Transform type keys to union with optional parameters

Question

Is there a way type to transform the Screens type into ScreensNew type?

What I want to do is:

• Transform all first level keys into strings (like keyof)
• Keep the other parameters as is (strip undefined)
• Make a union type of each property afterwards

type Screens = {
  First: undefined;
  Second: {
    name: string;
  }
}

type ScreensNew =
  | "First"
  | "Second": { name: string; }
}

Answer 1

The type in your post is not a valid TS type. We can create a discriminated union of the form type ScreensNew = | { type: "First" } | { type: "Second", name: string; }:

type Screens = {
  First: undefined;
  Second: {
    name: string;
  }
}

type CreateDiscriminatedProps<T> = T extends undefined ? {} : T
type CreateDiscriminated<T> = {
  [P in keyof T]: { type: P } & CreateDiscriminatedProps<T[P]>
}

type ScreensNew = CreateDiscriminated<Screens>

//Same as
// type ScreensNew = {
//     First: {
//         type: "First";
//     };
//     Second: {
//         type: "Second";
//     } & {
//         name: string;
//     };
// }

Playground Link