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Wasm By Example


Hello World!

Before getting started, be sure to check out all of the languages available, by clicking the "languages" dropdown in the header.


For our first program, we will be doing a "Hello world" type of program in AssemblyScript. AssemblyScript, is essentially a TypeScript to WebAssembly compiler. This is great for TypeScript and JavaScript developers who want to write WebAssembly, without learning a new language. Though, to be clear, you can not compile our TypeScript React apps to WebAssembly with AssemblyScript because of its differences from TypeScript. But is still a great language for building WebAssembly applications.

To keep things simple with Wasm's limitations mentioned in the introduction example, instead of displaying a string, we will add two numbers together and display the result. Though, it is good to keep in mind, in later examples, a lot of these limitations will be abstracted away by your WebAssembly Language of choice (In this case, AssemblyScript).


So first, let's create our hello-world.ts AssemblyScript file:

// This exports an add function.
// It takes in two 32-bit integer values
// And returns a 32-bit integer value.
export function add(a: i32, b: i32): i32 {
  return a + b;

Then, we need the AssemblyScript Compiler, which can be installed by executing:

npm install -g assemblyscript

Now, let's compile that into a wasm module which will output a hello-world.wasm:

asc hello-world.ts -b hello-world.wasm

Next, lets create a hello-world.js JavaScript file, and add a function for loading Wasm modules using the WebAssembly Web APIs:

export const wasmBrowserInstantiate = async (wasmModuleUrl, importObject) => {
  let response = undefined;

  if (!importObject) {
    importObject = {
      env: {
        abort: () => console.log("Abort!")

  // Check if the browser supports streaming instantiation
  if (WebAssembly.instantiateStreaming) {
    // Fetch the module, and instantiate it as it is downloading
    response = await WebAssembly.instantiateStreaming(
  } else {
    // Fallback to using fetch to download the entire module
    // And then instantiate the module
    const fetchAndInstantiateTask = async () => {
      const wasmArrayBuffer = await fetch(wasmModuleUrl).then(response =>
      return WebAssembly.instantiate(wasmArrayBuffer, importObject);
    response = await fetchAndInstantiateTask();

  return response;

Next, Lets use the above Javascript function from hello-world.js to load and instantiate our wasm module, hello-world.wasm. Then, continuing with hello-world.js, we will call our exported add() function from our Wasm module:

const runWasmAdd = async () => {
  // Instantiate our wasm module
  const wasmModule = await wasmBrowserInstantiate("./hello-world.wasm");

  // Call the Add function export from wasm, save the result
  const addResult = wasmModule.instance.exports.add(24, 24);

  // Set the result onto the body
  document.body.textContent = `Hello World! addResult: ${addResult}`;

Lastly, lets load our ES6 Module, hello-world.js Javascript file in our index.html:

<!DOCTYPE html>
    <meta charset="UTF-8" />
    <title>Hello World - AssemblyScript</title>
    <script type="module" src="./hello-world.js"></script>

And we should have a working Wasm Add (Hello World) program! Congrats!

You should have something similar to the demo (Source Code) below:


Next let's take a deeper look at WebAssembly Exports.