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.getElementsByTagName("ol")[0];
var olLi = ol.getElementsByTagName("li");
var handleClick = function(e) {
    e = e || window.event;
    var target = e.target || e.srcElement;
    var index = 0;

    while (target = target.previousSibling) {
        (target.nodeType == 1) && (index++);   
    }
    
    yourFunction(index);
    
};

ol.addEventListener && ol.addEventListener("click", handleClick);
ol.attachEvent && ol.attachEvent("onclick", handleClick);