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    WebAssembly. Scary name, exciting applications.

Table of contents
  1. A problem
    1. Demo
  2. A solution
    1. Demo
  3. How
  4. The main thread is still needed
    1. Demo
  5. Importing packages
  6. Third-party-scripts
  7. Performance
    1. Load
      1. Transferring objects
  8. It's time to stop
  9. Show progress
    1. Demo
  10. WebAssembly
    1. WASM threading
  11. Advent of Code Solver
  12. Realworld examples
  13. Comlink
  14. Do it

Underused gems of the web: WebWorkers

Underused gems of the web: WebWorkers

The browsers we use to surf the web are amazing pieces of technology. But they do a lot of work to make it possible for me to watch cat videos.

Over the years, website have gotten orders of magnitude more complicated. Without going into detail, many sites are now classified as “apps”. They’re no longer static documents. Those sites have to do much more work.

Nowadays, each tab in a browser has its own thread.

Within that tab, you are bound to that one thread (pretty much) that does nearly everything.

It parses the document, executes JavaScript, lays out all things and paints them to the screen. All these steps take time.

Oh, it also processes user interactions (things like a hover). It’s a lot of work, and we keep adding to it.

A large part of the work is (in)directly caused by executing JavaScript.

A problem

JavaScript is a single threaded language.

That’s a problem.

JavaScript was designed around the concept of having access to a single thread.

This greatly simplifies a lot of programming in JavaScript (before writing the post, I had literally never thought about race conditions in JS). But it also comes with a bunch of downsides.

The biggest downside comes in the form of user experience.

When that main thread is busy, it’s can’t do anything else, like responding to your clicks or scrolls.


This demo has a button that blocks the main thread for three seconds with a while loop. During the time the browser is blocked, several things are noticible.

  1. You can no longer select text
  2. The JS Animation froze
  3. Your browser cursor, and the hover styles don’t change
  4. You cannot increment the counter on the page

A second button exists that doesn’t fully block the browser for three seconds, but causes what’s known as “jank”. It causes some chunks of JavaScript work. During each chunk, the browser is unresponsive as before, but in between, it can ship a frame and react to user actions.

That is why you can still increment the counter. The animation doesn’t come to a complete standstill, but it has an ugly “start-stop”-behaviour.

Main thread

Main thread

Main thread

JS Animation


walking rainbow corgi



A solution

In other words, not threading makes your event listeners sad.

Eventhough I just said JavaScript is a single threaded language and doesn’t have threads, you can still use multiple threads on the web to do work in parallel!

The browser has the Web Worker API for this.

It’s not a JavaScript -the language- thing, it’s a browser thing.

A Web Worker executes your code in a new thread that has an entire seperate execution context. You can think of it like a seperate tab without a UI. It shares nothing with the main thread.

It’s has its own event loop, but this loop isn’t responsible for shipping frames to the browser, only the main thread is.

While blocking the event loop on the main thread will stop new frames from being shipped, blocking a Web Worker leaves the main thread free to respond to user interaction and ship new frames.

A real-world analogue would be a street with two lanes. One lane represents the main thread and the second lane represents a web worker. The second lane is a dedicated bus lane. When the main lane is blocked, that doesn’t affect the bus lane and vice versa.


The same demo as before, only this time the first two buttons do not affect the main thread, but a worker thread.

The main thread is entirely unaffected.

I promise that a worker thread is doing the same heavy work as the main thread did in the previous demo. This isn’t a magic trick. Open your system monitor and you will see a jump in CPU usage when you click on a button.

Web Worker

Web Worker

Main thread

JS Animation


walking rainbow corgi




The Web Worker API has been around for a long time. Since IE10.

The browser support for web workers is excellent, it’s even better than the browser support for fetch.

A worker can be created by passing the path to a seperate JavaScript file to its constructor.

const worker = new Worker("./worker.js");

Everything in that file will be executed in a seperate thread. Because of the required isolation between the main thread and a worker thread, the threads don’t have access to any variables or other code from eachother.

This combined with the fact that workers have an entirely seperate execution context means they don’t have access to a bunch of things you expect to “just be there”, like the DOM. That means web workers can’t update the UI directly!

