<style>
* {
margin: 0;
padding: 0;
}
div {
width: 1200px;
margin: 10px auto;
}
.header {
margin-top: 10px;
height: 100px;
background-color: hsl(39, 87%, 55%);
}
.banner {
height: 500px;
background-color: hsl(216, 81%, 58%);
}
.main {
height: 1000px;
background-color: hsl(337, 62%, 57%);
}
.slider-bar {
position: absolute;
top: 300px;
right: 120px;
height: 200px;
width: 40px;
background-color: #45d80ba9;
}
span {
display: none;
text-decoration-line: none;
color: #000;
font-weight: bold;
font-size: 20px;
}
</style>
</head>
<body>
<div class="header"></div>
<div class="banner"></div>
<div class="main"></div>
<div class="slider-bar"><span class="goBack">返回顶部</span></div>
<script>
var banner = document.querySelector(".banner");
var main = document.querySelector(".main");
var sliderbar = document.querySelector(".slider-bar");
var goBack = document.querySelector(".goBack");
var bannerTop = banner.offsetTop;
var mainTop = main.offsetTop;
var sliderbarTop = sliderbar.offsetTop - bannerTop;
document.addEventListener("scroll", function () {
// 1、页面被卷去的头部大于等于banner的offsetTop时 让left变为固定定位 改变left的top值【如果不修改left的边偏移 变成固定定位后 还是会以原来的边偏移进行偏移】
if (window.pageYOffset >= banner.offsetTop) {
sliderbar.style.position = "fixed";
sliderbar.style.top = sliderbarTop + "px";
} else {
sliderbar.style.position = "absolute";
sliderbar.style.top = "300px";
}
// 2、页面被卷去的头部大于等于footer的offsetTop时 显示a标签
if (window.pageYOffset >= mainTop) {
goBack.style.display = "block";
} else {
goBack.style.display = "none";
}
});
goBack.addEventListener("click", function (e) {
// 需要先禁止goBack的默认行为，点击goBack才能实现页面滚动
e.preventDefault();
//因为是窗口滚动，所以对象是 window
animate(window, 0);
});
// 带有动画的返回顶部
//动画函数
function animate(object, target, callback) {
clearInterval(object.timer);
object.timer = setInterval(function () {
var step = (target - window.pageYOffset) / 10;
step = step > 10 ? Math.ceil(step) : Math.floor(step);
window.scroll(0, window.pageYOffset + step);
if (window.pageYOffset == target) {
clearInterval(object.timer);
callback && callback();
}
//把每次加1 这个步长值改为一个慢慢变小的值 步长公式：（目标值-现在的位置）/ 10
//obj.style.left = obj.offsetLeft + step + "px";
//window.scroll(x,y) x y 不写单位 直接写数字即可
window.scroll(0, window.pageYOffset + step);
}, 15);
}
</script>
</body>
