The postMessage() method of the DedicatedWorkerGlobalScope interface sends a message to the main thread that spawned it. This accepts a single parameter, which is the data to send to the worker. The data may be any value or JavaScript object handled by the structured clone algorithm, which includes cyclical references.

The main scope that spawned the worker can send back information to the thread that spawned it using the Worker.postMessage method.


postMessage(aMessage, transferList);


The object to deliver to the main thread; this will be in the data field in the event delivered to the Worker.onmessage handler. This may be any value or JavaScript object handled by the structured clone algorithm, which includes cyclical references.
transferList Optional
An optional array of Transferable objects to transfer ownership of. If the ownership of an object is transferred, it becomes unusable (neutered) in the context it was sent from and it becomes available only to the main thread it was sent to.
Only MessagePort and ArrayBuffer objects can be transferred.




The following code snippet shows worker.js, in which an onmessage handler is used to handle messages from the main script. Inside the handler a calculation is done from which a result message is created; this is then sent back to the main thread using postMessage(workerResult);

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');

In the main script, onmessage would have to be called on a Worker object, 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).

Note: postMessage() can only send a single object at once. As seen above, if you want to pass multiple values you can send an array.


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 4 ? 3.5 10 10.6 4
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support Yes Yes ? 4 11.5 5.1 Yes

See also

The DedicatedWorkerGlobalScope interface it belongs to.

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.