ServiceWorker: postMessage() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2018.

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is available in Web Workers.

The postMessage() method of the ServiceWorker interface sends a message to the worker. The first parameter is the data to send to the worker. The data may be any JavaScript object which can be handled by the structured clone algorithm.

The service worker can send back information to its clients by using the postMessage() method. The message will not be sent back to this ServiceWorker object but to the associated ServiceWorkerContainer available via navigator.serviceWorker.

Syntax

postMessage(message)
postMessage(message, transfer)
postMessage(message, options)

Parameters

message

The object to deliver to the worker; this will be in the data field in the event delivered to the message event. This may be any JavaScript object handled by the structured clone algorithm.

The message parameter is mandatory. If the data to be passed to the worker is unimportant, null or undefined must be passed explicitly.

transfer Optional

An optional array of transferable objects to transfer ownership of. The ownership of these objects is given to the destination side and they are no longer usable on the sending side. These transferable objects should be attached to the message; otherwise they would be moved but not actually accessible on the receiving end.

options Optional

An optional object containing the following properties:

transfer Optional: Has the same meaning as the transfer parameter.

Return value

None (undefined).

Exceptions

SyntaxError: Thrown if the message parameter is not provided.

Examples

In this example a ServiceWorker is created and a message is immediately sent:

navigator.serviceWorker.register("service-worker.js");

navigator.serviceWorker.ready.then((registration) => {
  registration.active.postMessage(
    "Test message sent immediately after creation",
  );
});

In order to receive the message, the service worker, in service-worker.js has to listen to the message event on its global scope.

// This must be in `service-worker.js`
addEventListener("message", (event) => {
  console.log(`Message received: ${event.data}`);
});

Note that the service worker can send back messages to the main thread using the postMessage() method. To receive it, the main thread needs to listen for a message event on the ServiceWorkerContainer object.

Specifications

Specification
Service Workers # dom-serviceworker-postmessage
Service Workers # dom-serviceworker-postmessage-message-options

Browser compatibility

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