JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li><a href="#">link 1</a></li>
    <li><a href="#">link 2</a></li>
    <li><a href="#" class="active">link 3</a></li>
    <li><a href="#">link 4</a></li>
</ul>

CSS

.active {
    color: pink;
}

JavaScript

var index = $('ul').find('a').index('.active'); // -1

alert(index);