window.addEventListener('message', event => { // IMPORTANT: check the origin of the data! if (event.origin.startsWith('http://your-first-site.com')) { // The data was sent from your site. // Data sent with postMessage is stored in event.data: console.log(event.data); } else { // The data was NOT sent from your site! // Be careful! Do not use it. This else branch is // here just for clarity, you usually shouldn't need it. return; } });