An other result of that isolation is that the threads have to exchange data by passing messages.

This is done through the postMessage API. It will copy the message, and fire an event in the other thread with that copied data.

postMessageEventLoopMain threadWorker threadEventLooppostMessage

On the main thread, the postMessage function is available on that worker object you got from calling the new Worker() constructor.

In the worker file, postMessage is one of the functions global to the worker.

On both sides, you can listen to the "message" event this causes. The data you provided as an argument to postMessage will be available on the .data property of the event.

To listen to that event, add an addEventListener method. On the main thread, that method is available on the worker object.

In the worker, it’s the same deal as with postMessage. The addEventListener method is available on the global web worker scope.

This way, the threads can still communicate with eachother while adhering to that needed strict seperation, great!

This method of using web workers can be summed up in the same way the Go programming language approaches multithreading.

Don’t communicate by sharing memory; share memory by communicating.

You can handle different messages with your favourite method of controlling code flow. In this example, I used a switch statement to respond differently to certain messages.

const worker = new Worker("./worker.js");
worker.addEventListener("message", (event) => {
addEventListener("message", (event) => {
switch ( {
case "tic": {
case "ping": {
default: {
postMessage("A hotdog is a sandwich");

This example used strings as arguments to postMessage, but it can handle way more.

postMessage uses the structured clone algorithm to copy the messages. It can handle a lot!

A message can be a Set, Map, Blob, or even some cyclical data structure!
It can’t handle functions, trying to copy those will cause an error. A class will be converted to a standard JavaScript object, losing methods in the process.

The main thread is still needed

Sometimes, even if your code doesn’t touch the DOM. You still need to run it on the main thread as a bunch of APIs aren’t available in workers.

But because we cannot move everything in a chunk of work to a worker doesn’t mean we should abandon it altogether. A web worker still might be able to perform most of the work before passing off a result to the main thread that then handles the needed mainthread work.

The following example expands on the previous one. It plays a sound-clip in response to a message of a certain type.

The messages sent to and from the worker are now objects with a type property. This is done to make it possible to send messages with seperate data attached to them.

If you ever worked with the Flux-pattern (like Redux), these messages might seem familiar.

They’re actions. In fact, the actor model is a perfect fit here.

Because Audio is one of the APIs that need the main thread, it is created and listened to on the main thread. But everything involved in getting the things that audio object needs to be created in the first place is done on the worker thread.

1const worker = new Worker("./worker.js");
3worker.addEventListener("message", (event) => {
4 switch ( {
5 // ...
6 case "hiResponse": {
7 const audio = new Audio(;
8 audio.addEventListener("ended", () => {
9 console.log("It's quiet...");
10 });
12 break;
13 }
14 default: {
15 console.log(;
16 }
17 }
20worker.postMessage({ type: "hi" });

The callback that is passed to the event listener is now async in order to be able to use await in combination with the call to fetch.

1addEventListener("message", async (event) => {
2 switch ( {
3 // ...
4 case "hi": {
5 const res = await fetch("./cereal-is-soup.json");
6 const json = await res.json();
7 // ...lots of data janitor work here...
8 const audioPath = json.audioPath;
9 postMessage({ type: "hiResponse", audioPath });
10 break;
11 }
12 // ...
13 }


Communicate with worker

Message from Worker

It's quiet

Importing packages

Using code you didn’t write is ubiquitous. Web workers originally weren’t built with module imports in mind, because modules didn’t exist yet.

That changed!

The Worker constructor takes an options object where you can specify it will use modules.

You can now use import inside a web worker, yay!

const worker = new Worker("worker.js", {
type: "module",

Expanding on the example again, let’s import a heavy machine vision library that detects what object is in a photo.

The photo is grabbed from a canvas element as an ImageData object.

