<div class="container">
<div class="header">頭部</div>
<div class="content">主要內(nèi)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(nèi)聯(lián)樣式，具有最高的優(yōu)先級(jí);
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(shù)可調(diào)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(dòng)條的豎直距離
return document.documentElement.scrollTop || document.body.scrollTop;
}
function setScrollTop(value) { //設(shè)置滾動(dòng)條的豎直距離,實(shí)現(xiàn)效果的關(guān)鍵就是在很短的間隔時(shí)間內(nèi)不斷地修改滾動(dòng)條的豎直距離,以實(shí)現(xiàn)滾動(dòng)效果
document.documentElement.scrollTop = value;
document.body.scrollTop = value;
}
window.onscroll = function() {
var scrollTop = getScrollTop();
if (scrollTop > 100) { //判斷滾動(dòng)條距離窗口頂部多遠(yuǎn)時(shí)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(guān)鍵，取得滾動(dòng)條豎直距離，除以speed后再給滾動(dòng)條設(shè)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(dòng)條豎直距離
speed = (0 - now) / 10;
speed = Math.floor(speed);
if (now == 0);
clearInterval(timer);
document.documentElement.scrollTop = now + speed; //標(biāo)準(zhǔn)模式下的瀏覽器
document.body.scrollTop = now + speed; //怪異模式下的瀏覽器
}, 15);
}