rust-app.js

import 'regenerator-runtime/runtime'; // specific parcel (missing for async/await)

function convertUint8ArrayToString(uint8array) {
  return new TextDecoder('utf-8').decode(uint8array);
}

// based on https://github.com/wasmerio/wasmer-js/tree/master/packages/wasi#quick-start
import { WASI } from '@wasmer/wasi';
import wasiBindings from '@wasmer/wasi/lib/bindings/browser';
import { lowerI64Imports } from '@wasmer/wasm-transformer';

import { WasmFs } from '@wasmer/wasmfs';

// Instantiate a new WASI Instance
const wasmFs = new WasmFs();
window.wasmFs = wasmFs; // expose wasmFs instance to debug
let wasi = new WASI({
  // first arg is the name of the command called
  args: [
    './rust-app.wasm',
    new Date().toGMTString(),
    'Running with wasmer on the Browser (emulating File System)',
  ],
  env: {},
  bindings: {
    ...wasiBindings,
    fs: wasmFs.fs,
  },
  // same as the node example, map a fake FileSystem
  preopens: {
    '.': '.',
  },
});

const startWasiTask = async () => {
  // Fetch our Wasm File
  const response = await fetch('./rust-app.wasm');
  const responseArrayBuffer = await response.arrayBuffer();

  // Instantiate the WebAssembly file
  const wasm_bytes = new Uint8Array(responseArrayBuffer).buffer;
  const lowered_wasm = await lowerI64Imports(wasm_bytes);
  let module = await WebAssembly.compile(lowered_wasm);
  let instance = await WebAssembly.instantiate(module, {
    ...wasi.getImports(module),
  });

  // Start the WebAssembly WASI instance!
  wasi.start(instance);

  // Output what's inside of /dev/stdout!
  const stdout = await wasmFs.getStdOut();
  console.log(stdout);
};
startWasiTask()
  .then((...rest) => {
    const output = convertUint8ArrayToString(wasmFs.fs.readFileSync('tmp.txt'));
    document.querySelector('.output').textContent = output;
    console.log('Generated tmp.txt file on the location ./ 👇');
    console.log(output);
  })
  .catch((e) => {
    console.error('failure', e);
    const error = `
OOPS, there was an error, your device might not support WebAssembly
or something went wrong in the execution of the wasm file.

This demo works well on Chrome and Firefox, it does not work yet on Safari.

---

${e.message}

${e.stack}
    `;
    document.querySelector('.output').textContent = error;
  });