calss class name operation

    <script>
         // className can operate the class name, defects: there will be overwriting problems when setting the class name 

        // H5 provides a classList class name collection with methods available 
        var box = document.querySelector ("# box" );
         // console.dir (box); 

        // classList.add method ==> add class name 
        // classList.remove method ==> delete class name 
        // classList.contains method ==> determine whether there is an included class name, and return if there is true. Otherwise, return false 
        // classList.toggle method ==> switch class name (remove if there is, add if not) 

        // add ff 
        // box.classList.add ("ff"); 
        // box.className = "aa bb cc ff"; 

        // box.classList.remove ("bb"); 

        // console.log (box.classList.contains ("cc"));