JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
</ul>
<ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
</ul>
<ul>
    <li>1</li>
    <li>2</li>
</ul>

CSS

.marked { font-weight: bold; color: red; }

JavaScript

$("ul li:last-child").addClass('marked');