JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
 <li>Anton</li>
 <li class="abc">Victor</li>
 <li class="abc">Simon</li>
 <li>Adam</li>
 <li>Peter</li>
 <li class="abc">Tom</li>
</ul>

JavaScript

alert($('li:contains(Simon)').index('.abc'));
//Or this...
alert($('li').filter(':contains(Simon)').index('.abc'));