JSFiddle - React, Tailwind, and code Playground

HTML

<div style="height: 200vh;">
    <div id="back-top"></div>
</div>

CSS

#back-top {
    position: fixed;
    width: 100px;
    height: 100px;
    opacity:0;
    background-color: black;
}

JavaScript

$(window).scroll(function () {
    if ($(this).scrollTop() > 100) {
        $('#back-top').fadeTo("fast", 0.6);
    } else {
        $('#back-top').fadeTo("fast", 0);
    }
});

$('#back-top').hover(function () {
    $(this).fadeTo("slow", 1);
}, function () {
    $(this).fadeTo("slow", 0.6);
});