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What are Web Workers in JavaScript?

Silvija Prozinger

Web Workers are a simple API that runs in the background of a web application, without affecting the performance of the application. They allow long-running scripts to handle their tasks without blocking the UI or other scripts that handle user interactions. JavaScript is a single-threaded language and Web Workers allow multi-threading.

As Web Workers run in an isolated thread, the code that they execute needs to be contained in a separate file. However, before writing a Web Worker, it’s necessary to create a new Worker object in the main page script.

The object is created using the Worker constructor:

var worker = new Worker('workerExample.js');

Web Workers and their parent page communicate via postMessage() method, which can accept either a string or JSON object as its single argument depending on the browser version.

This is how a string is passed from the main page script to the worker script:

var worker = new Worker('workerExample.js');

worker.addEventListener('message', function(event) {
  console.log('Worker prints out: ',; //the message 'Hello World' is sent through
}, false);

worker.postMessage('Hello World'); 

The workerExample.js file returns the message that the main script passed to it:

self.addEventListener('message', function(event) {
  self.postMessage(; //the worker accepts 'Hello World' through 
}, false);

There are several things to note about Web Workers:

  • Web Workers do not have access to the DOM, window object, document object or parent object.
  • They can be stopped using worker.terminate() from the main script or self.close() inside the worker script itself.
  • A Worker's global scope object has a distinct event loop and its task queues have events, callbacks, and networking activity as tasks.
  • While executing, Workers never block main event loop.



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