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运用javascript知识，实现根据class name获取对象并返回、给页面元素增加class等功能函数，并实现在滚动时根据滚动条的位置自动设置导航菜单的焦点。

温馨提示 : 完成任务后，请验证是否与实践描述效果一致，如一致，恭喜您，你已经掌握此技能，否则，请重复学习此节内容。

任务

1.定义getByClassName函数，让函数实现根据class name获取对象并返回

提示：先获取obj中的所有标签，赋值给elements，然后遍历elements，找到className相符的元素存入数组result，最后返回result

2.定义addClass函数，让函数实现给对象增加class

提示：首先要判断是否已经有名为cls的class，如果没有，添加即可

3.补充代码，给正确的menu下的a元素class赋值current

提示：此处要遍历menus，对menus中的每一个元素判断href属性值和currentId是否相符，并根据判断结果决定是移除名为current的class，还是增加名为current的class。

?不会了怎么办

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index.html

<!DOCTYPE html> <html> <head> <meta charset="utf-8"/> <title>地狗购物网——网页定位导航效果</title> <style> * { margin: 0; padding: 0; } body { font-size: 12px; line-height: 1.7; } li { list-style: none; } #content { width: 800px; margin: 0 auto; padding: 20px; } #content h1 { color: #0088bb; } #content .item { padding: 20px; margin-bottom: 20px; border: 1px dotted #0088bb; } #content .item h2 { font-size: 16px; font-weight: bold; border-bottom: 2px solid #0088bb; margin-bottom: 10px; } #content .item li { display: inline; margin-right: 10px; } #content .item li a img { width: 230px; height: 230px; border: none; } #menu { position: fixed; top: 100px; left: 50%; margin-left: 400px; width: 80px; } #menu ul li a { display: block; margin: 5px 0; font-size: 14px; font-weight: bold; color: #333; width: 80px; height: 50px; line-height: 50px; text-decoration: none; text-align: center; } #menu ul li a:hover, #menu ul li a.current { color: #fff; background: #0088bb; } /*ie6 hack*/ * html, * html body { background-image: url(about:blank); background-attachment: fixed; } * html #menu { /*position: fixed;*/ position: absolute; top: expression(((e=document.documentElement.scrollTop)?e:document.body.scrollTop)+100+'px'); } </style> <script> //定义getByClassName函数，让函数实现根据class name获取对象并返回 function hasClass( obj, cls ){ return obj.className.match(new RegExp("(\\s|^)" + cls + "(\\s|$)")); } function removeClass( obj, cls ){ if( hasClass( obj, cls )){ //remove var reg = new RegExp("(\\s|^)" + cls + "(\\s|$)"); obj.className = obj.className.replace(reg, ""); } } //定义getByClassName函数，让函数实现给对象增加class window.onload = function(){ window.onscroll = function(){ var top = document.documentElement ? document.documentElement.scrollTop : document.body.scrollTop; var menus = document.getElementById("menu").getElementsByTagName("a"); var items = getByClassName(document.getElementById("content"), "item"); var currentId = ""; for ( var i=0; i< items.length; i++ ){ var _item = items[i]; var _itemTop = _item.offsetTop; if( top > _itemTop - 200 ){ currentId = _item.id; } else { break; } } //请补充此处代码，给正确的menu下的a元素class赋值current } } </script> </head> <body> <div id="menu"> <ul>  <li><a href="#" class="current">1F 男装</a></li> <li><a href="#">2F 女装</a></li> <li><a href="#">3F 美妆</a></li> <li><a href="#">4F 数码</a></li> <li><a href="#">5F 母婴</a></li> </ul> </div> <div id="content"> <h1>地狗购物网</h1> <div id="item1" class="item"> <h2>1F 男装</h2> <ul> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//53660fce0001111903990422.jpg" alt=""/></a></li> </ul> </div> <div id="item2" class="item"> <h2>2F 女装</h2> <ul> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//5366104c0001e5c403990422.jpg" alt=""/></a></li> </ul> </div> <div id="item3" class="item"> <h2>3F 美妆</h2> <ul> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610820001cf5b03990422.jpg" alt=""/></a></li> </ul> </div> <div id="item4" class="item"> <h2>4F 数码</h2> <ul> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610ac00012df703990422.jpg" alt=""/></a></li> </ul> </div> <div id="item5" class="item"> <h2>5F 母婴</h2> <ul> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> <li><a href="#"><img src="http://img1.sycdn.imooc.com//536610cd0001f34603990422.jpg" alt=""/></a></li> </ul> </div> </div> </body> </html>

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