Intersection of mapped types

Question

Consider the following:

type Properties = {
    foo: { n: number };
    bar: { s: string };
    baz: { b: boolean };
};

declare function retrieveValues

        

Answer 1

Using conditional types and type inference in conditional types it is possible to transform { n: number } | { s: string } directly into { n: number } & { s: string }.

type GetKeys = U extends Record ? K : never

type UnionToIntersection = {
   [K in GetKeys]: U extends Record ? T : never
}

type Transformed = UnionToIntersection<{ a: string } | { b: number }>
// Transformed has type {a: string, b: number}

Playground Link

The reason this works is basically because conditional types are distributed over union types. From the conditional types pull request:

Conditional types in which the checked type is a naked type parameter are called distributive conditional types. Distributive conditional types are automatically distributed over union types during instantiation. For example, an instantiation of T extends U ? X : Y with the type argument A | B | C for T is resolved as (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y).