A Comparison of Web Workers

Thomas Hunter II

CEO, Working on Radar.Chat

Adapted from Multithreaded JavaScript:


  1. Dedicated Workers
  2. Shared Workers
  3. Service Workers


Multithreading in JavaScript

It is the nature of JavaScript – and its
ecosystem – to be single-threaded.
  • For the longest time no multithreading in JS
  • Could pull off basic message passing via iframes
  • Now we have Web Workers and shared memory
  • Presentation from perspective of multithreading

What is a JavaScript Environment?

  • Isolated collection of variables, globals
  • Different versions of Object, prototypes
  • Each environment incurs overhead (~6MB Node.js)
  • Object instances cannot be shared
  • Serialized versions, JSON, can be passed
  • No Web Workers can access DOM, document
  • SharedArrayBuffer data can be shared
  • Hand-waving over complexities of contexts/realms

Dedicated Workers

What is a Dedicated Worker?

  • Dedicated Workers have exactly one parent
  • Can be loaded as a hierarchy
  • Each worker is a new JavaScript environment
Dedicated Workers have exactly one parent and can form a hierarchy

Dedicated Workers in the Page

console.log('hello from main.js');

const worker = new Worker('worker.js');

worker.onmessage = (msg) => {
  console.log('from worker:', msg.data);

worker.postMessage('message to worker');

console.log('hello from end of main.js');

Dedicated Workers in the Worker

// worker.js

console.log('hello from worker.js');

self.onmessage = (msg) => {
  console.log('from main:', msg.data);

  // perform a heavy calculation

  postMessage('message from worker');

Dedicated Worker Output

hello from main.js main.js
hello from end of main.js main.js
hello from worker.js worker.js
from main: message to worker worker.js
from worker: message from worker main.js

Why use a Dedicated Worker?

  • Gives access to an additional thread
  • Offload CPU intensive work
  • Prevent scroll-jank
  • Note: Worker dies when parent dies

Shared Workers

What is a Shared Worker?

  • Shared Workers can have multiple parents
  • Allows communication across same-origin windows

Shared Workers in the Page(s)

// red.html and blue.html
const worker = new SharedWorker('shared.js');

worker.port.onmessage = (event) => {
  console.log('EVENT', event.data);
  • Later, trigger a message
worker.port.postMessage('hello, world');

Shared Workers in the Worker

const ID = Math.floor(Math.random() * 999999);
console.log('shared.js', ID);
const ports = new Set();
self.onconnect = (event) => {
  const port = event.ports[0];
  console.log('CONN', ID, ports.size);
  port.onmessage = (event) => {
    console.log('MESSAGE', ID, event.data);
    for (let p of ports) {
      p.postMessage([ID, event.data]);

Shared Worker Output

shared.js 123456shared.js
CONN 123456 1shared.js
CONN 123456 2shared.js
MESSAGE 123456 hello, worldshared.js
EVENT [ 123456, "hello, world" ]red.html
EVENT [ 123456, "hello, world" ]blue.html

Why use a Shared Worker?

  • You need to communicate across pages
  • You want variable contexts to outlive a page
  • You want a cross-page singleton source of truth
  • ❌ You don't need to support Safari
    • Consider BroadcastChannel as an alternative
  • Note: Worker dies when last parent dies

Service Workers

What is a Service Worker?

  • The most complex of the Web Workers
  • Intercept / proxy requests made to server
  • Can have zero parents, run in background
  • Can share state between same-origin windows

Service Workers in the Page(s)

navigator.serviceWorker.register('/sw.js', {
  scope: '/' // URL range that worker can control

navigator.serviceWorker.oncontrollerchange = () => {
  console.log('controller change');

async function makeRequest() {
  const result = await fetch('/data.json');
  const payload = await result.json();

Service Workers in the Worker

// sw.js part 1
let counter = 0;

self.oninstall = (event) => {
  console.log('service worker install');

self.onactivate = (event) => {
  console.log('service worker activate');
  // allow immediate control of opened pages

Service Workers in the Worker

// sw.js, part 2
self.onfetch = (event) => {
  console.log('fetch', event.request.url);
  if (event.request.url.endsWith('/data.json')) {
    return void event.respondWith(
      new Response(JSON.stringify({counter}), {
        headers: { 'Content-Type': 'text/json' }
  // fallback to normal HTTP request

Service Worker Output

service worker installsw.js
service worker activatesw.js
controller changemain.js
fetch http://localhost:5000/data.jsonsw.js
Object { counter: 1 }main.js

Why use a Service Worker?

  • Cache network assets when offline
  • Perform background syncs of updated content
  • Push notifications
  • PWA / "Add to Homescreen" on Android & iOS
  • Note: Worker might die when last parent dies

Web Worker Comparison Matrix

HTTP Proxy
Parents1>= 1>= 0
DeathWith ParentLast ParentTricky

A Comparison of Web Workers