Hello World

Wasm modules can’t do anything on their own — they need a host to run them. This page shows a few small WAT programs and how to call them from JavaScript.

A minimal module

Every WAT file starts with a (module ...) wrapper. Here’s the simplest possible module — it does nothing:

(module)

Returning a value

A function that takes no arguments and returns the number 42:

(module
  (func (export "hello") (result i32)
    (i32.const 42))
)

(export "hello") makes the function available to the host under the name "hello". (result i32) declares a 32-bit integer return value. (i32.const 42) pushes 42 onto the stack, which becomes the return value.

To run this from JavaScript:

const response = await fetch('hello.wasm');
const { instance } = await WebAssembly.instantiateStreaming(response);
console.log(instance.exports.hello()); // 42

Adding two numbers

A function that takes parameters and computes a result:

(module
  (func (export "add") (param $a i32) (param $b i32) (result i32)
    (i32.add (local.get $a) (local.get $b)))
)

(param $a i32) declares a parameter named $a. local.get reads it from the local scope. i32.add pops two values off the stack and pushes their sum.

console.log(instance.exports.add(3, 4)); // 7

Calling an import

Wasm modules can call functions provided by the host. Here the module imports a log function and calls it with the result of a computation:

(module
  (import "env" "log" (func $log (param i32)))
  (func (export "run") (param $a i32) (param $b i32)
    (call $log (i32.add (local.get $a) (local.get $b))))
)

The host must supply the import at instantiation time:

const importObject = {
  env: { log: (value) => console.log('result:', value) },
};
const response = await fetch('imports.wasm');
const { instance } = await WebAssembly.instantiateStreaming(response, importObject);
instance.exports.run(10, 20); // logs "result: 30"

Compiling and running

To go from .wat to .wasm, you can use wat2wasm from the WebAssembly Binary Toolkit or wasm-tools from the Bytecode Alliance:

wat2wasm hello.wat -o hello.wasm
# or
wasm-tools parse hello.wat -o hello.wasm

You can also try modules instantly in the browser at wat2wasm demo.

Keep learning

For a hands-on tutorial, check out watlings — a set of small exercises where you learn WAT by fixing and completing programs, similar to rustlings.