1const worker = new Worker("./worker.js", { type: "module" });
3worker.addEventListener("message", (event) => {
4 switch ( {
5 // ...
6 case "detectionResult": {
7 console.log(
8 `There was ${
9 === true ? "a" : "no"
10 } sandwich in this picture`
11 );
12 break;
13 }
14 default: {
15 console.log(;
16 }
17 }
20const input = document.querySelector("input");
21const canvas = document.querySelector("canvas");
22const ctx = canvas.getContext("2d");
24input.addEventListener("change", async (e) => {
25 const file =[0];
26 const bitmap = await createImageBitmap(file);
28 canvas.height = bitmap.height;
29 canvas.width = bitmap.width;
30 ctx.drawImage(bitmap, 0, 0);
31 const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
33 worker.postMessage({ type: "isSandwich", imageData });
1import { whatIsThis } from "some-fictional-machine-vision-library";
3addEventListener("message", async (event) => {
4 switch ( {
5 // ...
6 case "isSandwich": {
7 const item = await whatIsThis(;
8 postMessage({ type: "detectionResult", result: item === "hotdog" });
9 break;
10 }
11 default: {
12 postMessage("A hotdog is a sandwich");
13 }
14 }


Including third-party scripts in your website is an other very common way JavaScript you didn’t write ends up executing. These are often code snippets for analytics, metrics, tracking, advertising, …

Executing on the main thread. Consuming valuable performance budget you might need to ensure a smooth experience.

Because these script are outside of your control, they’re a common source of frustration. You can carefully design an entire webapp that’s tuned for performance, and a (few) third-party script(s) can easily dwarf all of those efforts.

But chucking those scripts in a webworker can’t easily be done because sometimes you still need the main thread. One call to anything on window or document, and it needs to run that piece of code on the main thread.

Partytown offers a solution to that problem. It lets you run third-party scripts in a webworker.

They use an ingenious method to make synchronous calls to things on the main thread.

The method they use is explained in this video. The TL;DW is: it uses a synchronous XMLHttpRequest (remember those?) to block the worker thread until data the script needs from the main thread arrives.


Runtime performance is just as important as startup performance.

If your app loads in 2ms but stutters on every action I try to take, it’s not a performant app.

It might score straight 100s on a Lighthouse test, but using it will suck.

In a talk from way back in 2014, Paul Lewis discusses runtime performance.

Performance doesn’t always mean doing work faster. I consider a responsive page that does a task slightly slower more performant than a blocked page that completes the task a little faster.

Using more than one thread won’t make your app any faster if the logic it uses isn’t parallellizable. On the contrary, calls to postMessage aren’t magic, they aren’t free.

In most cases, using a web worker means moving a chunk of work to a worker thread unchanged. The overall amount of work will rise slightly due to the added overhead between the worker and the main thread. The big difference is that the main thread is able to react to user events and ship updated frames while that chunk of work is happening in a worker.

This behaviour has a significant impact on how an app feels. Sometimes, slower feels faster.

It’s often better to make the user wait a little bit longer than to drop a frame.

  • The time to drop a frame is on the order of milliseconds.
  • The time to maka a user wait is on the oder os 100s of milliseconds.

This becomes even more important when you consider the devices your code will run on are unknown to you.

The web is the most accessible UI in the world.

Your code might be ran on a high-end desktop computer, or on a low-end smartphone. A task that took the machine you write code on 5ms to complete might take 100ms on a different device.

You don’t know, you can’t know.

Different screens run at different refresh rates. 60Hz is a very popular frequency for a screen (60 images per second). But that’s far from a universal number. And guess what, that number determines the time that is available for a frame to either be shown on the screen, or not.

For a 60Hz screen, that time is 16.666ms. Remember, everything has to happen inside that timeframe if you want the browser to ship a frame: from running your JS, to calculating the layout, to painting the screen and compositing.

Got a screen with a faster refresh rate? Cool. Every frame is shorter now and you have less time to not drop a frame.

The slower the computer/phone running your code is, the less work it will be able to do within that time.

Dropping a frame here and there might not be a big deal, it might even be unnoticable to visitors of the site. But do it too often and you’ve got a case of my pick for word of the year: jank.


Remember that time I had that mini-rant about loadtime performance? Of course you do, it was a paragraph ago. This is the part where I tell you that you can have your cake and eat it too.

When you move logic to a worker thread, you are not only moving execution costs off the main thread. If that logic uses a module the main thread doesn’t need, you’re also moving parsing costs off the main thread.

