javascript性能优化

<script>
//减少http请求，实现js非阻塞模式
    function loadScript(url,callback){
        var script=document.createElement("script");
        script.type="text/javascript";
        if(script.readyState){//IE
            script.onreadystatechange=function(){
                if(script.readyState=="loaded"||script.readyState=="complete"){
                    script.onreadystatechange=null;
                    callback();
                }
            }
        }else{//others
            script.onload=function(){
                callback();
            }
        }
        script.src=url;
        var body=document.getElementsByTagName("body")[0];
        body.appendChild(script);
    }
    loadScript("js/index.js",function(){
//        Application.init();
        alert("this is error")
    })
</script>