function getCity(person) {
  return person?.address?.city ?? 'Unknown';
}

const data = [
  null,
  { name: 'Alice' },
  { name: 'Bob', address: null },
  { name: 'Charlie', address: { street: '123 Main' } },
  { name: 'David', address: { city: 'Boston' } }
];

const cities = data.map(getCity);
console.log(cities);