滚动案例
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.slider-bar {
position: absolute;
left: 50%;
top: 300px;
margin-left: 600px;
45px;
height: 130px;
background-color: pink;
}
.w {
1200px;
margin: 10px auto;
}
.header {
height: 150px;
background-color: purple;
}
.banner {
height: 250px;
background-color: skyblue;
}
.main {
height: 1000px;
background-color: yellowgreen;
}
span {
display: none;
position: absolute;
bottom: 0;
}
</style>
</head>
<body>
<div class="slider-bar">
<span class="goBack">
<a href="#header">返回顶部</a>
</span>
</div>
<div class="header w" id="header">头部区域</div>
<div class="banner w">banner区域</div>
<div class="main w">主体部分</div>
<script>
// 1 获取元素
var sliderbar = document.querySelector('.slider-bar');
var banner = document.querySelector('.banner');
// 被滚动距离的大小
var barnerTop = banner.offsetTop;
// 侧边栏固定定位之后 应该变化的值
var sliderbarTop = sliderbar.offsetTop - barnerTop;
// 获取main 主体元素
var main = document.querySelector('.main');
var goBack = document.querySelector('.goBack');
var mainTop = main.offsetTop;
// 2 页面滚动事件 scroll
document.addEventListener('scroll', function() {
// 页面滚动的头部
// console.log(window.pageYOffset);
// 3 当滚动距离大于 172 侧边栏改为固定定位
if (window.pageYOffset >= barnerTop) {
sliderbar.style.position = 'fixed';
sliderbar.style.top = sliderbarTop + 'px';
} else {
sliderbar.style.position = 'absolute';
sliderbar.style.top = '300px';
}
// 4 当也没滚动到 main 盒子 显示 返回顶部模块
if (window.pageYOffset >= mainTop) {
goBack.style.display = 'block';
} else {
goBack.style.display = 'none';
}
})
</script>
</body>
</html>