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');
console.log(myClass.indexOf(fourth));