JSFiddle - React, Tailwind, and code Playground

HTML

<ol>
    <li>Some value</li>
    <li>another</li>
    <li>more</li>
    <li>more</li>
</ol>

JavaScript

var yourFunction = function (index) {
    alert(index);
};

var ol = document.querySelector("ol");
var olLi = ol.querySelectorAll("li");
var handleClick = function(e) {
    var target = e.target;
    var index = 0;

    while (target = target.previousElementSibling) {
        index++;   
    }
    
    yourFunction(index);
    
};

ol.addEventListener("click", handleClick);