javascript中的简单常量在我的函数中不起作用

Question

我试图用const简化我的代码，但它不起的作用

// my const 'myscroll' is not working ?? Why ??
const myscroll = document.querySelector('.scrollup');
// 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 > 800 || document.documentElement.scrollTop > 800) {
myscroll.style.display = "block";
} else {
myscroll.style.display = "none";
}
};

我必须这样做

window.onscroll = function () {scrollFunction()};
function scrollFunction() {
if (document.body.scrollTop > 800 || document.documentElement.scrollTop > 800) {
document.querySelector('.scrollup').style.display = "block";
} else {
document.querySelector('.scrollup').style.display = "none";
}
};

这一定很简单，但我是盲人！感谢您的帮助

Answer 1

const myscroll = document.querySelector('.scrollup');
window.onscroll = scrollFunction;
function scrollFunction(e) {
const state = document.documentElement.scrollTop > 200;
myscroll.style.display = state ? 'block' : 'none';
};

body {
height: 500vh;
}
.scrollup {
width: 50px;
height: 50px;
position: fixed;
bottom: 10px;
right: 10px;
background-color: red;
border-radius: 50%;
display: none;
}

<div class="scrollup"></div>