JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="foo">
 <li class="item-1">Cheval</li>
 <li class="item-2">Cochon</li>
 <li class="item-3">Lapin</li>
 <li class="item-4">Requin</li>
</ul>

JavaScript

$(function(){
ll=$('#foo li').length
$('#foo li:eq(2)').after( $('<li/>').html("Nouveau Li").addClass('item-'+ll)) 
})