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.

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 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 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 (run dedicated worker).

Specifications

Specification	Status	Comment
WHATWG HTML Living Standard The definition of 'Worker.onmessage' in that specification.	Living Standard	No change from Web Workers.
Web Workers The definition of 'Worker.onmessage' in that specification.	Editor's Draft	Initial definition.

Browser compatibility

See also

The Worker interface it belongs to.