css实现 偶数（even） 奇数 （odd）
  1.         li:nth-child(even){
  2.             background-color: pink;
  3.         }
  4.         li:nth-child(odd){
  5.             background-color: aqua;
  6.         }
  1. <ul>
  2.         <li>1</li>
  3.         <li>2</li>
  4.         <li>3</li>
  5.         <li>4</li>
  6.         <li>5</li>
  7.     </ul>
  8.     <script>
  9.         var lis = document.getElementsByTagName("li");
  10.         for(i=0;i<lis.length;i++){
  11.             if(i%2==0){
  12.                  lis[i].style.backgroundColor = "pink"
  13.             }else{
  14.                 lis[i].style.backgroundColor = "blue"
  15.             }
  16.         }
  17.     </script>

toggle

  1. <div style="display: block;">
  2.         div
  3.     </div>
  4.     <button id="btn">btn</button>
  5.     <script>
  6.         var div = document.getElementsByTagName("div")[0];
  7.         var btn = document.getElementById("btn");
  8.         btn.onclick = function(){
  9.             var d = div.style.display;
  10.             // if(d=="block"){
  11.             //     div.style.display = "none";
  12.             // }else{
  13.             //     div.style.display = "block";
  14.             // }
  16.         }
  17.         div.style.display = (d=="block")?"none":"block";