DeepPick.ts

type RequiredKeys<O extends unknown> = {
  // eslint-disable-next-line @typescript-eslint/ban-types
  [Key in keyof O]: {} extends Pick<O, Key> ? never : Key;
}[keyof O];

type DeepPick<O extends unknown, Keys extends unknown[]> = Keys extends [
  keyof O,
  ...infer Rest
]
  ? Keys[0] extends RequiredKeys<O>
    ? { [K in Keys[0]]: DeepPick<O[Keys[0]], Rest> }
    : { [K in Keys[0]]?: DeepPick<O[Keys[0]], Rest> }
  : O;

type A = { a: 1; b: 2 };
type A2 = DeepPick<A, ['a']>;
type A3 = DeepPick<A, ['b']>;
type A4 = DeepPick<A, ['c']>;
type A5 = DeepPick<A, [1]>;

type B = { a: 1; b: { c: 2; d: 3 } };
type B1 = DeepPick<B, ['b', 'c']>;
type B2 = DeepPick<B, ['b', 'e']>;

type C = { a: 1; b: { c: 2; d: { e: 3; f: 4 } } };
type C1 = DeepPick<C, ['b', 'c']>;
type C2 = DeepPick<C, ['b', 'd']>;
type C3 = DeepPick<C, ['b', 'd', 'e']>;
type C4 = DeepPick<C, ['b', 'd', 'g']>;

type D = { a: 1; b?: { c?: 2; d: { e?: 3; f?: 4 } } | null };
type D1 = DeepPick<D, ['b', 'c']>;
type D2 = DeepPick<D, ['b', 'd']>;
type D3 = DeepPick<D, ['b', 'd', 'e']>;
type D4 = DeepPick<D, ['b', 'd', 'e', 'f']>;
type D5 = DeepPick<D, ['b', 'd', 'g']>;