<!doctype html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link href="/assets/reset.css" rel="stylesheet">
<link href="setup.css" rel="stylesheet">
<link href="style.css" rel="stylesheet">
<script defer src="script.js"></script>
</head>
<body>
<section>
<p>I’ve just added some text here to have another element, and also added some CSS to style it a bit—nothing too fancy.</p>
</section>
<button id="example">Click here!</button>
</body>
</html>
index.html
// Set up our variables.
let highlightClass = 'highlight' // “Strings” (like a class name) are wrapped in quotes.
let textBlock = document.querySelector('section') // Any selector.
let switchButton = document.querySelector('#example')
switchButton.onclick = () => { // “Listen” for clicks.
textBlock.classList.toggle(highlightClass) // Toggle the class!
}