MessagePort

const channel = new MessageChannel(); const output = document.querySelector(".output"); const iframe = document.querySelector("iframe"); // Wait for the iframe to load iframe.addEventListener("load", onLoad); function onLoad() { // Listen for messages on port1 channel.port1.onmessage = onMessage; // Transfer port2 to the iframe iframe.contentWindow.postMessage("Hello from the main page!", "*", [ channel.port2, ]); } // Handle messages received on port1 function onMessage(e) { output.innerHTML = e.data; }