Web/API/Worker/onmessage

The onmessage property of the Worker interface represents an EventHandler, that is a function to be called when the message event occurs. These events are of type MessageEvent and will be called when the worker's parent receives a message (i.e. from the DedicatedWorkerGlobalScope.postMessage method).

Note: The message payload is available in the message event's data property.

Syntax

myWorker.onmessage = function(e) { ... }

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 that 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, an onmessage handler is used to the 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 (DedicatedWorkerGlobalScope).

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 'Worker.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 4	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 11	Safari iOS Full support 5.1	Samsung Internet Android Full support 1.0

Legend

Full support: Full support