How to safely load javascript files asynchronously_javascript tips

(function() {
  __safeLoadScript("http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js", function() {
    alert(jQuery);
  });
})();

window.__safeLoadScript = function(src, callback) {
  function addEvent(obj, type, fn) {
    if (obj.attachEvent) {
      obj['e' + type + fn] = fn;
      obj[type + fn] = function() { obj['e' + type + fn](window.event); }
      obj.attachEvent('on' + type, obj[type + fn]);
    } else
      obj.addEventListener(type, fn, false);
  }
  function async_load(src, callback) {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    var protocol = (("https:" == document.location.protocol) ? "https://" : "http://");
    s.src = protocol + src;
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);
    s.onload = s.onreadystatechange = function() {
      if(callback && (!this.readyState || this.readyState == "loaded" || this.readyState == "complete")) {
        callback();
      }
    };
  }
  addEvent(window, "load", function() { 
    async_load(src, callback);
  });
};