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 (run dedicated worker).

Specifications

Specification	Status	Comment
WHATWG HTML Living Standard The definition of 'DedicatedWorkerGlobalScope.onmessage' in that specification.	Living Standard	No change from Web Workers.
Web Workers The definition of 'DedicatedWorkerGlobalScope.onmessage' in that specification.	Candidate Recommendation	Initial definition.

Browser compatibility

See also

• The DedicatedWorkerGlobalScope interface it belongs to.
• WorkerGlobalScope