基于JS实现仿京东搜索栏随滑动透明度渐变效果

<header class="module-layer"> 
 <div class="module-layer-content"> 
  <div class="search-box-cover"></div> 
  <p class="layer-return"></p> 
  <h1 class="layer-head-name"> 
   <div class="pr search-box"> 
    <img class="shop-search" src="images/search.png"/> 
    <input id="shop-input" type="text" placeholder="搜索店内商品" value="" /> 
   </div> 
  </h1> 
  <p class="layer-share"></p> 
 </div> 
</header> 

.module-layer { 
 width:100%; 
 position:fixed; 
 top:0; 
 left:0; 
 z-index:100000; 
} 
.module-layer-content { 
 position:relative; 
 min-width:320px; 
 max-width:750px; 
 width:100%; 
 margin:0 auto; 
} 
.module-layer-bg { 
 width:100%; 
 height:100%; 
 background:#000; 
 opacity:.7; 
 position:absolute; 
 top:0; 
 left:0; 
 z-index:-1; 
} 
.layer-head-name { 
 height:50px; 
 line-height:50px; 
 text-align:center; 
 padding:0 50px; 
 font-size:20px; 
} 
.layer-return,.layer-share { 
 width:50px; 
 height:50px; 
 line-height:50px; 
 text-align:center; 
 position:absolute; 
 top:0; 
 z-index:1; 
} 
.layer-return { 
 left:0; 
} 
.layer-share { 
 right:0; 
} 
.pr { 
 position:relative; 
} 
#shop-input::-webkit-input-placeholder { 
 color:#fff; 
} 
#shop-input:-moz-placeholder { 
 color:#fff; 
} 
#shop-input::-moz-placeholder { 
 color:#fff; 
} 
#shop-input:-ms-input-placeholder { 
 color:#fff; 
} 
#shop-input { 
 border:none; 
 outline:none; 
 background:transparent; 
} 
.search-box { 
 height:30px; 
 border-radius:20px; 
 top:10px; 
 overflow:hidden; 
 z-index:10; 
} 
.search-box:after { 
 content:''; 
 display:block; 
 width:100%; 
 height:30px; 
 background:#fff; 
 opacity:.5; 
 position:absolute; 
 top:0; 
 left:0px; 
 z-index:-1; 
} 
#shop-input { 
 height:28px; 
 line-height:28px; 
 font-size:16px; 
 position:absolute; 
 top:0; 
 left:30px; 
} 

$(function(){ 
 var $body = $('body'); 
 var setCoverOpacity = function() { 
  $body.find('.search-box-cover').css({ 
   opacity: ((($body.scrollTop() / 150) > 0.9) ? 0.9 : ($body.scrollTop() / 150)) 
  }) 
 } 
//初始化设置背景透明度 
 setCoverOpacity(); 
//监听滚动条事件，改变透明度 
 $(window).scroll(function() { 
  setCoverOpacity(); 
 }); 
})

((($body.scrollTop() / 150) > 0.9) ? 0.9 : ($body.scrollTop() / 150))