var  timer=null,

     index=0,

     pics = ById("banner").getElementsByTagName("div"),

     size = pics.length,

     dots = ById("dots").getElementsByTagName("span"),

     banner=ById("banner"),

    prev=ById("prev"),

    next=ById("next");

function addHander(Element,type,hander){

if(Element.addEventListener){

         Element.addEventListener(type,hander,false);

}

else if(Element.attachEvent){

         Element.addEventListener("on"+type,hander);

}

else{

Element["on"+type]=hander;

}

}

 function ById(id){

if(typeof(id)==="string") {return document.getElementById(id);}

else {return id;} 

}

function changeimg(){

for(var i=0,len=pics.length;i<len;i++){

pics[i].style.display="none";

dots[i].className="";

}

pics[index].style.display="block";

dots[index].className="active";

}

addHander(next,"click",function(){

index++;

if(index>=3) index=0;

changeimg();

})

addHander(prev,"click",function(){

index--;

if(index<0) index=2;

changeimg();

})

for(var j=0,len1=dots.length;j<len1;j++){

dots[j].setAttribute("dotsId",j);

addHander(dots[j],"click",function(){

         index=this.getAttribute("dotsId");

         changeimg();

})

}

 function stopAtoplay(){

    if(timer){

            clearInterval(timer);

    }

    }

 function startAtoplay() {

  /*timer=setInterval(function(){

     index++;

     if(index>=3) {index=0;}

      changeimg();

     },3000)*/

     timer = setInterval(function(){

      console.log(index);

       index++;

        if(index >= size){

          index = 0;

       }

       changeimg();

   },3000)

 }

addHander(banner,"mouseout",startAtoplay);

addHander(banner,"mouseover",stopAtoplay);

html和css与老师代码相同，js轮播图自动播放时，要等待很长时间才开始自动播放，有时甚至不会播放，麻烦老师帮我找一下原因。