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Worker: message event

files/en-us/web/api/worker/message_event/index.md

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{{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.

js-nolint
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():

js
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:

js
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():

js
// static/scripts/worker.js

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

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also