ServiceWorkerGlobalScope: message event

The message event of the ServiceWorkerGlobalScope interface occurs when incoming messages are received. Controlled pages can use the ServiceWorker.postMessage() method to send messages to service workers. The service worker can optionally send a response back via the Client.postMessage(), corresponding to the controlled page.

This event is not cancelable and does not bubble.

Syntax

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("message", (event) => { })

onmessage = (event) => { }

Event type

An ExtendableMessageEvent. Inherits from ExtendableEvent.

Event properties

Inherits properties from its parent, ExtendableEvent.

ExtendableMessageEvent.data Read only

Returns the event's data. It can be any data type. If dispatched in messageerror event, the property will be null.

ExtendableMessageEvent.origin Read only

Returns the origin of the Client that sent the message.

ExtendableMessageEvent.lastEventId Read only

Represents, in server-sent events, the last event ID of the event source.

ExtendableMessageEvent.source Read only

Returns a reference to the Client object that sent the message.

ExtendableMessageEvent.ports Read only

Returns the array containing the MessagePort objects representing the ports of the associated message channel.

Examples

In the below example a page gets a handle to the ServiceWorker object via ServiceWorkerRegistration.active, and then calls its postMessage() function.

// main.js
if (navigator.serviceWorker) {
  navigator.serviceWorker.register("service-worker.js");

  navigator.serviceWorker.addEventListener("message", (event) => {
    // event is a MessageEvent object
    console.log(`The service worker sent me a message: ${event.data}`);
  });

  navigator.serviceWorker.ready.then((registration) => {
    registration.active.postMessage("Hi service worker");
  });
}

The service worker can receive the message by listening to the message event:

// service-worker.js
addEventListener("message", (event) => {
  // event is an ExtendableMessageEvent object
  console.log(`The client sent me a message: ${event.data}`);

  event.source.postMessage("Hi client");
});

Alternatively, the script can listen for the message using onmessage:

// service-worker.js
self.onmessage = (event) => {
  // event is an ExtendableMessageEvent object
  console.log(`The client sent me a message: ${event.data}`);

  event.source.postMessage("Hi client");
};

Specifications

Browser compatibility

See also

• Using Service Workers
• Service workers basic code example
• Using web workers