JSFiddle - React, Tailwind, and code Playground

HTML

<div class="pop-up">x</div>

CSS

body, html {
    width: 100%;
    height: 100%;
}
.pop-up {
    width: 50%;
    height: 50%;
    border: 1px solid red;
    position: fixed;
}

JavaScript

$('.pop-up').css({
     marginLeft: $(this).width() / 4,
     marginTop: $(this).height() / 4
 });