emscripten - How to use WebAssembly from node.js?

Question

I am currently working on a personal Node.js (>=8.0.0) project which requires me to call C subroutines (to improve execution time). I am trying to use WebAssembly to do this since I need my final code to be compatible when opened in a browser.

I have used Emscripten to compile C code into WebAssembly, and do not know how to proceed after this.

Any help in the right direction would be great. Thanks!

Answer 1

You can build a .wasm file (standalone) without JS glue file. Someone has answered the similar question.

Create a test.c file:

int add(int a, int b) {
  return a + b;
}

Build the standalone .wasm file:

emcc test.c -O2 -s WASM=1 -s SIDE_MODULE=1 -o test.wasm

Use the .wasm file in Node.js app:

const util = require('util');
const fs = require('fs');
var source = fs.readFileSync('./test.wasm');
const env = {
    memoryBase: 0,
    tableBase: 0,
    memory: new WebAssembly.Memory({
      initial: 256
    }),
    table: new WebAssembly.Table({
      initial: 0,
      element: 'anyfunc'
    })
  }

var typedArray = new Uint8Array(source);

WebAssembly.instantiate(typedArray, {
  env: env
}).then(result => {
  console.log(util.inspect(result, true, 0));
  console.log(result.instance.exports._add(9, 9));
}).catch(e => {
  // error caught
  console.log(e);
});

The key part is the second parameter of WebAssembly.instantiate(). Without it, you will get the error message:

TypeError: WebAssembly Instantiation: Imports argument must be present and must be an object at at process._tickCallback (internal/process/next_tick.js:188:7) at Function.Module.runMain (module.js:695:11) at startup (bootstrap_node.js:191:16) at bootstrap_node.js:612:3