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

Note: that there is a disagreement among browser manufacturers about whether a data URI is of the same origin or not. Though Gecko 10.0 (Firefox 10.0 / Thunderbird 10.0 / SeaMonkey 2.7) and later accept data URIs, that's not the case in all other browsers.


var myWorker = new Worker(aURL, options);


A USVString representing the URL of the script the worker will execute. It must obey the same-origin policy.
options Optional
An object containing option properties that can be set when creating the object instance. Available properties are as follows:
  • type: A DOMString specifying the type of worker to create. The value can be classic or module. If not specified, the default used is classic.
  • credentials: A DOMString 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 omit (no credentials required).
  • name: A DOMString specifying an identifying name for the DedicatedWorkerGlobalScope representing the scope of the worker, which is mainly useful for debugging purposes.


  • A SecurityError is raised 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.
  • A NetworkError is raised if the MIME type of the worker script is incorrect. It should always be text/javascript.
  • A SyntaxError is raised if aURL cannot be parsed.


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

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

first.onchange = function() {
  console.log('Message posted to worker');

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


Specification Status Comment
HTML Living Standard
The definition of 'Worker()' in that specification.
Living Standard

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
Worker() constructorChrome Full support 4Edge Full support 12Firefox Full support 3.5IE Full support 10Opera Full support 10.6Safari Full support 4WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11Safari iOS Full support 5.1Samsung Internet Android Full support 1.0
Strict MIME type checks for worker scriptsChrome ? Edge ? Firefox Full support 70IE No support NoOpera ? Safari ? WebView Android ? Chrome Android ? Firefox Android No support NoOpera Android ? Safari iOS ? Samsung Internet Android ?
Constructor name optionChrome Full support 70Edge Full support 18Firefox Full support 55IE No support NoOpera Full support 57Safari No support No
No support No
Notes Supported in Safari Technology Preview 64
WebView Android No support NoChrome Android Full support 70Firefox Android Full support 55Opera Android Full support 49Safari iOS No support No
No support No
Notes Supported in Safari Technology Preview 64
Samsung Internet Android Full support 10.0


Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
See implementation notes.
See implementation notes.

Note: A browser can be marked as providing full support for Worker() even though it does not support worker scripts written as modules. As of Mar 1, 2019, only Chrome 80+ supports this feature, while Firefox has an open feature request.  No other browsers are known to have support for production usage of worker scripts written as modules. Without that support, worker scripts written as modules and modules used by worker scripts have to be transpiled or otherwise converted to non-module code in order to run.

See also

The Worker interface it belongs to.