原生js写轮播图效果

<script>

            var picarr=[
                "pic/lb1.jpg",
                "pic/lb2.jpg",
                "pic/lb3.jpg",
                "pic/lb4.jpg"
            ];

            //全局变量index控制当前所在第几张图片
            var index=;
            var timer;
            var radiobtn= document.getElementsByClassName("tubiao")[].getElementsByTagName("span");

            function  autochang(index) {
                var index=parseInt(index);
                document.getElementsByClassName("content_1")[].style.backgroundImage="url("+picarr[index]+")";
                for(var j=;j<radiobtn.length;j++){
                    radiobtn[j].className="";
                }
                radiobtn[index].className="actived";
            }

            function change(index){
                setInterval("autochang((index++)%radiobtn.length);",);
            }

            //页面加载成功初始效果
            change(index);

            //当鼠标点击点点的时候切换效果
              for(var i=;i<radiobtn.length;i++){
               radiobtn[i].index = i;
                radiobtn[i].onclick=function () {
                    clearInterval(timer);
                   autochang(this.index);
                   index=this.index;
                }
             }

             //点击左箭头
             document.getElementsByClassName("left_jiantou")[].onclick=function(){
                 index--;
                 if(index<){
                     index=radiobtn.length-;
                 }
                 autochang(index);
             }

             //点击右箭头
             document.getElementsByClassName("right_jiantou")[].onclick=function(){
                 index++;
                 if(index>=radiobtn.length){
                     index=;
                 }
                 autochang(index);
             }
    </script>