Mapped Types: removing optional modifier

Question

Given this code:

interface Foo{
  one?: string;
  two?: string;
}

type Foo2 = {
  [P in keyof Foo]: number;
}

I would expect the type of Foo2 to be { one: number; two: number; } However, instead it seems to keep the optional modifier { one?: number; two?: number; }

Is it possible to remove the optional modifier when using mapped types?

Answer 1

In Typescript 2.8 you can explicitly eliminate the modifier:

type Foo2 = {
  [P in keyof Foo]-?: number;
}

Or use the Required type that is built into newer versions.

If you are using an older version you can use this workaround:

type Helper<T, TNames extends string> = { [P in TNames]: (T & { [name: string]: never })[P] };
type Foo3 = Helper<Foo, keyof Foo>;