Defer loading of Javascript files untill the page is already loaded

Defer.html

<script type="text/javascript">
function downloadJSAtOnload() {
  var element = document.createElement("script");
  element.src = "defer.js";
  document.body.appendChild(element);
}
if (window.addEventListener)
  window.addEventListener("load", downloadJSAtOnload, false);
else if (window.attachEvent)
  window.attachEvent("onload", downloadJSAtOnload);
else 
  window.onload = downloadJSAtOnload;
</script>

/*
  This was taken from:
    http://www.feedthebot.com/pagespeed/defer-loading-javascript.html
    
  Also have a look at the following URLs
    http://ablogaboutcode.com/2011/06/14/how-javascript-loading-works-domcontentloaded-and-onload/
    http://www.feedthebot.com/pagespeed/critical-render-path.html
    http://vimeo.com/71362583
*/