JSFiddle - React, Tailwind, and code Playground

HTML

<div id="extendedinfobox">
   <ul class="prodtabmenu">
    <li>1</li>
    <li>2</li>
    <li class="active">3</li>
    <li>4</li>
    <li>5</li>
  </ul>
 </div>
<div class="body">
    
</div>

CSS

.body.newClass
{
    background: red;
    padding: 10px;
}

JavaScript

if($('ul.prodtabmenu').find('li:nth-child(3)').hasClass('active'))
{
   $('.body').addClass('newClass');   
}