Helper types to resolve selectables

ts-deep-entity-select.ts

interface User {
    id: number;
    userName: string;
    fullName: string;
}

interface Comment {
    id: number;
    body: string;
    author: User;
}

interface Post {
    id: number;
    author: User;
    body: string;
    content: string;
    comments: Comment[];
}

type Selectable<T> = {
    [P in keyof T]?: 
        T[P] extends Array<infer U> ? (Selectable<U>  | boolean) :
        T[P] extends object ? (Selectable<T[P]> | boolean) :
        boolean;
}

type Select<T, Q extends Selectable<T>> = {
    [P in keyof T as Q[P] extends boolean ? P : Q[P] extends object ? P : never ]: 

        // Property
        Q[P] extends true ? T[P] : 

        // Array
        T[P] extends Array<infer U> ? Select<U, Q[P]>[] :

        // Object
        T[P] extends object ? Select<T[P], Q[P]> : never
}

type SelectablePost = Selectable<User>;
type Selected = Select<Post, {
    id: true;
    author: {
        fullName: true;
    };
    comments: {
        body: true;
        author: {
            userName: true;
        }
    }
}>

const result: Selected = {
    id: 1231,
    author: {
        fullName: "Hello world"
    },
    comments: {
        body: "Foo",
        author: {
            userName: "Johnson"
        }
    }
};