index.html
<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>My JavaScript</title>
</head>
<body>
<select>
<option value="red">Red</option>
<option value="green">Green</option>
<option value="blue">Blue</option>
</select>
<button>OK</button>
<script src="main.js"></script>
</body>
</html>main.js
'use strict';
{
document.querySelector('button').addEventListener('click', () => {
alert(document.querySelector('select').value);
});
}