How-To: Run wasm32-wasi Output in Node.js

This guide shows how to:

• compile a Silk program to wasm32-wasi, and
• run it under Node’s built-in WASI runtime (node:wasi), and
• forward process arguments into Silk.

The wasm32-wasi backend emits a _start () -> void entrypoint that calls Silk fn main () -> int and then calls wasi_snapshot_preview1.proc_exit(exit_code).

1) Build a WASI Module#

Create main.slk:

import io from "std/io";

fn main () -> int {
  io::println("hello from silk wasm wasi");
  return 0;
}

Build:

silk build main.slk --target wasm32-wasi -o out.wasm

2) Run it with node:wasi#

Create run.js:

const fs = require('node:fs');
const { WASI } = require('node:wasi');

async function main() {
  const wasmPath = process.argv[2];
  const userArgs = process.argv.slice(3);
  const wasi = new WASI({ version: 'preview1', args: [wasmPath, ...userArgs], env: {}, preopens: {} });
  const bytes = fs.readFileSync(wasmPath);
  const { instance } = await WebAssembly.instantiate(bytes, wasi.getImportObject());

  try {
    wasi.start(instance);
  } catch (err) {
    // Some Node versions throw on proc_exit; the exit code is still available.
  }

  const exitSym = Object.getOwnPropertySymbols(wasi).find((s) => s.toString() === 'Symbol(kExitCode)');
  const code = (exitSym && typeof wasi[exitSym] === 'number') ? wasi[exitSym] : (process.exitCode ?? 0);
  process.exit(code);
}

main().catch((err) => {
  console.error(err);
  process.exit(1);
});

Run:

node --no-warnings run.js out.wasm
echo $?

Expected output:

• stdout contains hello from silk wasm wasi
• exit code is 0

3) Access argv from fn main () -> int#

The WASI entrypoint remains parameterless. Read arguments through std::args inside main:

import { current } from "std/args";
import { println } from "std/io";

fn main () -> int {
  let args = current();
  println("argc={}", args.count());
  if args.count() > 1 {
    println("argv[1]={}", args.get(1));
  }
  return 0;
}

Pass user arguments after the .wasm path:

node --no-warnings run.js out.wasm alpha beta

Silk then sees out.wasm, alpha, and beta as argv[0], argv[1], and argv[2].

Troubleshooting#

• If you see missing-import errors mentioning wasi_snapshot_preview1, confirm you built with --target wasm32-wasi (not wasm32-unknown-unknown).
• task / yield and async / await do not yet have a WASM concurrency runtime.
• WASI is a constrained environment. Shipped modules such as std::args, std::env, and parts of std::fs use the WASI runtime surface; unsupported OS facilities can still fail code generation or return runtime errors.
• For deeper backend details and entrypoint behavior, see: WASM backend.