Question mark Typescript variable

Question

I\'ve seen code snippets like these:

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

But why

Answer 1

In TypeScript, ?: a shorthand for : | 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.