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

<!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>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) 0 0;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>
<a title="返回顶部" href="javascript:void(0)" class="back_top"></a>
<script src="js/jquery-1.11.0.min.js"></script>
<script>
var mousewheelevt=(/Firefox/i.test(navigator.userAgent))? "DOMMouseScroll" : "mousewheel"; //FF doesn't recognize mousewheel as of FF3.x
if (document.attachEvent) //if IE (and Opera depending on user setting)
document.attachEvent("on"+mousewheelevt, function(e){
var t=document.body.scrollTop+document.documentElement.scrollTop;
if(t==0){
$(".back_top").css("display","none");
}
else{
$(".back_top").css("display","block");
}
});
else if (document.addEventListener) //WC3 browsers
document.addEventListener(mousewheelevt, function(e){
var t=document.body.scrollTop+document.documentElement.scrollTop;
if(t==0){
$(".back_top").css("display","none");
}
else{
$(".back_top").css("display","block");
}
}, false);

</script>
</body>
</html>