原生JavaScript实现Tab选项卡切换功能

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Document</title>
 <style>
  *{
   margin:0;
   padding:0;
   list-style: none;
   box-sizing: border-box;
  }
  .menu{
   position: relative;
   display: flex;
   height: 20px;
   justify-content:space-around;
  }
  .menu span{
   display: block;
   width: 100%;
   height: 100%;
   text-align: center;
  }
  .menu .line{
   position: absolute;
   bottom:0;
   left: 0;
   width: 33.33%;
   height: 1px;
   background: red;
   -webkit-transition: all .2s;
   transition: all .2s;
  }
  .main{
   position: relative;
   width: 100%;
  }
  .main li{
   position: absolute;
   top:0;
   left:0;
  }
  .hide{
   display: none;
  }
  .show{
   display: block;
  }
 </style>
</head>
<body>
 <div class="menu">
   <span>menu1</span>
   <span>menu2</span>
   <span>menu3</span>
   <div class="line"></div>
 </div>
 <ul class="main">
  <li>menu1</li>
  <li class="hide">menu2</li>
  <li class="hide">menu3</li>
 </ul>
<script>
 window.onload=function(){
  var oMenu=document.querySelectorAll(".menu span");
  var oMain=document.querySelectorAll(".main li");
  var oLine=document.querySelector(".line");
  for(var i=0;i<oMenu.length;i++){
   oMenu[i].index=i;
   oMenu[i].onclick=function(){
    var thisIndex=this.index;
    for(var j=0;j<oMain.length;j++){
     oMain[j].style.display="none";
     oMain[thisIndex].style.display="block";
    }
    oLine.style.left=thisIndex*33.33+"%";
   }
  }
 }
</script>
</body>
</html>