JSFiddle - React, Tailwind, and code Playground

HTML

<div class="test">
    
</div>

CSS

.test {
    background-color: red;
    height: 100%;
}

.test:before {
    content: '';
    display: block;
    width: 20px;
    height: 20px;
    background-color: green;
}

JavaScript

$(function () {
    alert($('.test:before').height()); 
});