回到顶部效果：
可以中途暂停，达到一屏幕才出现totop按钮，不到一屏按钮消失。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>回到顶部效果</title>
<style>
body {
height: 10000px;
}
p {
position: fixed;
right: 20px;
bottom: 20px;
50px;
height: 50px;
background: rgba(102,102,102,0.3);
font-size: 18px;
line-height: 50px;
text-align: center;
color: #333;
cursor: pointer;
display: none;
}
p:hover {
background: rgba(102,102,102,1);
color: #fff;
}
</style>
</head>
<body>
<p id="totop">Totop</p>
<script>
window.onload = function () {
var totop = document.getElementById('totop');
var clientHeight = document.documentElement.clientHeight;
console.log(clientHeight);
var timer = null;
var isTop = true;
window.onscroll = function () {
var t = document.documentElement.scrollTop || document.body.scrollTop;
console.log(t);
if (t >= clientHeight) {
totop.style.display ='block';
}else if(t < clientHeight) {
totop.style.display ='none';
}
if(!isTop) {
clearInterval(timer);
}
isTop = false;
}
totop.onclick = function () {
clearInterval(timer);
timer = setInterval(function () {
//获取滚动条距离顶部的距离
var t = document.documentElement.scrollTop || document.body.scrollTop;
var speed = Math.floor(-t / 30);
document.documentElement.scrollTop = document.body.scrollTop = t + speed;
console.log(t - speed);
isTop = true;
if (t == 0) {
clearInterval(timer);
}
}, 30);
}
}
</script>
</body>
</html>