JSFiddle - React, Tailwind, and code Playground

HTML

<div class="trow">
  <span class="group">1</span>
  <span class="group">2</span>
  <span class="group">3</span>
  <span>4</span>
  <span>5</span>
  <span>6</span>
</div>

<div class="trow">
  <span class="group">1</span>
  <span class="group">2</span>
  <span class="group">3</span>
  <span>4</span>
  <span>5</span>
</div>

CSS

.trow .group.last{
  background:blue;
  color:#fff;
}

JavaScript

$(".trow").each(function(){
	$(this).children(".group").last().addClass("last");
})