Question mark Typescript variable

Question

I've seen code snippets like these:

export interface IUser {
    email?: string;
    firstName?: string;
    lastName?: string;
}

But why are the variable names suffixed by a question mark? This snippet is part of an example of using mongodb with Typescript.

The answer is probably somewhere out there but I seem to be using the wrong keywords since I can't find it.

Answer 1

In TypeScript, <name>?: <typename> a shorthand for <name>: <typename> | undefined.

This indicates to the type system that a symbol may contain a value of the indicated type or it may contain the value undefined (which is like null).

This is important when the (new in TypeScript 2) --strictNullChecks option is enabled. The documentation on Null- and undefined-aware types option is probably where you should start to understand why this is useful.

Answer 2

It means they can be there but dont have to be. It allows for optional field names. It can be quite common to use.

An example use is allowing users on a website to have an optional display name.