

import { WASI, useStdio } from "uwasi";
const wasi = new WASI({
    features: [useStdio({
        outputBuffers: true,
        stdin: () => new Uint8Array([1, 2, 3, 4, 5]),
        stdout: (buf) => console.log(buf),
        stderr: (buf) => console.error(buf),
    })],
});

Implementation Status

Some of WASI system calls are not implemented yet. Contributions are welcome!

Syscall	Status	Notes
args_XXX	✅
clock_XXX	✅	Monotonic clock is unavailable due to JS API limitation
environ_XXX	✅
fd_XXX	🚧	stdin/stdout/stderr are supported
path_XXX	❌
poll_oneoff	❌
proc_XXX	✅
random_get	✅
sched_yield	❌
sock_XXX	❌