The MessageEvent interface represents a message received by a target object.

This is used to represent messages in:

• Server-sent events (see EventSource.message_event).
• Web sockets (see the onmessage property of the WebSocket interface).
• Cross-document messaging (see Window.postMessage() and Window.message_event).
• Channel messaging (see MessagePort.postMessage() and MessagePort.message_event).
• Cross-worker/document messaging (see the above two entries, but also Worker.postMessage(), Worker.message_event, ServiceWorkerGlobalScope.message_event, etc.)
• Broadcast channels (see Broadcastchannel.postMessage()) and BroadcastChannel.message_event).
• WebRTC data channels (see onmessage).

The action triggered by this event is defined in a function set as the event handler for the relevant message event (e.g. using an onmessage handler as listed above).

MessageEvent()

Creates a new MessageEvent.

This interface also inherits properties from its parent, Event.

MessageEvent.data Read only

The data sent by the message emitter.

MessageEvent.origin Read only

A string representing the origin of the message emitter.

MessageEvent.lastEventId Read only

A string representing a unique ID for the event.

MessageEvent.source Read only

A MessageEventSource (which can be a WindowProxy, MessagePort, or ServiceWorker object) representing the message emitter.

MessageEvent.ports Read only

An array of MessagePort objects representing the ports associated with the channel the message is being sent through (where appropriate, e.g. in channel messaging or when sending a message to a shared worker).

This interface also inherits methods from its parent, Event.

initMessageEvent() Deprecated

Initializes a message event. Do not use this anymore — use the MessageEvent() constructor instead.

In our Basic shared worker example (run shared worker), we have two HTML pages, each of which uses some JavaScript to perform a simple calculation. The different scripts are using the same worker file to perform the calculation — they can both access it, even if their pages are running inside different windows.

The following code snippet shows creation of a SharedWorker object using the SharedWorker() constructor. Both scripts contain this:

const myWorker = new SharedWorker("worker.js");

Both scripts then access the worker through a MessagePort object created using the SharedWorker.port property. If the onmessage event is attached using addEventListener, the port is manually started using its start() method:

myWorker.port.start();

When the port is started, both scripts post messages to the worker and handle messages sent from it using port.postMessage() and port.onmessage, respectively:

first.onchange = () => {
  myWorker.port.postMessage([first.value, second.value]);
  console.log("Message posted to worker");
};

second.onchange = () => {
  myWorker.port.postMessage([first.value, second.value]);
  console.log("Message posted to worker");
};

myWorker.port.onmessage = (e) => {
  result1.textContent = e.data;
  console.log("Message received from worker");
};

Inside the worker we use the onconnect handler to connect to the same port discussed above. The ports associated with that worker are accessible in the connect event's ports property — we then use MessagePort start() method to start the port, and the onmessage handler to deal with messages sent from the main threads.

onconnect = (e) => {
  const port = e.ports[0];

  port.addEventListener("message", (e) => {
    const workerResult = `Result: ${e.data[0] * e.data[1]}`;
    port.postMessage(workerResult);
  });

  port.start(); // Required when using addEventListener. Otherwise called implicitly by onmessage setter.
};

• ExtendableMessageEvent — similar to this interface but used in interfaces that needs to give more flexibility to authors.