<div class="container">
<div class="header">頭部</div>
<div class="content">主要內容，高度是2000px</div>
<div class="footer>底部</div>
<div id="btn">返回頂部</div>
</div>

.container { width: 980px; margin: 0 auto; height: auto; background: #aaa;}
.content { height: 2000px; border: 1px solid blue; }
#btn {
position:fixed;
bottom: 50px;
right: 0;
width: 54px;
height: 55px;
background: url(icons.png) no-repeat 0 -110px; } //背景圖可隨意找一張
font-size: 12px;
line-height: 55px;
text-align: center;
text-indent: -9999em;
cursor: pointer;
display: none;

window.addEventListener("load",function() {
var btn = document.getElementById("btn");
btn.addEventListener("mouseover",moveIn, false);
btn.addEventListener("mouseout", moveOut, false);

function moveIn() {
btn.style.color = "#ffffff"; //修改的是內聯樣式，具有最高的優先級;
btn.style.textIndent = "0em";
btn.style.backgroundImage = "none";
btn.style.backgroundColor = "#FF4401";
}
function moveOut() {
btn.style.textIndent = "-9999em";
btn.style.backgroundImage = "url(icons.png)";
}

function goTop(acceleration, time) { //修改參數可調整返回頂部的速度
acceleration = acceleration || 0.1;
time = time || 10;
var speed = 1 + acceleration;
function getScrollTop() { //取得滾動條的豎直距離
return document.documentElement.scrollTop || document.body.scrollTop;
}
function setScrollTop(value) { //設置滾動條的豎直距離,實現效果的關鍵就是在很短的間隔時間內不斷地修改滾動條的豎直距離,以實現滾動效果
document.documentElement.scrollTop = value;
document.body.scrollTop = value;
}
window.onscroll = function() {
var scrollTop = getScrollTop();
if (scrollTop > 100) { //判斷滾動條距離窗口頂部多遠時顯示出來，這里是100px
btn.style.display = "block";
} else {
btn.style.display = "none";
}
};
btn.onclick = function () {
var timer = setInterval(function() {
setScrollTop(Math.floor(getScrollTop() / speed)); //這行代碼是關鍵，取得滾動條豎直距離，除以speed后再給滾動條設置豎直距離
if (getScrollTop() == 0)
clearInterval(timer);
}, time);
};
}
goTop(0.2, 8);
}, false);

btn.onclick = function() {
clearInterval(timer);
var timer = setInterval(function() {
var now = scrollTop; //滾動條豎直距離
speed = (0 - now) / 10;
speed = Math.floor(speed);
if (now == 0);
clearInterval(timer);
document.documentElement.scrollTop = now + speed; //標準模式下的瀏覽器
document.body.scrollTop = now + speed; //怪異模式下的瀏覽器
}, 15);
}