<script>
// see http://www.CodeDesignPatterns.com/javascript/window-onload-function-chaining/
function onloadFunction(prevOnloadFunction) {
   return function() {
      alert('This happens before the previous window.onload function');
      if (prevOnloadFunction) prevOnloadFunction();
      alert('This happens after the previous window.onload function');
   };
}
window.onload = onloadFunction(window.onload);
</script>