Real sleep() in JavaScript

The JavaScript language is single-threaded, which means that blocking that single thread for any period of time will prevent importing things like input handling and rendering.

...but about blocking the thread when that thread is a Web Worker?

It's possible to implement sleep() in JavaScript. A naive implementation of sleep() might look like this:

function sleep(t) {  
  const start =;
  while ( - start < t);

This solution has an obvious problem: even if we do want to block a thread for a period of time, doing so using a loop will consume all of the CPU time available to the thread. For a Web Worker, that's generally going to be 100% of one of your machine's cores. The goal with sleep() is generally to pause our code for a period of time, but this naive implementation actually causes our code to take as much time as possible - the opposite of what we need!

XMLHttpRequest, old friend

Fortunately, there is another way to block a thread from JavaScript in the browser: synchronous XMLHttpRequest. Like a long-running while() loop, Synchronous XMLHttpRrequest blocks the thread and has a uniquely negative impact on performance and UX. However, a Web Worker isn't a UI thread though, and there are cases where being able to "pause" a worker thread is genuinely useful. One that I'm particularly interested in is imposing resource limitations on modules.

We can use Synchronous XMLHttpRequest to pause JavaScript execution while waiting on a network resource. Requesting a URL thousands of times isn't ideal though, and since sleep(t) accepts a duration in milliseconds it's important to be able to "unpause" the thread after that amount of time has passed. One way to "unpause" is to set the .timeout property of the Synchronous XMLHttpRequest to our delay:

function sleep(t) {  
  const xhr = new XMLHttpRequest();
  xhr.timeout = Math.max(t, 10);'GET', '/some-url-that-loads-forever', false);
  try {
    xhr.send();  // ignore the timeout error
  } catch (e) {}

If the URL requested above happens to return a response before we reach the timeout though, our sleep() function won't sleep long enough. We could use a loop to continually re-request the resource until we reach the specified sleep duration, but this solution would result in a potentially massive number of network requests. Deploying this in production would be a good way to DDoS whichever web server or CDN happens to be the target of those requests - not something I'd recommend! Thankfully, there's a way to intercept requests made by JavaScript - Service Worker.

Service Worker to prevent DDoS

A Service Worker could intercept the network requests made from our sleep() function's Synchronous XMLHttpRequest calls, preventing them from ever making their way out to the internet. Even more useful, the Service Worker can wait for a specific amount of time before returning an empty response for those network calls, which means we don't have to issue multiple requests in order to reach a given sleep(t) delay. Here's a Service Worker that does just that:

// activate immediately:
addEventListener('install', () => self.skipWaiting());  
addEventListener('activate', () => self.clients.claim());

addEventListener('fetch', e => {  
  // we only handle requests to a special /SLEEP url:
  const url = new URL(e.request.url);
  if (url.pathname !== '/SLEEP') return;

  // wait ?t=X milliseconds, then return a 304:
  e.respondWith(new Promise(resolve => {
    const t = new URLSearchParams('t');
    const response = new Response(null, {status:304});
    setTimeout(resolve, t, response);

Once registered by calling navigator.serviceWorker.register('/sw.js'), the Service Worker will begin intercepting requests to a special /SLEEP?t=0 URL. All we have to do is modify the sleep() function to request that URL with the correct duration:

function sleep(t) {  
  t = Math.max(10, t);
  const xhr = new XMLHttpRequest();
  xhr.timeout = t;'GET', `/SLEEP?t=${t}`, false);
  try{ x.send(); } catch(e) {}

Even better: Atomics.wait()

As pointed out by my astute co-workers Derek and Shu, the somewhat recent JavaScript addition Atomics.wait() implements an optimized sleep() solution while waiting for a value to be changed within a SharedArrayBuffer. We can use this in a Worker thread to implement sleep extremely effectively, without any Synchronous network requests or a Service Worker:

const AB = new Int32Array(new SharedArrayBuffer(4));  
function sleep(t) {  
  Atomics.wait(AB, 0, 0, Math.max(1, t|0));

Seeing it work

Great! Now we can use sleep(500) to pause a Web Worker for 500ms, without increased CPU usage:

Interested? Try the demo for yourself on Glitch.