MessagePort: postMessage() method

{{APIRef("Channel Messaging API")}} {{AvailableInWorkers}}

The postMessage() method of the {{domxref("MessagePort")}} interface sends a message from the port, and optionally, transfers ownership of objects to other browsing contexts.

Syntax

postMessage(message)
postMessage(message, transfer)
postMessage(message, options)

Parameters

• message

  • : The message you want to send through the channel. This can be of any basic data type. Multiple data items can be sent as an array.

    [!NOTE] Execution contexts that can message each other may not be in the same agent cluster, and therefore cannot share memory. {{jsxref("SharedArrayBuffer")}} objects, or buffer views backed by one, cannot be posted across agent clusters. Trying to do so will generate a {{domxref("BroadcastChannel/messageerror_event", "messageerror")}} event containing a DataCloneError {{domxref("DOMException")}} on the receiving end.

• transfer {{optional_inline}}

  • : An optional array of 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 should be attached to the message; otherwise they would be moved but not actually accessible on the receiving end.

• options {{optional_inline}}

  • : An optional object containing the following properties:
    • transfer {{optional_inline}}
      • : Has the same meaning as the transfer parameter.

Return value

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

Examples

In the following code block, you can see a new channel being created using the {{domxref("MessageChannel.MessageChannel", "MessageChannel()")}} constructor. When the IFrame has loaded, we pass {{domxref("MessageChannel.port2")}} to the IFrame using {{domxref("window.postMessage")}} along with a message. The iframe receives the message, and sends a message back on the MessageChannel using postMessage(). The handleMessage handler then responds to a message being sent back from the iframe using onmessage, putting it into a paragraph — {{domxref("MessageChannel.port1")}} is listened to, to check when the message arrives.

const channel = new MessageChannel();
const para = document.querySelector("p");

const ifr = document.querySelector("iframe");
const otherWindow = ifr.contentWindow;

ifr.addEventListener("load", iframeLoaded);

function iframeLoaded() {
  otherWindow.postMessage("Transferring message port", "*", [channel.port2]);
}

channel.port1.onmessage = handleMessage;
function handleMessage(e) {
  para.innerHTML = e.data;
}

// in the iframe…

window.addEventListener("message", (event) => {
  const messagePort = event.ports?.[0];
  messagePort.postMessage("Hello from the iframe!");
});

For a full working example, see our channel messaging basic demo on GitHub (run it live too).

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• Using channel messaging