动态脚本

//动态添加脚本，当脚本下载完成且准备就绪后执行回调函数。（这也是推荐的无阻塞的方法）
function loadScript(url,callback){
    var script=document.creatElement('script');
    script.type='text/javascript';
    if(script.readyState){ //IE
        script.onreadystatechange=function(){
            if(script.readyState == 'loaded' || script.readyState == 'complete'){
                script.onreadystatechange=null;
                callback();
            }
        }
    }else{  //非IE
        script.onload=function(){
            callback();
        }
    }

    script.src=url;
    document.getElementsByTagName('head')[0].appendChild(script);

}