Web/API/DedicatedWorkerGlobalScope/onmessage

The onmessage property of the DedicatedWorkerGlobalScope interface represents an EventHandler to be called when the message event occurs and bubbles through the Worker — i.e. when a message is sent to the worker using the Worker.postMessage method.

Syntax

self.onmessage = function() { ... };

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. A Worker.onmessage handler is also present, to deal with messages are passed back from the worker.

var myWorker = new Worker("worker.js");

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

myWorker.onmessage = function(e) {
  result.textContent = e.data;
  console.log('Message received from worker');
}

In the worker.js script, a DedicatedWorkerGlobalScope.onmessage handler is used to handle messages from the main script:

onmessage = function(e) {
  console.log('Message received from main script');
  var workerResult = 'Result: ' + (e.data[0] * e.data[1]);
  console.log('Posting message back to main script');
  postMessage(workerResult);
}

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 (the DedicatedWorkerGlobalScope, in this case).

For a full example, see ourBasic dedicated worker example ([[../../../../../../../mdn.github.io/simple-web-worker/index|run dedicated worker]]).

Specifications

Specification	Status	Comment
HTML Living StandardThe definition of 'DedicatedWorkerGlobalScope.onmessage' in that specification.	Living Standard

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`onmessage`	Chrome Full support 4	Edge Full support 12	Firefox Full support 3.5	IE Full support 10	Opera Full support 10.6	Safari Full support 4	WebView Android Full support 37	Chrome Android Full support Yes	Firefox Android Full support 4	Opera Android Full support 11	Safari iOS Full support 5.1	Samsung Internet Android Full support Yes

Legend

Full support: Full support