JavaScript: Web Workers

By Xah Lee. Date: . Last updated: .

2017-12-30 This page is work in progress. Ignore it.

What's Web Worker?

Web workers is a feature of browser that lets you run JavaScript process in the background in parallel.

JavaScript is single threaded. That is, all JavaScript code on a page run sequentially, including all even handler functions or setTimeout, setInterval. This is a problem, and Web workers is the solution.

Any task that requires heavy computation should run in a web worker, because otherwise it'll freeze browser.

Create Web Worker

let wk = new Worker("worker_script.js")

// receive message from worker
wk.onmessage = function (e) {
    console.log( "I'm main. I got this msg from worker: ",

// send message to worker
wk.postMessage("hey worker, take this.")

The Worker Script

here's the worker_script.js:

console.log( "this worker is happily working.")

// receive message from main
onmessage = function (e) {
    console.log( "I'm worker. I got this msg from main: ", )

// send message to main
postMessage("hey main, i need food.")

You can create many workers. 5 or 10 is typical. Worker can also create workers.

Terminate Worker

When done, you should terminate worker by:


Webworker also has a onerror property that can be useful.



but have setTimeout, setInterval, XMLHttpRequest


determine, postMessage plain string or json

JS in Depth