Optional parameters on typescript interfaces?

Question

As explained here the ? operator can be used to mark a function parameter as optional. What does the ? operator mean on interface parameters? For example if we have this typescript interface:

    export interface Person {
    phone?: number;
    name?: string;
}

And a class that implements the interface:

class Customer implements Person {
} 

Did Customer now implement Person correctly because all the properties on the Person interface are optional?

Answer 1

The short answer is yes, Customer correctly implements Person since all fields of the interface are optional any object will correctly implement the interface.

The usefulness of this interface is:

• On the implementer site, if any optional field is declared, the type must correspond (so phone has to be defined as number )
• On receiving side (for example as a function parameter) you can only access fields that are potentially part of Person (you should check if they are undefined ) but the function for example guarantees it will not access any other fields of a Person parameter.