The introductory guide to AssemblyScript

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The Introductory Guide To AssemblyScript

WebAssembly (or Wasm) is a relatively recent addition to web browsers, but it has the potential to drastically expand what the web is capable of as a platform for serving applications.

While there can be a steep learning curve for web developers to get started with WebAssembly, AssemblyScript provides a way to get around that. Let’s first take a look at why WebAssembly is such a promising technology, and then we’ll see how AssemblyScript can help to unlock its potential.


WebAssembly is a low-level language for browsers, giving developers a compilation target for the web besides JavaScript. It makes it possible for website code to run at near-native speed in a safe, sandboxed environment.

It was developed with input from representatives of all the major browsers (Chrome, Firefox, Safari, and Edge), who reached a design consensus in early 2017. All of these browsers now support WebAssembly, which is usable in about 87 percent of all browsers.

WebAssembly is delivered in a binary format, which means that it has both size and load time advantages over JavaScript. Yet it also has a textual representation that is human-readable.

When WebAssembly was first announced, some developers thought it had the potential to eventually supplant JavaScript as the primary language of the web. But it’s better to think of WebAssembly as a new tool that integrates well with the existing web platform, which is one of its high-level goals.

Rather than replacing JavaScript for existing use cases, WebAssembly is intriguing more because it enables new use cases. WebAssembly doesn’t have direct access to the DOM yet, and most existing websites will want to stick with JavaScript, which is already quite fast after going through years of optimization. Here’s a sample of WebAssembly’s own list of possible use cases:

  • Games
  • Scientific visualization and simulation
  • CAD applications
  • Image/video editing

The common attribute among these is that we would typically think of them as desktop applications. By providing near-native performance for CPU-intensive tasks, WebAssembly makes it feasible to move more of these applications to the web.

Existing websites can also benefit from WebAssembly. Figma provides a real-world example, having used WebAssembly to significantly improve their load time. If a website uses code that does a lot of computation, it can make sense to replace only that code with WebAssembly to improve performance.

So maybe now you’re interested in getting started with WebAssembly. You could learn the language itself and write it directly, but it was really intended to be a compilation target for other languages. It was designed to have good support for C and C++, Go added experimental support for it in version 1.11, and Rust is also investing heavily in it.

But maybe you don’t want to learn or use one of these languages in order to use WebAssembly. This is where AssemblyScript comes into play.


AssemblyScript is a TypeScript-to-WebAssembly compiler. Developed by Microsoft, TypeScript adds types to JavaScript. It has become quite popular, and even for people who are not familiar with it, AssemblyScript only allows for a limited subset of TypeScript features anyway, so it shouldn’t take long to get up to speed.

Because it’s so similar to JavaScript, AssemblyScript lets web developers easily incorporate WebAssembly into their websites without having to work with an entirely different language.

Trying it out

Let’s write our first AssemblyScript module (all of the following code is available in this GitHub repository). We need Node.js with a minimum version of 8 for WebAssembly support.

Change to an empty directory, create a package.json file, and install AssemblyScript. Note that we need to install it directly from its GitHub repository. It isn’t published on npm because the AssemblyScript developers don’t consider the compiler to be ready for general use yet.

mkdir assemblyscript-demo
cd assemblyscript-demo
npm init
npm install --save-dev github:AssemblyScript/assemblyscript

Generate scaffolding files using the included asinit command:

npx asinit .

Our package.json should now include these scripts:

  "scripts": {
    "asbuild:untouched": "asc assembly/index.ts -b build/untouched.wasm -t build/untouched.wat --sourceMap --validate --debug",
    "asbuild:optimized": "asc assembly/index.ts -b build/optimized.wasm -t build/optimized.wat --sourceMap --validate --optimize",
    "asbuild": "npm run asbuild:untouched && npm run asbuild:optimized"

The top-level index.js looks like this:

const fs = require("fs");
const compiled = new WebAssembly.Module(fs.readFileSync(__dirname + "/build/optimized.wasm"));
const imports = {
  env: {
    abort(_msg, _file, line, column) {
       console.error("abort called at index.ts:" + line + ":" + column);
Object.defineProperty(module, "exports", {
  get: () => new WebAssembly.Instance(compiled, imports).exports

It allows us to easily require our WebAssembly module just like a plain JavaScript module.

The assembly directory contains our AssemblyScript source code. The generated example is a simple addition function.

export function add(a: i32, b: i32): i32 {
  return a + b;

If you expect the function signature to look like add(a: number, b: number): number, as it would in TypeScript, the reason it uses i32 instead is that AssemblyScript uses WebAssembly’s specific integer and floating point types rather than TypeScript’s generic number type.

Let’s build the example.

npm run asbuild

The build directory should now include the following files:


We get plain and optimized versions of the build. For each build version, we get a .wasm binary, a source map, and a .wat textual representation of the binary. The textual representation is designed to be readable by humans, but for our purposes, we don’t need to read or understand it — part of the point of using AssemblyScript is that we don’t need to work with raw WebAssembly.

Fire up Node and use our compiled module just like any other module.

$ node
Welcome to Node.js v12.10.0.
Type ".help" for more information.
> const add = require('./index').add;
> add(3, 5)

And that’s all it took to call WebAssembly from Node!

