Using Events
<html>
    <head></head>
    <body>
        <label>
            Type something in:
            <input type="text" onkeydown="handleKey(event)">
        </label>
        <h1 id="letter-display">?</h1>

        <script>
            const letterDisplay = document.getElementById("letter-display");

            function handleKey(event) {
                letterDisplay.innerText = event.key;
            }
        </script>

    </body>
</html>