JS实现返回顶部功能：固定按钮与平滑滚动代码示例

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>JS点击按钮到页面最底部/返回页面顶部代码</title>
<style type="text/css">
 #back-to-top{ position: fixed; bottom: 50px; right: 10%; }
</style>
</head>
<body>
<div style="height: 3000px;"></div>
<div id="back-to-top">
 <a href="#top" rel="external nofollow" >返回顶部</a>
</div>
<script src="http://libs.baidu.com/jquery/2.0.0/jquery.js"></script>
 <script type="text/javascript">
 $(document).ready(function(){
  //首先将#back-to-top隐藏
  $("#back-to-top").hide();
  //当滚动条的位置处于距顶部100像素以下时，跳转链接出现，否则消失
  $(function () {
    $(window).scroll(function(){
    if ($(window).scrollTop()>100){
    $("#back-to-top").fadeIn(100);
    }
    else
    {
    $("#back-to-top").fadeOut(100);
    }
    });
    //当点击跳转链接后，回到页面顶部位置
    $("#back-to-top").click(function(){
    $('body,html').animate({scrollTop:0},"speed");
    return false;
    });
    });
    });
 </script>
</body>
</html>