js实现导航跟随效果

<style type="text/css">
 *{padding:0;margin:0;}
 a{text-decoration:none;}
 html,body{height:100%;width:100%;background:black;}
 ul{position:relative;width:990px;list-style:none;background:white;display: flex;flex-direction:row;justify-content: space-around;margin:50px auto;border-radius:10px;}
 ul li{position: relative;flex:1;text-align:center;}
 ul li a{font-size:18px;color:#333;padding:10px 0;display: block;}
 .cloud{position:absolute;left:32px;top:0;bottom:0;margin:auto;width:83px;height:42px;background:url('images/cloud.gif');}
 </style>

<ul>
 <span class="cloud"></span>
 <li> <a href="javascript:viod(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >首页 </a></li>
 <li><a href="javascript:viod(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >电视剧</a></li>
 <li><a href="javascript:viod(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >最新电影</a></li>
 <li><a href="javascript:viod(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >新闻头条</a></li>
 <li><a href="javascript:viod(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >八卦娱乐</a></li>
 <li><a href="javascript:viod(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >军事热点</a></li>
 </ul>

 <script type="text/javascript">
 //获得类为cloud的标签，
 var pic=document.getElementsByClassName('cloud')[0];
 //获得所有的 li 标签
 var liList=document.getElementsByTagName('li');
 //定义向右的移动初始距离
 var liLeft=32;
 //定义缓慢动画的初始值
 var header=32;
 //用于定义当鼠标点击时的初始位置
 var currentLeft=32;
 for(var i=0;i<liList.length;i++){
  //鼠标放上事件
  liList[i].onmouseover=function(){
  //获取目标距离
  liLeft = this.offsetLeft+this.offsetWidth/2-pic.offsetWidth/2;
  }
  //鼠标移除事件
  liList[i].onmouseout=function(){
  //当鼠标移除某个li的时候把目标距离改为初始状态
  liLeft=currentLeft;
  }
  //鼠标点击事件
  liList[i].onclick=function(){
  currentLeft=this.offsetLeft+this.offsetWidth/2-pic.offsetWidth/2;
  }
 
 }
 //定义缓慢动画
 setInterval(function(){
  header = header + (liLeft-header)/10;
  pic.style.left = header + 'px';
 },20);
 </script>