Get started with WebAssembly using JavaScript

Get started with WebAssembly using JavaScript

WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. WebAssembly is an exciting new language that many JavaScript engines have added support for.

WebAssembly is a brand new web technology with massive potential. It will have a significant impact on how web applications are **developed **in the future.

But, sometimes, I feel like it just doesn’t want to be understood… almost in a strangely passive-aggressive kind of way.

When I look at the documentation and the handful of tutorials that are already out there, I can’t help but feel like a farmer who prayed for rain, only to drown in a flood. I technically got what I wanted… just not in the way that I’d hoped. “You want rain?! Oh, I’ll give you rain!”

This is because WebAssembly makes so many new things possible and can be implemented in so many different ways. But, it has changed so much along the way to its official MVP release in February, that when you first get started learning about it, it’s easy to drown in a sea of details.

Continuing the rain metaphor, this article is my attempt to provide a light shower of an introduction to WebAssembly. Not the concepts or the nuts and bolts, but the actual implementation.

I’ll walk you through the steps to create and implement an extremely simple project, removing complexity wherever possible. After you’ve implemented it once, however simply, a lot of those higher level ideas are a lot easier to make sense of.

Let’s break it down

Everything will be much clearer if we step back and look at a list of the steps involved in implementing **WebAssembly **in a project.

When you’re first getting started, it’s easy to look at WebAssembly and just see a big wad of options and processes. Breaking it down into discrete steps will help us get a clear picture of what’s going on:

  1. Write: Write something (or use an existing project) in C, C++, or Rust
  2. Compile: Compile it into WebAssembly (giving you a binary .wasm file)
  3. Include: Get that .wasm file into a project
  4. Instantiate: Write a bunch of asynchronous **JavaScript **that will compile the .wasm binary and instantiate it into something that **JS **can play nicely with.

And that’s pretty much it. Granted, there are different permutations of this process, but that’s the gist of it.

Broadly speaking, it’s not all that complicated. However, it can get extremely complicated, because most of these steps allow for widely varying degrees of complexity. In each case, I’m going to err on the side of bare-bones simplicity.

For our project, we’ll be writing a simple function in C++ (don’t worry if you’re not familiar with C++, it’ll be** **extremely simple). The function will return the square of a given number.

Then, we’ll compile it into .wasm using an online tool (you won’t need to download or use any command line utilities). Next, we’ll instantiate it with 14 lines of JS.

When we’re done, you’ll be able to call a function written in C++ as if it were a JS function, and you’ll be amazed!

The sheer number of possibilities that this opens up are absolutely mind blowing.


Let’s start with our C++ code. Remember, we won’t be using a local dev environment to write or compile this.

Instead, we’ll be using an online tool called WebAssembly Explorer. It’s kind of like **CodePen **for WebAssembly, and it allows you to compile your C or C++ code right in the browser and download a .wasm file all in one place.

Once you’ve opened up WebAssembly Explorer, type this C++ code into the leftmost window:

int squarer(int num) {
  return num * num;

Like I said, we’re using a really simple example here. Even if you’ve never looked at C or C++ before, it’s probably not too difficult to tell what’s going on.


Next, click the button that says “compile” in the red bar above your C++ code. Here’s what you’ll see:

The middle column shows you a human-readable version of the .wasm binary that you’ve just created. This is called “WAT” or WebAssembly Text Format.

On the right is the resultant assembly code. Pretty cool.

I won’t go into much detail about either of these, but you do need to know at least a little bit about the WAT file in order to follow the next steps.

WAT exists because we humans generally have a hard time making sense of straight binary. It’s essentially a layer of abstraction that helps you understand and interact with your WebAssembly code.

In our case, what we want to understand is how our WebAssembly refers to the function that we just created. This because we’ll need to use that exact same name in our JS file later on to refer to it.

Any functions that you write in your C++ code will be available in WebAssembly as something called an “export.” We’ll talk a bit more about this later, but for now, all you need to know is that the exports are the things that you’ll be able to interact with and use.

Take a look at the WAT file and look for the word “export.” You’ll see it twice: once alongside the word memory and again alongside the word _Z7squareri. We don’t need to know about memory for now, but we’re definitely interested in _Z7squareri.

We used the function name squarer in our C++, but now that has somehow become _z7squareri. This can definitely be confusing the first time you see it.

As far as I can tell, the “_Z7” prefix and “i” suffix are debug markers introduced by the C++ compiler. This isn’t really important to understand in depth, though. You just need to be aware that this will happen, because you need to use this exact name in your JS file in order to call your C++ function.


