JavaScript 实现鼠标交互的 Banner 图片轮播功能

<style type="text/css">
body,img,span,ul,li{margin: 0;padding: 0;}
#div1{width: 600px;height: 350px;margin: 150px auto;border: 10px solid #eee;position: relative;}
img{width: 600px;height: 350px;}
span{display: inline-block;position: absolute;width: 50px;height: 50px;border-radius: 25px;background:rgba(0,0,0,.3); font-size: 30px;line-height: 50px;text-align: center;cursor: pointer;color: white;}
span:hover{background:rgba(255,255,255,.5);color:black;transform:scale(1.3);}
span#L{left: 10px;top: 150px;}
span#R{right: 10px;top: 150px;}
#div1 #ul{position: absolute;bottom: 10px;left: 250px;width: 125px;height: 20px;}
#div1 #ul li{list-style: none;float: left;height: 20px;width: 20px;border-radius: 10px;background:rgba(255,255,255,.5); margin-right: 5px;cursor: pointer;}
#div1 #ul .active{background:rgba(0,0,0,.5);}
</style>

<body>
  <div id="div1">
    <img src="" alt="">
    <span id="L">&lt;</span>
    <span id="R">&gt;</span>
    <ul id="ul">
     <li></li>
     <li></li>
     <li></li>
     <li></li>
     <li></li>
    </ul>
  </div>
</body>

<script type="text/javascript">
window.onload=function(){
  var div1=document.getElementById('div1');
  var oImg=div1.getElementsByTagName('img')[0];
  var spanL=document.getElementById('L');
  var spanR=document.getElementById('R');
  var oUl=document.getElementById('ul');
  var oLi=oUl.getElementsByTagName('li');
  var arrImg=['img2/1.jpg','img2/2.jpg','img2/3.jpg','img2/4.jpg','img2/5.jpg'];
  var num=0;
  var oldLi=0;
  function fn(num) {
    oImg.src=arrImg[num];
  }
  fn(0);
  function fnLi(num){
    oLi[oldLi].className='';
    oLi[num].className='active';
    oldLi=num;
  }
  fnLi(0);
  /*点击左右span图片切换*/
  /*点击左右span时li切换*/
  spanL.onclick=function(){
    if (num<1) {
      num=arrImg.length-1;
      fn(num);
      fnLi(num);
    }else{
      num--;
      fnLi(num);
      fn(num);
    }    
  }
  spanR.onclick=function(){
    if (num==arrImg.length-1) {
      num=0;
      fn(num);
      fnLi(num);
    }else{
      num++;
      fnLi(num);
      fn(num);
    }    
  }
  /*点击li实现图片切换*/
  for (var i = 0; i < oLi.length; i++) {
    oLi[i].index=i;
    oLi[i].onclick=function(){
      fn(this.index);
      fnLi(this.index);
    }
  }
}
</script>