Object interfaces

The label function assumes the object it receives has a title and a price, and nothing guarantees that. An interface gives that shape a name once, and it becomes a contract the compiler checks at every call.

The matching here is structural: any object that has the required fields with the right types matches the interface, wherever it came from, so you don't need a class or implements.

An object literal, meaning an object you write directly where it's used, gets a stricter check: an extra field in it is an error, because it's usually a slip, like a misspelled name or a field nobody will ever read. This check applies only to object literals, though. A variable you already have that carries extra fields passes without complaint.