<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
</head>
<body>
<div id="box" style="background-color: salmon; width: 100px; height: 100px">
Box 1
</div>
<button id="btn">Hide div</button>
<script src="index.js"></script>
</body>
</html>