WebAssembly Beginner Workshop

My recap of what I learned with Jem Young regarding WebAssembly

Taran "tearing it up" Bains • February 20, 2021


Image by Comfreak from Pixabay

The Why

True to form, I like to start with why (something I picked up from Simon Sinek) and thus, a question that I was wanting to have answered in this workshop was why should I, as a Front-end Engineer, care about WebAssembly (WASM).

In a nutshell 🌰, WASM is a low-level language that is meant to be a compile target for high-level languages. It is designed to be portable and ran in many different environments. It’s a compliment to JavaScript… not a replacement, ye damn trolls 👹. Admittedly, my day to day is spent working in one of the highest of the high-level languages (JavaScript) but the “high-level” languages referred to above would be language such as C++, Java, Rust, etc.

At the time of this blog post, all major browsers ship with a WebAssembly global object that allow us to load in and utilize WASMs.

So again, why, as a Front-end Engineer, do I care about the fact that people who know Rust and other traditionally non client side languages can now ship code that can run in the browser? The answer is simple: we can now offload work that JavaScript has been historically terrible at to a module written in a more memory efficient and battle tested language.

Listen, I love JavaScript, but like all things in the world, it has its shortcomings; never forget, [] == ![] evaluates to true… how the heck does the empty array equal the parody of the empty array 🤣. I mean, I know why and I’ll post it at the bottom of this article, but if you’re just picking up the language and you saw this piece of code, you’d maybe just say, “fuck it, I don’t want to JS”. So WASM make senses for things that require memory efficiency and powerful computing capabilities, including, but not limited to:

  1. Graphics
  2. Compression
  3. Video/Image Processing
  4. Physics simulations

During my day to day, I don’t have to worry about creating amazing visual animations or processing massive images to create unique browser experiences, but I do know that if I tried to do this in native JavaScript, the experience and the resulting code would be clunky and bug ridden.

For the Javascripter Who Wants to Write Wasm

If you’re like me and other Front-end Engineers, you’re probably proficient with JavaScript and know a little bit about another language but not enough to do any crazy hectic computing with that secondary language. So how does someone like myself get started with WASM without having to put in the backbreaking work of refamiliarising myself or learning a new programming language? The answer: AssemblyScript

AssemblyScript is a variant of TypeScript that allows us to easily compile to WASM without learning a new language (yay) 😊! That being said, this doesn’t mean, assuming you’re someone who hasn’t delved into languages like C/C++, that we don’t have to learn some new things and think about our programs differently. The big thing that WASM and AssemblyScript ask of the programmer is to think about memory allocation and how said memory is being used; if you’re a native JS dev, you’ve never probably had to think about garbage collection and efficiency (unless you’re a performance engineer).

Here’s an example of what I’m talking about:

// somefile.js
export function minusOne(n) {
  return n - 1;

// assembly/index.ts
export function minusOne(n: i32): i32 {
  return n - 1;

Did you notice the difference? It was subtle! The second minusOne had the type parameter for the n argument set as i32 and the return type as i32… but what the heck is i32?. Essentially, the i32 is a data type defined in WASM and it stands for 32-bit integer. In JavaScript, we’re generally not thinking about how many bits comprise our numbers… a number is a number, right? No. Not right. Even if we’ve never thought about it, the fact remains, the JavaScript Number type is actually a 64 bit signed floating point number; this means numbers in JavaScript can’t be greater than 2^53 - 1 (it’s not 2^64 because some bits have to be reserved for headers etc).

WASM and AssemblyScript demand that the programmer think about memory allocation and memory efficiency… it comes with the territory.

Enough about the theory, give me some code…


Step 1 - Installs

I’m assuming you have the latest version of node installed (we need a version > 14). Create a new directory to huck all this code.

$ mkdir ./wasm-fun && cd ./wasm-fun
$ npm init --yes
$ npm i --save-dev assemblyscript
$ npx asinit .
$ npm run asbuild
# need this for serving content since we need
# to set a header of application/wasm on wasm files
$ npm i express

Step 2 - The Files

Your directory structure should look something like this

├── asconfig.json
├── assembly
├── build
├── index.js
├── node_modules
├── package-lock.json
├── package.json
└── tests

In the package.json make these changes so that we utilize the glue code that comes with the AssemblyScript loader:

"asbuild:untouched": "asc assembly/index.ts --target debug --exportRuntime",
"asbuild:optimized": "asc assembly/index.ts --target release --exportRuntime"

In the assembly directory, copy this into index.ts

export function minusOne(n: i32): i32 {
  return n - 1;

In the root create the following files:

// Basic interface to abstract away loading logic (glue code essentially)
class WasmLoader {
  constructor() {
    // functions we want available as globals to wasm modules
    this._imports = {
      env: {
        abort() {
          throw new Error("Abort called from wasm file");
      index: {
        log(x) {

  async wasm(path, imports = this._imports) {
    console.log(`fetching ${path}`);

    // loader being the assembly script loader that we imported in index.html
    if (!loader.instantiateStreaming) {
      return this.wasmFallback(path, imports);

    const instance = await loader.instantiateStreaming(fetch(path), imports);

    return instance?.exports;

  // since safari doesn't have great WASM support
  async wasmFallback(path, imports) {
    console.log("using fallback");
    const response = await fetch(path);
    const bytes = await response?.arrayBuffer();
    const instance = await loader.instantiate(bytes, imports);

    return instance?.exports;
// this is needed so we can serve application/wasm
const express = require('express');
const app = express();
// Serve static files from root (note: do not do this in production code)

app.listen(3000, () => console.log('Server up on port 3000!'));
<!-- index.html -->
<!DOCTYPE html>
  <div id="main"></div>
  <!-- Need this to actually be able to use assembly script in the browser -->
  <script src="https://cdn.jsdelivr.net/npm/@assemblyscript/loader/umd/index.js"></script>

  <script src=/js/loader.js></script>
    const WL = new WasmLoader();
      .then(instance => {
          const { minusOne } = instance;

Step 3 - Build and Run

First, use node to run your server with node server.js so that when you hit localhost:3000, you’ll be served something.

Then, we need to actually compile our wasm code so run npm run asbuild.

Finally, visit localhost:3000 and behold the glory.

Closing Thoughts

WASM is a pretty damn cool idea that will make web applications way better in the future! But, again, like a lot of things in computer science, it depends on the use case. If I were to create a case management application, for example, it may be more performant to do it in AssemblyScript but not as programatically friendly as something done in JavaScript/TypeScript. However, if I was making something like a virtual tour viewer like for a museum or something, I’d consider doing some of the processing in WASM and the navigation or other items in JavaScript.

Where to learn more:

What’s Going on with[] == ![]

  1. Remember that there is operator precedence
  2. First the ! Operator does Boolean([]) which evaluates to true
  3. Then the negate operator says: I need to give you the parody of this value
  4. So now the right side is false
  5. So what we have is [] == false.

Now both left and right are coerced into Number as == prefers numbers. So what we get is 0 == 0.