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


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


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 =;
  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: ' + ([0] *[1]);
  console.log('Posting message back to main script');

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).


HTML Standard (HTML)
# handler-dedicatedworkerglobalscope-onmessage

Browser compatibility

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See also