JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    width:100px;
    height:100px;
    background:gray;
    position:relative;
}
div:after {
    content:'';
    position:absolute;
    width:50px;
    height:50px;
    background:red;
    right:-50px;
    top:0;
}
.lol:after {
    background:blue;
}

JavaScript

$("div").click(function(){
    $(this).addClass('lol');
});