ServiceWorker: postMessage() method

Secure contextAvailable in 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 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.

    [!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 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.