Transform youtube's new handle url into the old /channel/id format

turns youtube handles into a channel id in the url

// ==UserScript==
// @name     Transform youtube's new handle url into the old /channel/id format
// @description turns youtube handles into a channel id in the url
// @author clickednebula3
// @match    *.youtube.com/@*
// @run-at   document-start
// @version  1.0
// @namespace https://greasyfork.org/users/1030152
// ==/UserScript==

window.onload = (event) => {
    var entirePage = document.getElementsByTagName("html")[0].innerHTML;
    var restOfThePage = entirePage.slice( entirePage.indexOf("},\"channelUrl\"") +48);
    var justTheId = restOfThePage.slice(0, restOfThePage.indexOf("\""));
    window.location.replace("http://www.youtube.com/channel/"+justTheId);
};