When the user scrolls down 1020px from the top of the document, show a button that will scroll back to the top of the page

scroll-to-top-bttn.css

/* Back to Top Icon */
#btt-icon {
    width: 46px;
    height: 50px;
    position: fixed;
    bottom: 40px;
    right: 40px;
    background-color: rgba(136, 136, 136, 0.18);
    cursor: pointer;
    border-radius: .2rem;
    display: none;
    align-items: center;
    justify-content: center;
}

#btt-arrow {
    position: relative;
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 0 15px 20px 15px;
    border-color: transparent transparent #0075C9 transparent;
    z-index: 99;
}

@keyframes bounceUp {
  0% {
    -webkit-transform: translateY(0px);
    -moz-transform: translateY(0px);
    -o-transform: translateY(0px);
    -ms-transform: translateY(0px);
    transform: translateY(0px); }
  100% {
    -webkit-transform: translateY(-12px);
    -moz-transform: translateY(-12px);
    -o-transform: translateY(-12px);
    -ms-transform: translateY(-12px);
    transform: translateY(-12px); } }
/* Back to Top Icon Ends */

scroll-to-top-bttn.html

<!-- Back to Top Icon -->
<div id="btt-icon">
	<div id="btt-arrow"></div>
</div>

scroll-to-top.js

// Scroll to Top ============================================= //
// When the user scrolls down 1020px from the top of the document, show the button
window.onscroll = function() {scrollFunction()};

var bttIcon = document.getElementById("btt-icon");
var bttArrow = document.getElementById("btt-arrow");

function scrollFunction() {

    if (document.body.scrollTop > 1820 || document.documentElement.scrollTop > 1820) {
        bttIcon.style.display = "flex";
    } else {
        bttIcon.style.display = "none";
    }
}

bttIcon.addEventListener('click', function (e) {
    e.preventDefault();

    document.querySelector('header').scrollIntoView({
        behavior: 'smooth'
    });
});

bttIcon.addEventListener("mouseover", bounceArrow, false);
function bounceArrow() {
    bttArrow.style.animation = "bounceUp .4s ease-in-out infinite";
}
bttIcon.addEventListener("mouseout", bounceStop, false);
function bounceStop() {
    bttArrow.removeAttribute("style");
}