<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <button id="button">Ekle</button>
    <ul id="users"></ul>
    <script>
        const users = Object.create({});
        const addUser = () => {
            let userId = (Object.keys(users).length + 1);
            if( userId < 256 ) {
                let username = prompt("Kullanıcı Adınız:");
                let password = prompt("Şifreniz:");
                users[userId] = Object.create({ username, password });
                return document.getElementById("users").innerHTML += `<li>${userId}: ${users[userId].username}, ${users[userId].password}`;
            }
            return alert("Maalesef, slot yok.");
        }
        document
            .getElementById("button")
            .addEventListener("click", addUser);
    </script>
</body>
</html>
Bu şekilde daha net olabilir belki