JSFiddle - React, Tailwind, and code Playground

HTML

<div class="d">Test</div>
<div class="d">Test</div>
<div class="d">Test</div>
<div class="d">Test</div>
<div class="d">Test</div>
<div class="d">Test</div>

CSS

.d
{
    background: red;
    color: white;
}

.first-d
{
    background: blue;
}

JavaScript

$(document).ready(function(){
    $($(".d")[0]).addClass("first-d");
});