MessageChannel: port2 プロパティ

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;
}