JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>1</li>
    <li class="ok">2</li>
    <li>3</li>
    <li id="test">4</li>
    <li>5</li>
</ul>

JavaScript

var elem = document.getElementById('test')

console.log(getElementIndex(elem))
console.log(getElementIndex('.ok'))



function getElementIndex(elem) {
    elem = elem.tagName ? elem : document.querySelector(elem)
	return [].indexOf.call(elem.parentNode.children, elem)
}