Fire JS after page is totally loaded

readystatechange.js

document.addEventListener("readystatechange", function () {
  switch (document.readyState) {
    case "complete":
      {
        // put your code here that will run after the page loads completely.
        // if there is "flashing" and complaints are raised you can choose an earlier readyState 
        // always use "complete" if loading external resources
      }
  }
});