返回顶部按钮响应滚动且动态显示与隐藏
很多的网站上都有返回顶部功能，判断滚动参数动态显示与隐藏，比较适合初学者
<!DOCTYPEhtmlPUBLIC"-//W3C//DTDXHTML1.0Transitional//EN""http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<htmlxmlns="http://www.w3.org/1999/xhtml">
<head>
<metahttp-equiv="Content-Type"content="text/html;charset=utf-8"/>
<title>实现动态的显示回到顶部与否</title>
<style>
/*返回顶部*/
.back_top{width:45px;height:45px;background:url(back_top_new.png)00;position:fixed;left:50%;margin-left:500px;bottom:105px;display:none;}
.back_top:hover{background:url(images/back_top_new.png)0-46px;}
</style>
</head>
<body>
<atitle="返回顶部"href="javascript:void(0)"class="back_top"></a>
<scriptsrc="js/jquery-1.11.0.min.js"></script>
<script>
varmousewheelevt=(/Firefox/i.test(navigator.userAgent))?"DOMMouseScroll":"mousewheel";//FFdoesn'trecognizemousewheelasofFF3.x
if(document.attachEvent)//ifIE(andOperadependingonusersetting)
document.attachEvent("on"+mousewheelevt,function(e){
vart=document.body.scrollTop+document.documentElement.scrollTop;
if(t==0){
$(".back_top").css("display","none");
}
else{
$(".back_top").css("display","block");
}
});
elseif(document.addEventListener)//WC3browsers
document.addEventListener(mousewheelevt,function(e){
vart=document.body.scrollTop+document.documentElement.scrollTop;
if(t==0){
$(".back_top").css("display","none");
}
else{
$(".back_top").css("display","block");
}
},false);
</script>
</body>
</html>