Exporting a function from wasm

Let’s write a small wasm module which only exports one function named double. This function doubles its argument.

You can write it with any language that can be compiled to wasm, but I wrote it in wbt (WebAssembly Text Format) directly.

(module
  (func $double (param $n i32) (result i32)
    local.get $n
    local.get $n
    i32.add
  )
  (export "double" (func $double))
)

This module exports double function which takes one i32 as a parameter and returns i32. It pushes the parameter to the stack twice using local.get, and call i32.add which pops these values from the stack, adds them and pushes the result to the stack. This value on the stack will be returned to the caller of double directly (In WebAssembly, we return a value from a function by pushing it to the stack).

When you name this file double.wbt and run wbt2wasm double.wbt, you’ll get double.wasm.

Now, let’s call this function. This time, we’ll call it from JavaScript.

import { readFile } from 'fs/promises';

const wasm = await readFile('double.wasm');
const module = await WebAssembly.instantiate(wasm);
const double = module.instance.exports.double;
const result = double(5);
console.log(result);

When you instantiate this module using WebAssembly.instantiate, you can get a function via its instance.exports property.

Saving this as export.mjs and running node export.mjs will print 10.