JSFiddle - React, Tailwind, and code Playground

HTML

<div class="aaa" id="block"></div>

CSS

.aaa{
  background: green;
  width: 100px;
  height: 100px;
}

.bbb{
  background: blue;
  width: 100px;
  height: 100px;
}

JavaScript

var div = document.getElementById("block");

div.setAttribute("class", "bbb")


var div2 = document.createElement("div");
div2.className = "aaa"
document.body.appendChild(div2)