toggle toggleClass();
    contains hasClass() 判断是否包含某个class

    1. <!DOCTYPE html>
    2. <html lang="en">
    3. <head>
    4.     <meta charset="UTF-8">
    5.     <meta name="viewport" content="width=device-width, initial-scale=1.0">
    6.     <title>Document</title>
    7.     <style>
    8.         .current{
    9.             color:red;
    10.         }
    11.     </style>
    12. </head>
    13. <body>
    14.     <!-- 
    15.         toggle toggleClass();
    16.         contains      hasClass()  判断是否包含某个class
    17.      -->
    18.     <p id="app">hello world</p>
    19.     <script>
    20.         var app =  document.getElementById("app");
    21.         app.onclick =  function(){
    22.             // if(this.classList.contains("current")){
    23.             //     this.classList.remove("current")
    24.             // }else{
    25.             //     this.classList.add("current")
    26.             // }
    27.             this.classList.toggle("current")
    28.         }
    29.     </script>
    30. </body>
    31. </html>