The port2 read-only property of the MessageChannel interface returns the second port of the message channel — the port attached to the context at the other end of the channel, which the message is initially sent to.

A MessagePort object representing the second port of the channel, the port attached to the context at the other end of the channel.

In the following code block, you can see a new channel being created using the MessageChannel.MessageChannel constructor. When the IFrame has loaded, we pass port2 to the IFrame using MessagePort.postMessage along with a message. 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.

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

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

ifr.addEventListener("load", iframeLoaded, false);

function iframeLoaded() {
  otherWindow.postMessage("Hello from the main page!", "*", [channel.port2]);
}

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

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

• Using channel messaging