Worker: Worker() constructor

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

* Some parts of this feature may have varying levels of support.

Note: This feature is available in Web Workers, except for Service Workers.

The Worker() constructor creates a Worker object that executes the script at the specified URL. This script must obey the same-origin policy.

Note: There is a disagreement among browser manufacturers about whether a data URL is of the same origin or not. Though Firefox 10 and later accept data URLs, that's not the case in all other browsers.

Syntax

new Worker(url)
new Worker(url, options)

Parameters

url

A string representing the URL of the script the worker will execute. It must obey the same-origin policy. The URL is resolved relative to the current HTML page's location.

Note: Bundlers, including webpack, Vite, and Parcel, recommend passing URLs that are relative to import.meta.url to the Worker() constructor. For example:

const myWorker = new Worker(new URL("worker.js", import.meta.url));

This way, the path is relative to the current script instead of the current HTML page, which allows the bundler to safely do optimizations like renaming (because otherwise the worker.js URL may point to a file not controlled by the bundler, so it cannot make any assumptions).

options Optional

An object containing option properties that can be set when creating the object instance. Available properties are as follows:

type: A string specifying the type of worker to create. The value can be classic or module. If not specified, the default used is classic.
credentials: A string specifying the type of credentials to use for the worker. The value can be omit, same-origin, or include. If not specified, or if type is classic, the default used is same-origin (only include credentials for same-origin requests).
name: A string specifying an identifying name for the DedicatedWorkerGlobalScope representing the scope of the worker, which is mainly useful for debugging purposes.

Exceptions

SecurityError DOMException: Thrown if the document is not allowed to start workers, e.g., if the URL has an invalid syntax or if the same-origin policy is violated.
NetworkError DOMException: Thrown if the MIME type of the worker script is incorrect. It should always be text/javascript (for historical reasons other JavaScript MIME types may be accepted).
SyntaxError DOMException: Thrown if aURL cannot be parsed.

Examples

The following code snippet shows creation of a Worker object using the Worker() constructor and subsequent usage of the object:

const myWorker = new Worker("worker.js");
const first = document.querySelector("input#number1");

first.onchange = () => {
  myWorker.postMessage(first.value);
  console.log("Message posted to worker");
};

For a full example, see our Basic dedicated worker example (run dedicated worker).