JSFiddle - React, Tailwind, and code Playground

HTML

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

<ul>
    <li>Item 1</li>
    <li class="active">Item 2</li>
    <li>Item 3</li>
</ul>

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

JavaScript

alert($('li.active').index());