JAVASCRIPT

get response from window.open

function openWindowPopup(url, windowname, w, h, x, y) {
	return window.open(url, windowname, "resizable=no, toolbar=no, scrollbars=no, menubar=no, status=no, directories=no, width=" + w + ", height=" + h + ", left=" + x + ", top=" + y);
}

let Popup = openWindowPopup('url', 'Title', 500, 600, 500, 500 );
Popup.addEventListener('load', () => {
  console.log(Popup.document.body.innerHTML);
  // or
  console.log(Popup.document.body.innerText);
});