强制滚动到顶部按钮进入页脚的最简单方法是什么？我希望它始终保持在页脚上方，因为我计划在其中添加链接和文本，它最终会覆盖这些链接和文本。它应该在击中页脚之前停止，不要再往下走。
JSFiddle：https://jsfiddle.net/9s3kRT5tFBBHFVc8eT/9rwn1qub/5/
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
font-family: Arial, Helvetica, sans-serif;
font-size: 20px;
}
#myBtn {
display: none;
position: fixed;
bottom: 20px;
right: 30px;
z-index: 99;
font-size: 18px;
border: none;
outline: none;
background-color: red;
color: white;
cursor: pointer;
padding: 15px;
border-radius: 4px;
}
#myBtn:hover {
background-color: #555;
}
footer {
margin: 100px;
}
</style>
</head>
<body>
<button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>
<div style="background-color:black;color:white;padding:30px">Scroll Down</div>
<div style="background-color:lightgrey;padding:30px 30px 2500px">This example demonstrates how to create a "scroll to top" button that becomes visible
<strong>when the user starts to scroll the page</strong>.</div>
<footer>
This is a footer
</footer>
<script>
//Get the button
var mybutton = document.getElementById("myBtn");
// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {scrollFunction()};
function scrollFunction() {
if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
mybutton.style.display = "block";
} else {
mybutton.style.display = "none";
}
}
// When the user clicks on the button, scroll to the top of the document
function topFunction() {
document.body.scrollTop = 0;
document.documentElement.scrollTop = 0;
}
</script>
</body>
</html>
//Get the button
var mybutton = document.getElementById("myBtn");
// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {
scrollFunction()
};
function scrollFunction() {
if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
mybutton.style.display = "block";
} else {
mybutton.style.display = "none";
}
// check that button is not lower than footer.
let footerElement = document.getElementById('footer');
let footerElementRect = footerElement.getBoundingClientRect();
let mybuttonPositionBottom = mybutton.offsetTop + mybutton.offsetHeight;
if (footerElementRect.y < mybuttonPositionBottom) {
// if button is lower than footer.
// modify css bottom.
let diffheight = mybuttonPositionBottom - footerElementRect.y;
let style = window.getComputedStyle(mybutton);
let addBottom = parseInt(style.getPropertyValue('bottom')) + diffheight;
mybutton.style.bottom = addBottom + 'px'; // maybe add more 10 px for bottom space of a button.
} else {
// if button is heigher than footer. this including scroll up.
// remove custom css bottom.
mybutton.style.bottom = '';
}
}
// When the user clicks on the button, scroll to the top of the document
function topFunction() {
document.body.scrollTop = 0;
document.documentElement.scrollTop = 0;
}body {
font-family: Arial, Helvetica, sans-serif;
font-size: 20px;
}
#myBtn {
display: none;
position: fixed;
bottom: 20px;
right: 30px;
z-index: 99;
font-size: 18px;
border: none;
outline: none;
background-color: red;
color: white;
cursor: pointer;
padding: 15px;
border-radius: 4px;
}
#myBtn:hover {
background-color: #555;
}<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>
<div style="background-color:black;color:white;padding:30px">Scroll Down</div>
<div style="background-color:lightgrey;padding:30px 30px 2500px">This example demonstrates how to create a "scroll to top" button that becomes visible
<strong>when the user starts to scroll the page</strong>.</div>
<div id="footer">
This is footer.
</div>
</body>
</html>在jsfiddle 上看到它的作用
//Get the button
const mybutton = document.getElementById("myBtn");
const footer = document.getElementById('footer');
const btnMarginBottom = parseInt(window.getComputedStyle(mybutton).getPropertyValue('bottom'));
// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {scrollFunction()};
function scrollFunction() {
if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
mybutton.style.display = "block";
} else {
mybutton.style.display = "none";
}
const footStartPos = footer.getBoundingClientRect().y;
if (window.innerHeight > footStartPos) {
mybutton.style.bottom = `${(window.innerHeight - footStartPos) + btnMarginBottom}px`;
} else {
mybutton.style.bottom = '';
}
}
// When the user clicks on the button, scroll to the top of the document
function topFunction() {
document.body.scrollTop = 0;
document.documentElement.scrollTop = 0;
}body {
font-family: Arial, Helvetica, sans-serif;
font-size: 20px;
}
#myBtn {
display: none;
position: fixed;
bottom: 20px;
right: 30px;
z-index: 99;
font-size: 18px;
border: none;
outline: none;
background-color: red;
color: white;
cursor: pointer;
padding: 15px;
border-radius: 4px;
}
#myBtn:hover {
background-color: #555;
}
footer {
padding: 100px;
}<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>
<div style="background-color:black;color:white;padding:30px">Scroll Down</div>
<div style="background-color:lightgrey;padding:30px 30px 2500px">This example demonstrates how to create a "scroll to top" button that becomes visible
<strong>when the user starts to scroll the page</strong>.</div>
<footer id="footer">
This is a footer
</footer>
</body>
</html>