This makes that initial load performance better. Your UI thread is ready to show something faster: better First Contentful Paint and Time to Interactive.

David East dropped 95% of their bundle size by loading the heaviest part of their site in a web worker.


About that cost of a call to postMessage. Surma wrote a great post called is postMessage slow.

The TL;DR is: Nope. Payloads up to 10KiB are risk-free. (and 10 kilobytes is quite a lot)

Transferring objects

In situations where the message you want to send is huge, calls to postMessage can become slow, because copying a huge object takes time. Thankfully, there is a method to make these calls fast again, very fast, regardless of the message-size.

Transferable objects can be sent to a different thread near instantly.

These objects aren’t copied. Instead, a pointer to the same piece of memory is created.

Because the required isolation between threads in JavaScript to ensure thread safety, transferred objects are no longer available to the thread that sent them after they are transferred.

In the last example in this post, we sent an image through postMessage. Because that image could be huge in size, cloning all that data might take a large amount of time. It’s a great candidate to be transferred instead of copied.

- worker.postMessage({ type: "isSandwich", imageData });
+ worker.postMessage({ type: "isSandwich", imageData }, [
+ ]);

It’s time to stop

A great feature of web workers is the ability to stop them at a whim. No. Really.

A web worker might be responsible for a type of work that can block an entire thread while an algorithm crunches numbers.

If a new input from the main thread comes in that causes the same type of work, a blocked thread won’t be able to start until it’s done with the previous work.

That’s where a .terminate() on the worker comes in. It can be called on the main thread and stops a web worker regardless of what it’s doing. In the middle of a million long for-loop? Don’t care, it will stop the web worker immediately.

That means a partially done heavy calculation that became useless because of a new user input can be stopped, saving wasted CPU cycles.

A neat pattern is terminating a worker that’s busy and spinning up a new identical one to handle a new input.

A way to stop the worker from within itself exists on the global scope of the worker called close. It’s annoying these two methods don’t have the same name, but, eeeeh 🤷‍♀️, web standards are weird sometimes.

Be it when a user navigates away from a page, or after a set idle period, you should terminate web workers that aren’t used. They do consume some memory after all.

The advice for event listeners also applies to web workers: clean up folks!

Show progress

Performing a large task in a web worker provides the opportunity to show the user that something is happening.

While a multi-second calculation is happening, the web worker can periodically call postMessage. The main thread is then free to display those updates to the screen. Be it in the form of a progress bar or a partially rendered picture.

If that task was done on the main thread, the UI would be frozen, and there would be no indication anything is really happening. (No, a CSS spinner you started before the operation doesn’t count.)


In this demo, a button starts a three second long while loop that increments a counter.

The web worker sends a message with the progress so far every 50,000 ticks.

addEventListener("message", (msg) => {
let count = 0;
const startTime =;
while ( < startTime + 3000) {
count += 1;
// communicate progress
if (count % 50_000 === 0) {
// final count

The message listener on the main thread updates the UI with the received number.

Trying to do the counting on the main thread would block the thread, preventing any updates from being shown.

Main Thread

Web Worker

Main thread

JS Animation


walking rainbow corgi




WASM is what got me so excited about bringing heavier applications to the web in the first place.

My blogpost on WASM basically said “WASM is good, use it”.
This post is very similar, “web workers are good, use them”.

Turns out WASM and web workers are an excellent match.

WASM is a first class programming language that runs on the web.

Since WASM is synchronous, I’d say it should almost always be run in workers, excluding those situations where it needs access to some main thread-only APIs.

WASM threading

WASM is the place where I expect a second method of threading on the web I’ve largely glossed over in this post to be used most often.

Shared memory.

The concept of shared memory was isolated to a single type in JavaScript: SharedArrayBuffer.

WASM threads use this data structure under the hood enable parallellism.

The shared memory method of multithreading is very powerful and is closely related to some methods languages like C and Rust use to handle multithreading.

Because of that necessary seperation between threads on the web, it comes with a bunch of cavaets. It’s possible to do, but I’d call it anything but ergonomic.

