JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="test">
    <li>one</li>
    <li>two</li>
    <li>three</li>
    <li>four</li>
</ul>

JavaScript

var ul = document.getElementById('test'),
    two = ul.getElementsByTagName('li')[2];

function getIndex(li, ul) {
    var lis = ul.getElementsByTagName('li');
    for (var i = 0, len = lis.length; i < len; i++) {
        if (li === lis[i]) {
            return i;
        }
    }
}
alert(getIndex(two, ul));