JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
    <div class="foo">yah</div>
 </div>

CSS

div {
    float: left;
    clear: both;
    padding:10px;
}

JavaScript

$('.foo:nth-child(6n-3)').css("background-color", "red");