JSFiddle - React, Tailwind, and code Playground

HTML

<div class="popupContent">popupContent</div>

CSS

.popupContent{border:solid black 1px;padding:5px;}

JavaScript

$.fn.center = function () {
    this.css("position","absolute");
    this.css("top", (($(window).height() - this.outerHeight()) / 2) + $(window).scrollTop() + "px");
    this.css("left", (($(window).width() - this.outerWidth()) / 2) + $(window).scrollLeft() + "px");
    return this;
}
    
    
    $(".popupContent").center();