JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li>My element 1</li>
  <li>My element 2</li>
  <li>My element 3</li>
</ul>

JavaScript

var mylist = document.querySelector('ul');
mylist.addEventListener('click', function(evt){
  alert( getElementIndex(evt.target) )
})

function getElementIndex(node) {
  var index = 0;
  while ((node = node.previousElementSibling)) {
    index++;
  }
  return index;
}