Js实现的网页返回顶部功能，自动判断当前页是不是第二屏，如果是，则自动出现“回到顶部”按钮，点击后即可回到网页第一屏的开始位置，貌似现在很多网站都有这功能，觉得炫，就自己动手实现一个，采用了lee-goToTop开源JS库，例子代码如下：
<!DOCTYPE html>
<html>
<head>
<title>返回顶部-Demo</title>
<style>
/*to-top.css*/
* html, * html body{background-image:url(about:blank);background-attachment:fixed;}
#toTop{
position:fixed;
_position:absolute;
right:30px;
bottom:40px;
_top:expression(document.documentElement.scrollTop+document.documentElement.clientHeight-this.offsetHeight-40);
cursor:pointer;
display:none;
}
</style>
</head>
<body>
<p>当拖动滚动条进入第二屏时，网页右下角会出现“返回顶部”图标</p>
<p>Item 1</p>
<p>Item 2</p>
<p>Item 3</p>
<p>Item 4</p>
<p>Item 5</p>
<p>你的网页内容</p>
<p>Item 96</p>
<p>Item 97</p>
<p>Item 98</p>
<p>Item 99</p>
<img src="images/to-top.png" id="toTop"/><!--“回到顶部”图标，丢失了，网上搜一下-->
<script style="text/javascript">
//to-top.js
(function(win, doc){
//变量
var compatMode = doc.compatMode,
isChrome = win.navigator.userAgent.indexOf("Chrome") === -1 ? false : true,
scrollEle = compatMode === "BackCompat" || isChrome ? doc.body : doc.documentElement,
clientEle = compatMode === "BackCompat" ? doc.body : doc.documentElement,
toTopImg = doc.getElementById("toTop"),
rate = 0.6,
timeGap = 10;
//点击返回顶部图标后的响应动作
toTopImg.onclick = function(){
var moveInterval = setInterval(moveScroll, timeGap);
function moveScroll(){
var scrollTop = scrollEle.scrollTop;
if(scrollTop === 0){
clearInterval(moveInterval);
return ;
}
scrollEle.scrollTop = scrollTop * rate;
}
};
//滚动时判断是否显示“返回顶部”图标
win.onscroll = function(){
var display = toTopImg.style.display;
if(scrollEle.scrollTop > clientEle.clientHeight){
if(display !== "block"){
toTopImg.style.display = "block";
}
}else{
if(display !== "none"){
toTopImg.style.display = "none";
}
}
};
})(window, document);
</script>
</body>
</html>注意，低于IE8的浏览器无效，火狐或Chrome下正常。
