MessagePort: postMessage() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.

Note: This feature is available in Web Workers.

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

Syntax

js
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. SharedArrayBuffer objects, or buffer views backed by one, cannot be posted across agent clusters. Trying to do so will generate a messageerror event containing a DataCloneError DOMException on the receiving end.

transfer Optional

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 are not automatically sent; they must either be contained in the message or be accessible to the recipient via other means, such as MessagePort via MessageEvent.ports.

options Optional

An optional object containing the following properties:

transfer Optional

Has the same meaning as the transfer parameter.

Return value

None (undefined).

Examples

In the following code block, you can see a new channel being created using the MessageChannel() constructor. When the IFrame has loaded, we pass MessageChannel.port2 to the IFrame using 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 — MessageChannel.port1 is listened to, to check when the message arrives.

js
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).