JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li class="active">6</li>
    <li>7</li>
</ul>

CSS

li.active { color:red; font-width: bold; }

JavaScript

var i = $("li.active").index();
$("li:lt("+i+")").addClass("active");