异步安全加载javascript文件的方法

(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);  });};