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导航及下拉菜单
小追
Original
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展示效果：

css代码：

  1. <style>
  2.         *{
  3.             margin:0;
  4.             padding:0;
  5.             box-sizing:border-box;
  6.         }
  7.         /*给a标签设置颜色和文本装饰*/
  8.         a {
  9.             color : #bbb;
  10.             text-decoration: none;
  11.         }
  12.         /*给#nav主导航设置背景色，高度，行内字体高度*/
  13.         #nav {
  14.             background-color:red;
  15.             height:50px;
  16.             line-height:50px;
  17.         }
  18.        /*设置所有li的样式*/
  19.         li{
  20.             list-style:none;
  21.             margin:0 10px;
  22.             float:left;
  23.         }
  24.         /*设置#nav的子元素的子元素a标签鼠标在导航菜单上悬浮时的效果*/
  25.         #nav > li > a:hover {
  26.             color:white;
  27.         }
  28.         /*主导航相对于它的父级#nav定位*/
  29.          #nav > li {
  30.              position:relative;
  31.          }
  32.          /*设置#nav子元素的子元素，子菜单的定位因为#nav>li时定位了，所以会依照#nav>li的定位进行定位*/
  33.          #nav > li > ul {
  34.              position:absolute;
  35.              top:50px;
  36.              width:180px;
  37.              border:1px solid #aaa;
  38.              border-top:none
  39.          }
  40.         /*设置子菜单里a标签的样式*/
  41.          #nav > li >ul > li a{
  42.              display:inline-block;
  43.              height:50px;
  44.              color:#444;
  45.          }
  46.         /*初始时子菜单不显示*/
  47.          #nav>li>ul{
  48.              display:none;
  49.          }
  50.     </style>

html代码：

  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>下拉菜单</title>  
  7. </head>
  8. <body>
  9.     <ul id="nav">
  10.         <li><a href="">首页</a></li>
  11.         <li><a href="">视频教程</a></li>
  12.         <li>
  13.             <a href="">资源下载</a>
  14.             <ul>
  15.                 <li><a href="">PHP工具</a></li>
  16.                 <li><a href="">在线手册</a></li>
  17.                 <li><a href="">学习课件</a></li>
  18.                 <li><a href="">网站源码</a></li>
  19.             </ul>
  20.         </li>
  21.         <li><a href="">社区问答</a></li>
  22.         <li>
  23.             <a href="">技术文章</a>
  24.             <ul>
  25.                 <li><a href="">头条</a></li>
  26.                 <li><a href="">博客</a></li>
  27.                 <li><a href="">PHP教程</a></li>
  28.                 <li><a href="">PHP框架</a></li>
  29.             </ul>
  30.         </li>
  31.     </ul>
  32. </body>
  33. </html>

js代码：

  1. <script>
  2.     //获取所有的主导航 "#nav>li"表示这个类的子元素，不包含子元素的下级子元素
  3.     const navs = document.querySelectorAll("#nav > li");
  5.     navs.forEach(function (nav){
  6.         //鼠标移入时显示子菜单
  7.         nav.addEventListener("mouseover", showSubMenu);
  8.         //鼠标移出时关闭子菜单
  9.         nav.addEventListener("mouseout",closeSubMenu);
  10.     });
  12.     //显示子菜单
  13.     function showSubMenu(ev){
  14.         //判断当前这个导航有没有子菜单,有就显示在块中
  15.         if(ev.target.nextElementSibling !==null){
  16.             ev.target.nextElementSibling.style.display = "block";
  17.         }
  18.     }
  19.     //关掉子菜单   判断有没有子菜单，同时满足有a标签的
  20.     function closeSubMenu(ev){
  21.         if(ev.target.nodeName ==="A" && ev.target.style.nextElementSibling !==null){
  22.             ev.target.nextElementSibling.style.display = "none";
  23.         }
  24.     }
  25. </script>
Correcting teacher：天蓬老师天蓬老师

Correction status：qualified

Teacher's comments：这个案例目前有点超前了， 大家先有点感觉吧
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