Untitled

<html>
    <body>
        <h1>Guestbook</h1>

        <div id='posts'>
        </div>

        <hr />

        <form action='/post' method='post'>
            <input type='text' name='username' placeholder='Username' /><br />
            <textarea name='post' placeholder='Post text'></textarea><br />
            <input type='submit' />
        </form>

        <script>
            document.body.onload = () =>
                void fetch('/posts')
                .then(posts => posts.json())
                .then(posts => posts.map(
                    ({username, post}) => `
                        <div>
                            <b>${username}</b><br />
                            ${post}
                        </div>
                    `))
                .then(posts => posts.join('<br />'))
                .then(posts => void (document.getElementById('posts').innerHTML = posts))
        </script>
    </body>
</html>