JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo"></div>

CSS

div { border: 3px solid red; width: 300px; height: 300px; }

JavaScript

var myDiv = document.getElementById('foo');

myDiv.onmouseover = function() { 
    alert('entered');
}

myDiv.onmouseout = function() { 
    alert('left');
}