recursive-conditional-types.ts

// Awaiting promises

type Awaited<T> =
    T extends null | undefined ? T :
    T extends PromiseLike<infer U> ? Awaited<U> :
    T;

type P1 = Awaited<Promise<string>>;  // string
type P2 = Awaited<Promise<Promise<string>>>;  // string
type P3 = Awaited<Promise<string | Promise<Promise<number> | undefined>>;  // string | number | undefined

// Flattening arrays

type Flatten<T extends readonly unknown[]> = T extends unknown[] ? _Flatten<T>[] : readonly _Flatten<T>[];
type _Flatten<T> = T extends readonly (infer U)[] ? _Flatten<U> : T;

type InfiniteArray<T> = InfiniteArray<T>[];

type A1 = Flatten<string[][][]>;  // string[]
type A2 = Flatten<string[][] | readonly (number[] | boolean[][])[]>;  // string[] | readonly (number | boolean)[]
type A3 = Flatten<InfiniteArray<string>>;
type A4 = A3[0];  // Infinite depth error

// Repeating tuples

type TupleOf<T, N extends number> = N extends N ? number extends N ? T[] : _TupleOf<T, N, []> : never;
type _TupleOf<T, N extends number, R extends unknown[]> = R['length'] extends N ? R : _TupleOf<T, N, [T, ...R]>;

type T1 = TupleOf<string, 3>;  // [string, string, string]
type T2 = TupleOf<number, 0 | 2 | 4>;  // [] | [number, number] | [number, number, number, number]
type T3 = TupleOf<number, number>;  // number[]
type T4 = TupleOf<number, 100>;  // Depth error