JSFiddle - React, Tailwind, and code Playground

HTML

<title>Untitled Document</title>
<body>
    <div id="main"></div>
    <footer>Footer content</footer>
</body>

CSS

#main {

        height: 8000px;

        max-width: 900px;

        background-color: blue;

        margin: auto;

    }

    footer {

        width: 100%;

        background-color: lightblue;

        position:absolute;

        color: white;

        bottom:0;

    }

JavaScript

$(function () {
    var mainHt = $('#main').height();
    $('footer').css('top',mainHt);
});