JSFiddle - React, Tailwind, and code Playground

HTML

<div> 
    <div onclick="countPos(this)">1</div> 
    <div onclick="countPos(this)">2</div> 
    <div onclick="countPos(this)">3</div> 
</div>

JavaScript

function countPos(elem){
    var nodeList = Array.prototype.slice.call( elem.parentNode.children );
    var index = nodeList.indexOf( elem ) + 1
    console.log(index);
}