Worker: message event

{{APIRef("Web Workers API")}} {{AvailableInWorkers("window_and_worker_except_service")}} 

The message event is fired on a {{domxref('Worker')}}  object when the worker’s parent receives a message from its worker (i.e., when the worker sends a message using DedicatedWorkerGlobalScope.postMessage()).

This event is not cancellable and does not bubble.

Syntax

Use the event name in methods like {{domxref("EventTarget.addEventListener", "addEventListener()")}} , or set an event handler property.

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

onmessage = (event) => { }

Event type

A {{domxref("MessageEvent")}} . Inherits from {{domxref("Event")}} .

{{InheritanceDiagram("MessageEvent")}} 

Event properties

This interface also inherits properties from its parent, {{domxref("Event")}} .

• {{domxref("MessageEvent.data")}}  {{ReadOnlyInline}} 
  • : The data sent by the message emitter.
• {{domxref("MessageEvent.origin")}}  {{ReadOnlyInline}} 
  • : A string representing the origin of the message emitter.
• {{domxref("MessageEvent.lastEventId")}}  {{ReadOnlyInline}} 
  • : A string representing a unique ID for the event.
• {{domxref("MessageEvent.source")}}  {{ReadOnlyInline}} 
  • : A MessageEventSource (which can be a {{glossary("WindowProxy")}} , {{domxref("MessagePort")}} , or {{domxref("ServiceWorker")}}  object) representing the message emitter.
• {{domxref("MessageEvent.ports")}}  {{ReadOnlyInline}} 
  • : An array of {{domxref("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).

Examples

This code creates a new worker and listens to messages from it using addEventListener():

const worker = new Worker("static/scripts/worker.js");

worker.addEventListener("message", (event) => {
  console.log(`Received message from worker: ${event.data}`);
});

Alternatively, it could listen using the onmessage event handler property:

const worker = new Worker("static/scripts/worker.js");

worker.onmessage = (event) => {
  console.log(`Received message from worker: ${event.data}`);
};

The worker posts messages using self.postMessage():

// static/scripts/worker.js

self.postMessage("I'm alive!");

Specifications

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

• Related events: messageerror.
• DedicatedWorkerGlobalScope.postMessage().