JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li class='listItem'>listitem1</li>
    <li class='listItem'>listitem2</li>
    <li class='listItem'>listitem3</li>
</ul>

JavaScript

var arr = document.getElementsByClassName('listItem');
for (var i = 0; i < arr.length; i++) {
    arr[i].onmouseover = function() {
        alert(i);
    }
}