JSFiddle - React, Tailwind, and code Playground

HTML

<div class="myClass"></div>
<div class="myClass"></div>
<div class="myClass"></div>
<div class="myClass" id="fourth"></div>
<div class="myClass"></div>

JavaScript

var myClass = document.getElementsByClassName('myClass');
var fourth = document.getElementById('fourth');

function indexInClass(node) {
    var className = node.className;
    var num = 0;
    for (var i = 0; i < className.length; i++) {
        if (className[i] === node) {
            return num;
        }
        num++;
    }
    return -1;
}

console.log(myClass[indexInClass(fourth)]);