Add a watch script

For development, I recommend using onchange to automatically rebuild the module whenever you change the source code because AssemblyScript doesn’t include a watch mode yet.

npm install --save-dev onchange

Add an asbuild:watch script to package.json. Include the -i flag to run an initial build as soon as you run the command.

  "scripts": {
    "asbuild:untouched": "asc assembly/index.ts -b build/untouched.wasm -t build/untouched.wat --sourceMap --validate --debug",
    "asbuild:optimized": "asc assembly/index.ts -b build/optimized.wasm -t build/optimized.wat --sourceMap --validate --optimize",
    "asbuild": "npm run asbuild:untouched && npm run asbuild:optimized",
    "asbuild:watch": "onchange -i 'assembly/**/*' -- npm run asbuild"

Now you can run asbuild:watch instead of having to constantly rerun asbuild.


Let’s write a basic benchmark test to get an idea of what kind of performance boost we can get. WebAssembly’s specialty is handling CPU-intensive tasks like numerical calculations, so let’s go with a function for determining whether an integer is a prime number or not.

Our reference implementation looks like this. It’s a naive, brute-force solution because our goal is to perform a high number of calculations.

function isPrime(x) {
    if (x < 2) {
        return false;

    for (let i = 2; i < x; i++) {
        if (x % i === 0) {
            return false;

    return true;

The equivalent AssemblyScript version just needs some type annotations:

function isPrime(x: u32): bool {
    if (x < 2) {
        return false;

    for (let i: u32 = 2; i < x; i++) {
        if (x % i === 0) {
            return false;

    return true;

We’ll use Benchmark.js.

npm install --save-dev benchmark

Create benchmark.js:

const Benchmark = require('benchmark');

const assemblyScriptIsPrime = require('./index').isPrime;

function isPrime(x) {
    for (let i = 2; i < x; i++) {
        if (x % i === 0) {
            return false;

    return true;

const suite = new Benchmark.Suite;
const startNumber = 2;
const stopNumber = 10000;

suite.add('AssemblyScript isPrime', function () {
    for (let i = startNumber; i < stopNumber; i++) {
}).add('JavaScript isPrime', function () {
    for (let i = startNumber; i < stopNumber; i++) {
}).on('cycle', function (event) {
}).on('complete', function () {
    const fastest = this.filter('fastest');
    const slowest = this.filter('slowest');
    const difference = ('hz') -'hz')) /'hz') * 100;
    console.log(`${'name')} is ~${difference.toFixed(1)}% faster.`);

On my machine, I got these results when I ran node benchmark:

AssemblyScript isPrime x 74.00 ops/sec ±0.43% (76 runs sampled)
JavaScript isPrime x 61.56 ops/sec ±0.30% (64 runs sampled)
AssemblyScript isPrime is ~20.2% faster.

Note that this test is a microbenchmark, and we should be careful about reading too much into it.

For some more involved AssemblyScript benchmarks, I recommend checking out this WasmBoy benchmark and this wave equation benchmark.

Loading the module

Next, let’s use our module in a website. Create index.html:

<!DOCTYPE html>
        <meta charset="utf-8" />
        <title>AssemblyScript isPrime demo</title>
        <form id="prime-checker">
            <label for="number">Enter a number to check if it is prime:</label>
            <input name="number" type="number" />
            <button type="submit">Submit</button>

        <p id="result"></p>

        <script src="demo.js"></script>

Create demo.js. There are multiple ways to load WebAssembly modules, but the most efficient is to compile and instantiate them in a streaming manner with the WebAssembly.instantiateStreaming function. Note that we need to provide an abort function, which is called if an assertion fails.

(async () => {
    const importObject = {
        env: {
            abort(_msg, _file, line, column) {
                console.error("abort called at index.ts:" + line + ":" + column);
    const module = await WebAssembly.instantiateStreaming(
    const isPrime = module.instance.exports.isPrime;

    const result = document.querySelector("#result");
    document.querySelector("#prime-checker").addEventListener("submit", event => {
        result.innerText = "";
        const number =;
        result.innerText = `${number} is ${isPrime(number) ? '' : 'not '}prime.`;

Now install static-server. We need a server because in order to use WebAssembly.instantiateStreaming, the module needs to be served with a MIME type of application/wasm.

npm install --save-dev static-server

Add a script to package.json.

  "scripts": {
    "serve-demo": "static-server"

Run npm run serve-demo and open the localhost URL in a browser. Submit a number in the form, and you should get a message indicating whether the number is prime or not. Now we’ve gone all the way from writing AssemblyScript to actually using it in a website.


WebAssembly, and by extension AssemblyScript, is not going to magically make every website faster, but that was never the point. WebAssembly is exciting because it can make the web viable for a much larger set of applications.

Similarly, AssemblyScript makes WebAssembly accessible for more developers, making it easy for us to stick with JavaScript by default but bring in WebAssembly when we have work that requires lots of number crunching.

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2 Replies to “The introductory guide to AssemblyScript”

  1. it returns an array made up of suits whenever i invoke this.filter(‘fastest’) or this.filter(‘slowest’) ,and resulting in the value of difference is always NaN, im so confused

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