SharedWorker

The SharedWorker interface represents a specific kind of worker that can be accessed from several browsing contexts, such as several windows, iframes or even workers. They implement an interface different than dedicated workers and have a different global scope, SharedWorkerGlobalScope.

Note: If SharedWorker can be accessed from several browsing contexts, all those browsing contexts must share the exact same origin (same protocol, host and port).

EventTarget SharedWorker

Constructors

SharedWorker()

Creates a shared web worker that executes the script at the specified URL.

Instance properties

Inherits properties from its parent, EventTarget.

SharedWorker.port Read only

Returns a MessagePort object used to communicate with and control the shared worker.

Events

error

Fires when an error occurs in the shared worker.

Instance methods

Inherits methods from its parent, EventTarget.

Example

In our Basic shared worker example (run shared worker), we have two HTML pages, each of which uses some JavaScript to perform a simple calculation. The different scripts are using the same worker file to perform the calculation — they can both access it, even if their pages are running inside different windows.

The following code snippet shows creation of a SharedWorker object using the SharedWorker() constructor. Both scripts contain this:

js
const myWorker = new SharedWorker("worker.js");

Note: Once a shared worker is created, any script running in the same origin can obtain a reference to that worker and communicate with it. The shared worker will be alive as long as its global scope's owner set (a set of Document and WorkerGlobalScope objects) is not empty (for example, if there is any live page holding a reference to it, maybe through new SharedWorker()). To read more about shared worker lifetime, see The worker's lifetime section of the HTML specification.

Both scripts then access the worker through a MessagePort object created using the SharedWorker.port property. If the onmessage event is attached using addEventListener, the port is manually started using its start() method:

js
myWorker.port.start();

When the port is started, both scripts post messages to the worker and handle messages sent from it using port.postMessage() and port.onmessage, respectively:

Note: You can use browser devtools to debug your SharedWorker, by entering a URL in your browser address bar to access the devtools workers inspector; for example, in Chrome, the URL chrome://inspect/#workers, and in Firefox, the URL about:debugging#workers.

js
first.onchange = () => {
  myWorker.port.postMessage([first.value, second.value]);
  console.log("Message posted to worker");
};

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

myWorker.port.onmessage = (e) => {
  result1.textContent = e.data;
  console.log("Message received from worker");
};

Inside the worker we use the onconnect handler to connect to the same port discussed above. The ports associated with that worker are accessible in the connect event's ports property — we then use MessagePort start() method to start the port, and the onmessage handler to deal with messages sent from the main threads.

js
onconnect = (e) => {
  const port = e.ports[0];

  port.addEventListener("message", (e) => {
    const workerResult = `Result: ${e.data[0] * e.data[1]}`;
    port.postMessage(workerResult);
  });

  port.start(); // Required when using addEventListener. Otherwise called implicitly by onmessage setter.
};

Specifications

Specification
HTML Standard
# shared-workers-and-the-sharedworker-interface

Browser compatibility

BCD tables only load in the browser

See also