ยูทิลิตี Flatten และ UnwrapPromise

type Flatten<T> = T extends (infer U)[] ? Flatten<U> : T;
type A = Flatten<number[][][]>; // number
type B = Flatten<string[]>;     // string

type FlatOnce<T> = T extends (infer U)[] ? U : T;
type A = FlatOnce<number[][]>; // number[]
type B = FlatOnce<string[]>;   // string

type Flatten<T> = T extends (infer U)[] ? Flatten<U> : T;
type C = Flatten<boolean[][][]>; // boolean

type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;
type D = UnwrapPromise<Promise<string>>; // string
type E = UnwrapPromise<number>;          // number

type DeepUnwrap<T> = T extends Promise<infer U> ? DeepUnwrap<U> : T;
type F = DeepUnwrap<Promise<Promise<number>>>; // number

type Simplify<T> =
  T extends Promise<infer U> ? Simplify<U>
  : T extends (infer V)[] ? Simplify<V>
  : T;

type G = Simplify<Promise<string[][]>>; // string

type H = Awaited<Promise<Promise<string>>>; // string
// Also handles custom thenables with .then()

type MapValue<T> = T extends Map<any, infer V> ? V : never;
type I = MapValue<Map<string, number[]>>; // number[]

// This requires recursive mapped types + intersection spreading
// See the DeepPartial lesson for the base pattern

type ApiData<T> = { data: T; status: number };
type ExtractData<T> = T extends ApiData<infer D> ? D : T;
type J = ExtractData<ApiData<User[]>>; // User[]