MessageChannel: port1 property

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 port1 read-only property of the MessageChannel interface returns the first port of the message channel — the port attached to the context that originated the channel.

Value

A MessagePort object, the first port of the channel, that is the port attached to the context that originated the channel.

In the following code block, you can see a new channel being created using the 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. The handleMessage method is associated to the port1 to listen 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("Hello from the main page!", "*", [channel.port2]);
}

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