Creating a worker

Learn to create a worker with the Worker constructor and bind its script.

The Worker constructor

You create a worker with new Worker(url). The url is the path to a separate JavaScript file that holds the code running in the worker thread.

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

The call returns a Worker instance. You send messages to it, listen for messages and, when done, close it with terminate(). You can start several workers on the same page; each runs in its own thread.

Dedicated worker and its file

A worker started with new Worker() is called a dedicated worker: it is bound only to the page that created it. When the page closes, the worker ends too.

The worker file is a regular JavaScript file loaded separately from the main thread. The browser fetches it following the same-origin policy. Loading a worker from another origin requires importScripts or CORS headers.

Exercise Start and close the sorter worker

Assign a new Worker from 'sorter.js' to a worker variable. Then close it with worker.terminate().

Show example solution

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

Test yourself

Close the worker
```
worker.____();
```
Show answer
```
worker.terminate();
```
Which statement is true about the worker file?
- It can access the DOM directly
- It can read variables from the main thread
- It is loaded following the same-origin policy
- It can contain HTML
Show answer

It is loaded following the same-origin policy — Correct! The worker file is a regular JS file bound by the same-origin policy.
Worker constructor parts

new Worker(url)

creates a worker
terminate()

closes the worker
sorter.js

worker file
dedicated worker

page-bound

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