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完成了day26的任务，更新了readme.md
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‎26 - Strip Follow Along Nav/README.md

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# 26 Strip Follow Along Nav 中文指南
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> 本篇作者：©[大史快跑Dashrun](https://github.com/dashrun)——Chinasoft Frontend Developer
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> 简介：[JavaScript30](https://javascript30.com) 是 [Wes Bos](https://github.com/wesbos) 推出的一个 30 天挑战。项目免费提供了 30 个视频教程、30 个挑战的起始文档和 30 个挑战解决方案源代码。目的是帮助人们用纯 JavaScript 来写东西，不借助框架和库，也不使用编译器和引用。现在你看到的是这系列指南的第 26 篇。完整指南在 [GitHub](https://github.com/soyaine/JavaScript30)，喜欢请 Star 哦♪(^∇^*)
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> 创建时间：2017-10-16
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最后更新：2017-10-17
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## 挑战任务
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初始文档`index-start.html`中提供了一组导航按钮，本次的编程任务需要实现的效果是当鼠标悬停于导航按钮时，显示对应下拉菜单的内容。（说明：下拉菜单内容及白色背景已写好，只需要根据需要改变其CSS属性使元素显示出来或改变其位置即可）。
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## 实现效果
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![结果展示](https://github.com/dashrun/vanilla-javascript-30/blob/master/26%20-%20Strip%20Follow%20Along%20Nav/effect.png)
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## 编程思路
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监听每一个导航栏按钮绑定鼠标移入和移出的事件，在对应的回调函数中为相应的元素增加已经编写好的类名即可。
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## 过程指南
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1.监听每一个导航栏按钮的鼠标移入移出事件
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```js
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var mainUl = document.querySelectorAll('.cool > li >a');
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var navArr = Array.from(mainUl);
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var bg = document.querySelector('.dropdownBackground');
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navArr.map(function(item,index){
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item.onmouseover = function (){
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item.parentNode.classList.add('trigger-enter');
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item.parentNode.classList.add('trigger-enter-active');
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toggleBackground(index+1);
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}
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item.onmouseout = function (){
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item.parentNode.classList.remove('trigger-enter');
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item.parentNode.classList.remove('trigger-enter-active');
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toggleBackground();
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}
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});
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```
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2.toggleBackground()函数为自定义函数，当传入整数参数时，根据对应的下拉菜单序号获取下拉菜单的尺寸，将绝对定位的白色背景衬底改编为对应大小，并移动至相应位置；当不传入任何参数时，表示鼠标没有悬停于任何按钮上，此时需要将下拉菜单的背景置为透明。
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```js
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function toggleBackground(item){
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var itemPos;
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if(item === 1 || item === 2 || item ===3){
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//计算位置
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itemPos = document.querySelector('.dropdown'+item).getBoundingClientRect();
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bg.style.left = `${itemPos.left}px`;
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bg.style.top = `${itemPos.top-60}px`;
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bg.classList.add('open');
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bg.style.width = `${itemPos.width}px`;
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bg.style.height = `${itemPos.height}px`;
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}else{
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bg.style.height = 0;
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bg.style.width = 0;
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bg.classList.remove('open');
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}
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}
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```
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## 延伸思考
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1.本例中将相应的样式编写在一个css类中，通过添加和移出类来实现需要的效果，js编程实践中非常推荐这种将DOM操作集中化的做法。
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2.除了使用js脚本来实现交互效果外，也可以使用下面的css来实现鼠标悬浮在按钮上时显示下拉菜单的效果（代码中`~`表示同级的元素,由于`index-start.html`中给定的结构中下拉菜单的白色沉底为独立元素，需要动态获取其对应尺寸，故使用js脚本更容易实现）
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```css
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.cool>li>a:hover ~.dropdown{
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display:block;
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opacity:1;
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}
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```

‎26 - Strip Follow Along Nav/effect.png

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‎26 - Strip Follow Along Nav/index-finished-Dashrun.html

