JavaScript实现居中弹出窗口特效代码

<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>弹出窗口</title>
<script src="js/jquery-1.11.1.js"></script>
<style>
*{
 margin: 0;
 padding: 0;
}
div{
 margin: 0 auto;
 /*border: 1px solid red;*/
}
.overBg{
 width: 100%;
 height: 100%;
 background: gray;
 opacity: 0.5;
 filter: alpha(opacity=50);
 position: fixed;
 top:0;
 z-index: 300;
}
.tc-con{
 width: 200px;
 height: 200px;
 padding: 30px;
 background: blanchedalmond;
 position: fixed;
 top: 0;
 z-index: 1000;
}
</style>
<script>
 $(function(){
  $('.tc').hide();
  $('.btn').click(function(){
   $('.tc').show();
  })
  $('.tc').click(function(){
   $(this).hide();
  })
 })
</script>
</head>
<body>
<div class="btn">点击出现</div>
<div class="tc">
 <div class="overBg"></div>
 <div class="tc-con" id="tc-con">22</div>
 <script>
  var a=document.getElementById('tc-con');
  a.style.left=(document.body.clientWidth/2- a.clientWidth/2)+'px';
  a.style.top=(document.documentElement.clientHeight/2- a.clientHeight/2)+'px';
 </script>
</div>
<div style="height:2000px">22</div>
</body>
</html><!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>弹出窗口</title>
<script src="js/jquery-1.11.1.js"></script>
<style>
*{
 margin: 0;
 padding: 0;
}
div{
 margin: 0 auto;
 /*border: 1px solid red;*/
}
.overBg{
 width: 100%;
 height: 100%;
 background: gray;
 opacity: 0.5;
 filter: alpha(opacity=50);
 position: fixed;
 top:0;
 z-index: 300;
}
.tc-con{
 width: 200px;
 height: 200px;
 padding: 30px;
 background: blanchedalmond;
 position: fixed;
 top: 0;
 z-index: 1000;
}
</style>
<script>
 $(function(){
  $('.tc').hide();
  $('.btn').click(function(){
   $('.tc').show();
  })
  $('.tc').click(function(){
   $(this).hide();
  })
 })
</script>
</head>
<body>
<div class="btn">点击出现</div>
<div class="tc">
 <div class="overBg"></div>
 <div class="tc-con" id="tc-con">22</div>
 <script>
  var a=document.getElementById('tc-con');
  a.style.left=(document.body.clientWidth/2- a.clientWidth/2)+'px';
  a.style.top=(document.documentElement.clientHeight/2- a.clientHeight/2)+'px';
 </script>
</div>
<div style="height:2000px">22</div>
</body>
</html>