ServiceWorker: postMessage() method

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.

Note: A service worker is not in the same agent cluster as its client, and therefore cannot share memory. SharedArrayBuffer objects, or buffer views backed by one, cannot be posted across agent clusters. Trying to do so will generate a messageerror event containing a DataCloneError DOMException on the receiving end.

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 are not automatically sent; they must either be contained in the message or be accessible to the recipient via other means, such as MessagePort via MessageEvent.ports.

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

Browser compatibility

See also

• The ServiceWorker interface it belongs to.
• Its counterpart, the postMessage() method that a service worker must use to send a message back to the associated ServiceWorkerContainer.