5.ClassList对象

classList 属性返回元素的类名，作为 DOMTokenList 对象。
该属性用于在元素中添加，移除及切换 CSS 类

5.1 add() — 增加class

<p id="app">hello world</p>
<button id="btn">移除class</button>
    <script>
        var app = document.getElementById("app");
        var btn =  document.getElementById("btn");
        app.onclick = function(){
            this.classList.add("current")
        }
    </script>

5.2 remove() — 移除class

btn.onclick =  function(){
            app.classList.remove("current")
        }

5.3 toggle( ) 切换 contains( ) 判断是否包含某个class

<p id="app">hello world</p>
var app = document.getElementById("app")
app.onclick = function(){
  /* if(this.classList.contains("current")){
         this.classList.remove("current")
     }else{
         this.classList.add("current")
  } */
  this.classList.toggle("current")
}