JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li><a href="#" id="0">Services</a></li>
    <li><a href="#" id="1">Portfolio</a></li>
    <li><a href="#" id="2">Contact</a></li>
</ul>

JavaScript

window.onload = function() {
    var links = document.getElementsByTagName("a");
    for (var i = 0; i < links.length; i++) {
        links[i].onclick = function() {
            var index = FindIndex(this);
            alert("index: " + index);
            return false;
        };
    }
};

function FindIndex(oLink) {
    var listItem = oLink.parentNode;
    var oList = listItem.parentNode;
    var allItems = oList.getElementsByTagName("li");
    for (var i = 0; i < allItems.length; i++) {
        if (allItems[i] === listItem)
            return i;
    }
    return -1;
}