La palabra reservada infer

type ExtraerRetorno<T> = T extends (...args: any[]) => infer R ? R : never;

type Fn = () => string;
type Retorno = ExtraerRetorno<Fn>; // string

type Awaited<T> = T extends Promise<infer U> ? U : T;

type A = Awaited<Promise<string>>;  // string
type B = Awaited<Promise<number>>;  // number
type C = Awaited<string>;           // string

type Elemento<T> = T extends (infer U)[] ? U : never;

type A = Elemento<string[]>;  // string
type B = Elemento<number[]>;  // number
type C = Elemento<(string | number)[]>; // string | number

type Primero<T> = T extends [infer First, ...any[]] ? First : never;
type Resto<T> = T extends [any, ...infer Rest] ? Rest : never;

type A = Primero<[1, 2, 3]>;  // 1
type B = Resto<[1, 2, 3]>;    // [2, 3]

type ValorPropiedad<T, K extends keyof T> = T extends Record<K, infer V> ? V : never;

type Obj = { nombre: string; edad: number };
type TipoNombre = ValorPropiedad<Obj, "nombre">; // string

type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
};

interface Config {
  api: {
    url: string;
    timeout: number;
  };
  debug: boolean;
}

type ConfigParcial = DeepPartial<Config>;
// { api?: { url?: string; timeout?: number }; debug?: boolean; }