Exercise: who and where from

Write a function function describeUser(user) that returns the string "<name> from <city>". The name is in the property user.name, the city in the property user.address.city. When there is no name, guest goes in its place, and when there is no address or no city in it, unknown.

• describeUser({ name: "Ann", address: { city: "Oslo" } }) → "Ann from Oslo"
• describeUser({ name: "Bob" }) → "Bob from unknown"
• describeUser({ address: { city: "Rome" } }) → "guest from Rome"

Constraints: user is an object, possibly empty and with extra properties; name and city, when present, are non-empty strings; the property address may be missing, may be null, or may be an object without city.

Do not add extra imports unless the task says so explicitly.