fizz_buzz.wat

(module
  (import "console" "write_bytes_utf8" (func $write_bytes_utf8 (param i32 i32)))
  (import "console" "write_object" (func $write_object (param i32)))
  (import "console" "write_newline" (func $write_newline))
  (import "console" "write_space" (func $write_space))

  (import "js" "mem" (memory 1))

  (data (i32.const 0)  "Fizz")
  (data (i32.const 10) "Buzz")

  (func $main 
    (local $i i32)

    (loop $loop
      local.get $i
      i32.const 1
      i32.add
      local.set $i

      local.get $i
      call $write_object
      call $write_space

      local.get $i
      i32.const 3
      i32.rem_s
      i32.const 0
      i32.eq
      (if
        (then
          i32.const 0
          i32.const 4
          call $write_bytes_utf8
          call $write_space))

      local.get $i
      i32.const 5
      i32.rem_s
      i32.const 0
      i32.eq
      (if
        (then
          i32.const 10
          i32.const 4
          call $write_bytes_utf8
          call $write_space))

      call $write_newline

      local.get $i
      i32.const 100
      i32.lt_s
      br_if $loop))

  (export "main" (func $main)))

main.js

const { readFileSync } = require("fs");

const memory = new WebAssembly.Memory({ initial: 1 });

let write_bytes_utf8 = (offset, length) => {
  const bytes = new Uint8Array(memory.buffer, offset, length);
  const string = new TextDecoder("utf8").decode(bytes);
  process.stdout.write(string);
};

let write_newline = () => {
  process.stdout.write("\n");
};

let write_space = () => {
  process.stdout.write(" ");
};

let write_object = (object) => {
  process.stdout.write(String(object));
};

let foreign_object = {
  console: {
    write_bytes_utf8: write_bytes_utf8,
    write_object: write_object,
    write_newline: write_newline,
    write_space: write_space,
  },
  js: {
    mem: memory,
  },
};

const run = async () => {
  const buffer = readFileSync("./test.wasm");
  const module = await WebAssembly.compile(buffer);
  const instance = await WebAssembly.instantiate(module, foreign_object);
  console.log(instance.exports.main());
};

run();