Now just click the “download” button at the top of the purple WAT section. You’ll get the .wasm binary file. Rename it squarer.wasm. Then create a new directory and put your squarer.wasm file in there, along with two other files:

  • index.html (boilerplate)
  • scripts.js (empty for now)


Now for the tricky part. Or, at least, the part that caused me a lot of confusion when I first started sifting through the documentation.

Although you’ll eventually be able to include .wasm modules like a regular old ES6 module (using <script type='module'> ), for the time being you need to “manually” set it up. This is done by making a bunch of asynchronous calls to the WebAssembly API. There are three steps:

  • Get your .wasm binary file into an array buffer*
  • Compile the bytes into a WebAssembly module*
  • Instantiate* the WebAssembly module

If all of this makes sense to you, then you can skip to the next section. But if you found yourself scratching your head a bit and want a more detailed explanation, then continue reading.

Array Buffer

A buffer is a temporary storage place for data while it’s being moved around. Generally, this is useful when data is being received and processed at different rates.

For example, when a video is buffering, the data is being received at a rate slower than the video player can play it. One of the things that our array buffer is doing is queueing up our binary data so that it can be compiled more easily.

But there’s something else very important going on here. In JavaScript, an array buffer is a typed array, which is something that’s used specifically for storing binary data.

The fact that it is explicitly typed means that the JS engine can interpret an array buffer much faster than it can a regular array, because it already knows the data type and doesn’t have to go through the process of figuring it out.

WebAssembly module

Once you’ve got all your binary data into an array buffer, you can compile it into a module. The WebAssembly module is, in itself, inert. It’s just the compiled binary, waiting for something to be done with it.

You can almost think of the module like a cake recipe. The recipe is just a format for storing information about how to make a cake. If you actually want a cake, you need to create an instance of the cake described in the recipe (instantiate the cake).

You do this by following the instructions laid out in the recipe. Alternatively, you could send the recipe to someone else (a “service worker”), or you could save it and use it later (“cache” it). Both of these are much more convenient to do with a recipe, than with an actual cake.


The last thing you need to do is create an instance of your WebAssembly module, which “brings it to life” and makes it actually usable.

The instance gives you access to the module’s exports (remember this from our WAT file?). This is an object that contains:

  • Memory (not relevant to us, but you can read more about it here)
  • Any functions that were present in your C++ code. This is how you will use the C++ function that you’ve written.

Finish up and run it!

Here’s the code that accomplishes all of the steps we just went over (this goes into your scripts.js file):

let squarer;

function loadWebAssembly(fileName) {
  return fetch(fileName)
    .then(response => response.arrayBuffer())
    .then(bits => WebAssembly.compile(bits))
    .then(module => { return new WebAssembly.Instance(module) });
  .then(instance => {
    squarer = instance.exports._Z7squareri;
    console.log('Finished compiling! Ready when you are...');

The loadWebAssembly() function fetches your .wasm file and then performs the operations mentioned above. Then it returns a new instance of your WebAssembly module.

Our C++ function (remember it’s referred to by the funky name that we mentioned before: _z7squareri ) lives in the exports property of our instance. You can see it being assigned to the global variable squarer on line 12. Now we can use squarer() as a regular JavaScript function!

Once you put this into your scripts.js file and hit save, you can pull it up on localhost and you should see the “Finished compiling…” message in the console.

Now, just call your function and pass in an argument from the console. Try something like squarer(9) . Hit return and you’ll see 81 . It works! You’re calling a function written in C++!

This is fantastic

You can just imagine all of the things that this makes possible.

For one, JavaScript is no longer your only option for “doing things” in the browser. That is absolutely huge.

Then there’s the performance improvements, since WebAssembly, unlike JS, runs at near-native speed.

And then there’s all the legacy code that’s now at your disposal. C and C++ have been around for a long time, and in that time, a lot of brilliant people have created some amazing open-source projects with it. Projects that can now be integrated into websites or apps.

From here, you can write more complex C, C++, or Rust code, or even adapt an existing project, and “wasm-it” into a web project.

One caveat, however, is that if you want to create functions that accept arguments or return values that are not numbers, then things start to get a bit more complicated. That’s when you’ll need to learn a bit more about the memory attribute of the .wasm instance’s exports.

This project is available on GitHub if you’d just like to clone a working copy in addition to following along with the article.

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI