The postMessage() method of the ServiceWorker interface sends a message to the worker. This accepts a single parameter, which 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.

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.

options Optional

An optional object containing a transfer field with an array of transferable objects to transfer ownership of.

transfer Optional

An optional array of transferable objects to transfer ownership of. If the ownership of an object is transferred, it becomes unusable in the context it was sent from and becomes available only to the worker it was sent to.

Transferable objects are instances of classes like ArrayBuffer, MessagePort or ImageBitmap objects that can be transferred. null is not an acceptable value for transfer.

None (undefined).

SyntaxError

Thrown if the message parameter is not provided.

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.

• 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.