jQuery仿京东商城楼梯式导航定位菜单

<!-- 导航菜单 -->
 <div id="menu">
  <ul>
   <li>1F<span>服饰</span></li>
   <li>2F<span>美妆</span></li>
   <li>3F<span>手机</span></li>
   <li>4F<span>家电</span></li>
   <li>5F<span>数码</span></li>
   <li>6F<span>运动</span></li>
   <li>7F<span>居家</span></li>
   <li>8F<span>母婴</span></li>
   <li>9F<span>食品</span></li>
   <li>10F<span>图书</span></li>
   <li>11F<span>服务</span></li>
  </ul>
 </div>
 <!-- 导航菜单 -->
 <!-- 产品内容 -->
 <div id="content">
  <img src="images/head.png" alt="" style="border:1px solid red;">
  <div class="louti" id="louti1"><img src="images/f1.png" alt=""></div>
  <div class="louti" id="louti2"><img src="images/f2.png" alt=""></div>
  <div class="louti" id="louti3"><img src="images/f3.png" alt=""></div>
  <div class="louti" id="louti4"><img src="images/f4.png" alt=""></div>
  <div class="louti" id="louti5"><img src="images/f5.png" alt=""></div>
  <div class="louti" id="louti6"><img src="images/f6.png" alt=""></div>
  <div class="louti" id="louti7"><img src="images/f7.png" alt=""></div>
  <div class="louti" id="louti8"><img src="images/f8.png" alt=""></div>
  <div class="louti" id="louti9"><img src="images/f9.png" alt=""></div>
  <div class="louti" id="louti10"><img src="images/f10.png" alt=""></div>
  <div class="louti" id="louti11"><img src="images/f11.png" alt=""></div>
 </div>
 <!-- 产品内容 -->
 <!-- 底部 -->
 <div id="footer" class="container"></div>

#menu{
  width:32px;height:360px;
  position:fixed;
  top:200px;left:0px;
  display: none;
 }
 #menu ul li{
  width:32px;height:32px;
  list-style-type:none;
  color:#8F8583;
  text-align:center;
  line-height: 32px;
  border-bottom:1px dotted #ddd;
  position:relative;
 }
 #menu ul li span{
  display:block;width:32px;height:32px;
  background:#C81623;
  position:absolute;
  top:0;left:0;
  color:#fff;font-size:12px;
  display: none;
 }
 #menu ul li:hover span{display:block; }
 #menu ul li span.active{color:#C81623;background:#fff;display:block;}
 #content{
  width:1220px;
  margin:0 auto;
 } 
 #footer{width:1220px;height:600px;background:#FE7678;}
 .container{margin:0 auto;}

$(function(){
  var _index=0;
  $("#menu ul li").click(function(){
   $(this).find("span").addClass("active").parent().siblings().find("span").removeClass("active");
   _index=$(this).index()+1;
   //通过拼接字符串获取元素，再取得相对于文档的高度
   var _top=$("#louti"+_index).offset().top;
   //scrollTop滚动到对应高度
   $("body,html").animate({scrollTop:_top},500);
  });
  var nav=$("#menu"); //得到导航对象
  var win=$(window); //得到窗口对象
  var sc=$(document);//得到document文档对象。
  win.scroll(function(){

   if(sc.scrollTop()>=600){
   $("#menu").show(); 
   //获取滚动元素对应的索引!!!重难点
   var index=Math.floor(sc.scrollTop()/600);

   $("#menu ul li span").eq(index-1).addClass("active").parent().siblings().find("span").removeClass("active");
   }else{
   $("#menu").hide();
   }
  });
 });