- `message`

- : The object to deliver to the main thread; this will be in the data field in the event delivered to the {{domxref("Window/message_event", "message")}} event.

This may be any value or JavaScript object handled by the [structured clone](/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm) algorithm, which includes cyclical references.

- `transfer` {{optional_inline}}

- : An optional [array](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) of [transferable objects](/en-US/docs/Web/API/Web_Workers_API/Transferable_objects) to transfer ownership of. The ownership of these objects is given to the destination side and they are no longer usable on the sending side. These transferable objects are not automatically sent; they must either be contained in the message or be accessible to the recipient via other means, such as {{domxref("MessagePort")}} via {{domxref("MessageEvent.ports")}}.

- `options` {{optional_inline}}

- : An optional object containing the following properties:

- `transfer` {{optional_inline}}

- : Has the same meaning as the `transfer` parameter.

None ({{jsxref("undefined")}}).

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 = (e) => {

console.log("Message received from main script");

const workerResult = `Result: ${e.data[0] * e.data[1]}`;

console.log("Posting message back to main script");

postMessage(workerResult);

};

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 ({{domxref("DedicatedWorkerGlobalScope")}}).

For a full example, see our [Basic dedicated worker example](https://github.com/mdn/dom-examples/tree/main/web-workers/simple-web-worker) ([run dedicated worker](https://mdn.github.io/dom-examples/web-workers/simple-web-worker/)).

{{Specifications}}

{{Compat}}