DedicatedWorkerGlobalScope: message event
The message
event is fired on a DedicatedWorkerGlobalScope
object when the worker receives a message from its parent (i.e. when the parent sends a message using Worker.postMessage()
).
This event is not cancellable and does not bubble.
Syntax
Use the event name in methods like addEventListener()
, or set an event handler property.
js
addEventListener("message", (event) => {});
onmessage = (event) => {};
Event type
A MessageEvent
. Inherits from Event
.
Event properties
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 aWindow
,MessagePort
, orServiceWorker
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).
Example
The following code snippet shows creation of a Worker
object using the Worker()
constructor. Messages are passed to the worker when the value inside the form input first
changes. An onmessage
handler is also present, to deal with messages are passed back from the worker.
js
// main.js
const myWorker = new Worker("worker.js");
first.onchange = () => {
myWorker.postMessage([first.value, second.value]);
console.log("Message posted to worker");
};
// worker.js
self.onmessage = (e) => {
console.log("Message received from main script");
const workerResult = `Result: ${e.data[0] * e.data[1]}`;
console.log("Posting message back to main script");
postMessage(workerResult);
};
In the main.js
script, an onmessage
handler is used to handle messages from the worker script:
js
// main.js
myWorker.onmessage = (e) => {
result.textContent = e.data;
console.log("Message received from worker");
};
Alternatively, the script can listen for the message using addEventListener()
:
js
// worker.js
self.addEventListener("message", (e) => {
result.textContent = e.data;
console.log("Message received from worker");
});
Notice how in the main script, onmessage
has to be called on myWorker
, whereas inside the worker script you just need onmessage
because the worker is effectively the global scope (DedicatedWorkerGlobalScope
).
For a full example, see our Basic dedicated worker example (run dedicated worker).
Specifications
Specification |
---|
HTML Standard # event-message |
HTML Standard # handler-dedicatedworkerglobalscope-onmessage |
Browser compatibility
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