typescript - Intersection of mapped types

Question

Consider the following:

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

declare function retrieveValues<K extends keyof Properties>(add?: K[]): Pick<Properties, K>[K];

// what happens
const x: { n: number } | { s: string } = retrieveValues(['foo', 'bar']);

// what I'm really trying to express (type error)
const y: { n: number } & { s: string } = retrieveValues(['foo', 'bar']);

Is there a way to get an intersection of the properties of Pick<Properties, K>? Or just a different way to get the intersection of a set of types based on the presence of relevant strings in an array?

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> = U extends Record<infer K, any> ? K : never

type UnionToIntersection<U extends object> = {
   [K in GetKeys<U>]: U extends Record<K, infer T> ? 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).