window.onscroll页面滚动条滚动事件

用途一："返回顶部"；

window.onscroll = function(){
    var t = document.documentElement.scrollTop || document.body.scrollTop;
    var top_div = document.getElementById( "top_div" );
    if( t >=  ) {
        top_div.style.display = "inline";
    } else {
        top_div.style.display = "none";
    }
} 

用途二："商品详情菜单漂浮"  当鼠标滚动到一定位置 会有一个导航漂浮起来

window.onscroll = function(){
      var t = document.documentElement.scrollTop || document.body.scrollTop;
      var Div = document.getElementById( "fix_goods" );
      if( t <=  ) {
          Div.style.position = "";
          Div.style.top = "";
      } else {
          //Div.style.display = "none";
          Div.style.position = "fixed";
          Div.style.top = "-10px";

      }
  }

原文来源：http://www.cnblogs.com/wesky/admin/EditPosts.aspx?postid=3303694&update=1