TypeScript: conditional types и `infer` — пример кода

// Rebuild the standard-library conditionals — a frequent whiteboard task
type MyReturnType<T>   = T extends (...args: any[]) => infer R ? R : never;
type MyParameters<T>   = T extends (...args: infer P) => any ? P : never;
type MyNonNullable<T>  = T extends null | undefined ? never : T;
type MyAwaited<T>      = T extends Promise<infer U> ? MyAwaited<U> : T; // recurse to unwrap nesting

type R = MyReturnType<(x: number) => string>;         // string
type A = MyAwaited<Promise<Promise<number>>>; // number

// The distribution gotcha, made explicit
type Wrap<T> = T extends any ? { v: T } : never;
type Distributed = Wrap<'a' | 'b'>;      // { v: 'a' } | { v: 'b' }
type Blocked = [('a' | 'b')] extends [any] ? { v: 'a' | 'b' } : never; // { v: 'a' | 'b' }

// Why NonNullable is written with a naked T: it must distribute to filter each member
type Clean = MyNonNullable<string | null>; // string