JS原生实现带指示点的自动轮播图

*{
  margin:0px;
  padding: 0px;
 }
 ul{
  width: 2500px;
  height: 300px;
  position: absolute;
 }
 li{
  float: left;
  list-style: none;
 }
 img{
  width: 500px;
  height: 300px;
 }
 div{
  width: 500px;
  height: 300px;
  margin: 50px auto;
  position: relative;
  overflow: hidden;
 
 }
 
/*小白点的ul*/
 #round_ul{
  width:300px;
  height: 30px;
  /*background:yellow;*/
  position: relative;
  margin: 250px auto;
 
 }
 
 #round_ul li{
  width: 20px;
  height:20px;
  border-radius: 50%;
  background: #2196f3;
  margin-left: 50px;
  cursor: pointer;
 
 }

<div>
 <ul>
  <li><img src="img/31.jpg"></li>
  <li><img src="img/32.jpG"></li>
  <li><img src="img/33.jpG"></li>
  <li><img src="img/34.jpg"></li>
  <li><img src="img/31.jpg"></li>
 </ul>
 <ul id="round_ul">
  <li></li>
  <li></li>
  <li></li>
  <li></li>
 </ul>

<script type="text/javascript">
//页面加载完成后 执行代码
 window.onload = function(){
  //获取 ul
  var imgUl = document.getElementsByTagName("ul")[0];
  var groundUl = document.getElementById("round_ul");

  //把第一个小白点修改成红色children 节点的子节点(不算空白节点)
  groundUl.children[0].style.backgroundColor = "red";

  var sId,x = 0;
  //开始计时器函数

  function fn(){
   sId = setInterval(abc,10);
  }
  function abc(){

   //每隔10秒修改ul的坐标,修改1px
   imgUl.style.left = x-- +"px";
   //如果一张图片完全进入到div中
   if(x % 500 == 0){
    //调用修改小白点函数
    if(x == -2000){
     x = 0;
     imgUl.style.left = 0 +"px";
    }
    changLi(Math.abs(x/500));//调用修改小白点方法
    clearInterval(sId);//暂定定时器
    setTimeout(fn,1000);//隔100毫秒在次启动定时器

   }
  }
  fn();
//修改小白点方法
  function changLi(num){
   //遍历小白点数组
   for(var x = 0;x<4;x++){

    //把所有的点修改成蓝色
    groundUl.children[x].style.backgroundColor = "#2196f3";
   }
   //把相对应的小白点修改成红色
   groundUl.children[num].style.backgroundColor = "red";
  }
 }
</script>