const user = {
  profile: {
    settings: {
      theme: 'dark'
    }
  }
};

const getTheme = (obj) => obj?.profile?.settings?.theme ?? 'light';
const getLanguage = (obj) => obj?.profile?.settings?.language ?? 'en';
const getNotifications = (obj) => obj?.profile?.notifications?.enabled ?? true;

console.log(getTheme(user));
console.log(getLanguage(user));
console.log(getNotifications(user));
console.log(getTheme(null));

let obj = { a: 1 };
let proto = { b: 2 };

Object.setPrototypeOf(obj, proto);

for (let key in obj) {
  console.log(key);
}