异步加载中按需加载的代码

function loadScript(url, callback){

   var script = document.createElement("script")

   script.type = "text/javascript";

   if (script.readyState){  //IE浏览器

      script.onreadystatechange = function(){

         if (script.readyState == "loaded" ||

            script.readyState == "complete"){

            script.onreadystatechange = null;

              callback();

         }

      };

   } else {    //其他浏览器: Firefox, Safari, Chrome, and Opera

      script.onload = function(){

            callback();

      };

   }

   script.src = url;

   document.body.appendChild(script);

  }