JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <li onclick="func(this)">abc</li>
    <li onclick="func(this)">def</li>
    <p onclick="func(this)">ghi</p>
</div>

JavaScript

function func(el) {
			    els = document.getElementsByTagName(el.tagName)
                // els = el.parentNode.children
			    num = -1
			    for (var i = 0; i < els.length; i++) {
			        if (els[i] == el) {
			            num = i
			            break
			        }
			    }
			    if (num >= 0) {
			        el.textContent += num
			    }
			}