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>Follow Along Nav</title>
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</head>
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<body>
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<h2>Cool</h2>
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<nav class="top">
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<div class="dropdownBackground">
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<span class="arrow"></span>
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</div>
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<ul class="cool">
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<li>
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<a href="#">About Me</a>
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<div class="dropdown dropdown1">
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<div class="bio">
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<img src="https://logo.clearbit.com/wesbos.com">
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<p>Wes Bos sure does love web development. He teaches things like JavaScript, CSS and BBQ. Wait. BBQ isn't part of web development. It should be though!</p>
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</div>
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</div>
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</li>
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<li>
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<a href="#">Courses</a>
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<ul class="dropdown courses dropdown2">
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<li>
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<span class="code">RFB</span>
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<a href="https://ReactForBeginners.com">React For Beginners</a>
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</li>
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<li>
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<span class="code">ES6</span>
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<a href="https://ES6.io">ES6 For Everyone</a>
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</li>
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<li>
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<span class="code">STPU</span>
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<a href="https://SublimeTextBook.com">Sublime Text Power User</a>
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</li>
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<li>
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<span class="code">WTF</span>
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<a href="http://flexbox.io">What The Flexbox?!</a>
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</li>
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<li>
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<span class="code">LRX</span>
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<a href="http://LearnRedux.com">Learn Redux</a>
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</li>
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<li>
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<span class="code">CLPU</span>
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<a href="http://CommandLinePowerUser.com">Command Line Power User</a>
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</li>
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<li>
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<span class="code">MMD</span>
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<a href="http://MasteringMarkdown.com">Mastering Markdown</a>
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</li>
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</ul>
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</li>
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<li>
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<a href="#">Other Links</a>
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<ul class="dropdown dropdown3">
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<li><a class="button" href="http://twitter.com/wesbos">Twiter</a></li>
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<li><a class="button" href="http://facebook.com/wesbos.developer">Facebook</a></li>
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<li><a class="button" href="http://wesbos.com">Blog</a></li>
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<li><a class="button" href="http://wesbos.com/courses">Course Catalog</a></li>
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</ul>
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</li>
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</ul>
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</nav>
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<style>
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html {
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box-sizing: border-box;
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font-family: "Arial Rounded MT Bold","Helvetica Rounded",Arial,sans-serif
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}
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*, *:before, *:after {
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box-sizing: inherit;
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}
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body {
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min-height: 100vh;
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background:
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linear-gradient(45deg, hsla(340, 100%, 55%, 1) 0%, hsla(340, 100%, 55%, 0) 70%),
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linear-gradient(135deg, hsla(225, 95%, 50%, 1) 10%, hsla(225, 95%, 50%, 0) 80%),
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linear-gradient(225deg, hsla(140, 90%, 50%, 1) 10%, hsla(140, 90%, 50%, 0) 80%),
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linear-gradient(315deg, hsla(35, 95%, 55%, 1) 100%, hsla(35, 95%, 55%, 0) 70%);
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}
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nav {
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position: relative;
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perspective: 600px;
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}
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.cool > li > a {
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color: yellow;
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text-decoration: none;
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font-size: 20px;
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background: rgba(0,0,0,0.2);
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padding:10px 20px;
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display: inline-block;
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margin:20px;
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border-radius:5px;
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}
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nav ul {
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list-style: none;
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margin: 0;
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padding: 0;
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display: flex;
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justify-content: center;
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}
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.cool > li {
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position: relative;
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display:flex;
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justify-content: center;
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}
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.dropdown {
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opacity: 0;
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position: absolute;
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overflow: hidden;
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padding:20px;
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top:-20px;
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border-radius:2px;
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transition: all 0.5s;
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transform: translateY(100px);
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will-change: transform;
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display: none;
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}
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.trigger-enter .dropdown {
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display: block;
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}
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.trigger-enter-active .dropdown {
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opacity: 1;
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}
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.dropdownBackground {
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width:100px;
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height:100px;
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position: absolute;
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background: #fff;
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border-radius: 4px;
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box-shadow: 0 50px 100px rgba(50,50,93,.1), 0 15px 35px rgba(50,50,93,.15), 0 5px 15px rgba(0,0,0,.1);
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transition:all 0.3s, opacity 0.1s, translate 0.2s;
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transform-origin: 50% 0%;
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display: flex;
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justify-content: center;
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opacity:0;
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}
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.dropdownBackground.open {
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opacity: 1;
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}
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.arrow {
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position: absolute;
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width:20px;
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height:20px;
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display: block;
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background:white;
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transform: translateY(-50%) rotate(45deg);
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}
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.bio {
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min-width:500px;
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display:flex;
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justify-content: center;
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align-items: center;
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line-height: 1.7;
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}
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.bio img {
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float:left;
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margin-right:20px;
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}
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.courses {
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min-width:300px;
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}
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.courses li {
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padding: 10px 0;
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display: block;
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border-bottom: 1px solid rgba(0,0,0,0.2);
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}
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.dropdown a {
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text-decoration: none;
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color: #ffc600;
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}
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a.button {
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background:black;
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display: block;
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padding:10px;
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color:white;
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margin-bottom: 10px;
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}
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/* Matches Twitter, TWITTER, twitter, tWitter, TWiTTeR... */
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.button[href*=twitter] { background: #019FE9; }
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.button[href*=facebook] { background: #3B5998; }
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.button[href*=courses] { background: #ffc600; }
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</style>
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<script>
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var mainUl = document.querySelectorAll('.cool > li >a');
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var navArr = Array.from(mainUl);
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var bg = document.querySelector('.dropdownBackground');
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navArr.map(function(item,index){
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item.onmouseover = function (){
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item.parentNode.classList.add('trigger-enter');
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item.parentNode.classList.add('trigger-enter-active');
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toggleBackground(index+1);
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}
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item.onmouseout = function (){
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item.parentNode.classList.remove('trigger-enter');
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item.parentNode.classList.remove('trigger-enter-active');
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toggleBackground();
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}
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});
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function toggleBackground(item){
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var itemPos;
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if(item === 1 || item === 2 || item ===3){
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//计算位置
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itemPos = document.querySelector('.dropdown'+item).getBoundingClientRect();
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bg.style.left = `${itemPos.left}px`;
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bg.style.top = `${itemPos.top-60}px`;
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bg.classList.add('open');
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bg.style.width = `${itemPos.width}px`;
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bg.style.height = `${itemPos.height}px`;
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}else{
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bg.style.height = 0;
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bg.style.width = 0;
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bg.classList.remove('open');
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}
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}
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</script>
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</body>
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</html>

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