JSFiddle - React, Tailwind, and code Playground

HTML

<div class='a'>im a div</div>
<div class='a'>im a div</div>
<div class='a'>im a div</div>

JavaScript

//var a = document.getElementsByClassName("a");
//var b = document.getElementsByClassName("b");

var a = $(".a");
var b = $(".b");

console.log("a is: "+a.length+", b is: "+b.length);
a[1].className="b";
console.log("a is: "+a.length+", b is: "+b.length);