JSFiddle - React, Tailwind, and code Playground

HTML

<div id="tes">X</div>

CSS

#tes {
  display: inline;
  background-color: silver;
  font-size: 32px;
  padding: 0.5em;
}

JavaScript

let i = 0;

let testi = document.getElementById('tes');

testi.onmouseover = function () {
testi.innerHTML = i++;
}