You need to make sure you don’t touch the bits that make up that shared buffer at the same time by using Atomics. The SharedArrayBuffer object itself was disabled for a long time because of some very scary exploits named Spectre and Meltdown.

It has since been re-enabled with the requirement of making your site cross-origin isolated to protect against those exploits.

All this to say: I think the shared memory method of concurrency on the web will mostly be used by packages (like the glue-code WASM uses) under the hood. You as website author will probably never touch it directly if you don’t want to.

Case study: Advent of Code Solver

I participated in Advent of Code in 2021. It’s a series of (programming) puzzles that can be solved by any method you would like. The answer to each problem is typically a single number or a small string.

I picked the Rust language to solve those puzzles. The source code for my solutions is available to view.

Sharing your solutions is done often and is encouraged, as the goal of this event is to become better at programming. Learning from reading each others source code is one of those ways to get better at programming.

With a varying amounts of effort, getting the solution for a day consists of: copy and paste someones code, run it on your problem-input.

I wanted to lower the amount of effort needed to do that.

So I made my solutions in Rust usable on my website by using WASM and created the AoC solver.

For most days running that WASM directly on the main thread was performant enough. The calculation function took a small enough time to return. This made it seem like the results appeared immediately.

But for some days, my code wasn’t so fast and a solution would be calculating for multiple seconds before it spit out the answer 😱.

The browser being blocked for so long was unacceptable. Putting the solution code into a web worker was the answer.

Not only did my page load faster because that big WASM-package was no longer loaded on the main thread. The main thread remained responsive while a calculation was happening.

If a user decides start a new calculation, the previous one (if running) is aborted by terminating the web worker and spinning up a new one.

Before using web workers, this wasn’t possible. If you started a calculation you had to wait until it completed, with an unresponsive main thread and no sign that something was happening.

This little project is more complex than it looks, and to manage that complexity I used state machines. More specifically, Xstate.

The actor model it uses is a perfect fit for communication with your worker with postMessages. My event handlers were a single line of code that sent an action to the state machine.

The state machine handled the app logic and called postMessage when it needed to. Any messages returned from the web worker fired an action that got sent to the state machine.

This flow significantly simplified state management. I write the whole thing using a useReducer before refactoring to a state machine. It worked, but it was way more complex, and full of impossible situations it could get into.

Realworld examples

That’s all neat, but my how do I use it in my NextJs app? What about my SvelteKit app?

I made a repository filled with webworker example usages.

Good news!

Most tools are able to use the same syntax. They use the Worker constructor. The path to the file is typically provided by creating a URL with import.meta.url as a base.

const worker = new Worker(new URL("./worker.js", import.meta.url), {
name: "my-worker",
type: "module",

postMessage isn’t the most ergonomic API to use. It can be awesome (especially when used as part of the actor pattern/combined with a state machine) but you have to have take into account it’s a fire and forget function. It sends a message to an other thread, that’s it.

If you want to wait for a response to that message specifically, you have to build that logic yourself.

Comlink makes it seem like objects from a worker are accessible from the main thread. But every “thing” that’s returned is a Promise to that “thing” now.

Combined with the async/await syntax, this leads to a very nice developer experience where you don’t need to think about postMessage to use code that’s in a worker.

A lightly modified version of the example in the Comlink docs, using the npm package and a module worker:

import { wrap } from "comlink";
const counter = wrap(new Worker("./worker.js", { type: "module" }));
console.log(await counter.count); // 0
await counter.increment();
console.log(await counter.count); // 1
import { expose } from "comlink";
count: 0,
increment() {

Do it

Go and use workers!

Web workers can (and should) be used in more situations than they are today. A while ago, Surma wrote “When should you be using Web Workers?”, and the answer is pretty much “all the time”.

Every time a piece of logic can run seperately from the UI, you should ask yourself if it makes sense to put into a worker.

Your entire app state is an example of this. It’s related to the UI, but the whole process of creating and changing it doesn’t need to live on the main thread. Surma also blogged about putting React and Redux into a worker.

While they are incredibly useful, web workers aren’t silver bullets that solve all problems.

Silver bullets only work on werewolf-shaped problems

Designed and developed by Nicky Meuleman

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