ReturnTypes.ts

example.ts

class Foo {
  name = 'bar';

  fun1(): number {
    return 12;
  }

  async func2(): Promise<{foo: string}> {
    return {foo: 'foo'};
  }
}

type FooTypes = ReturnTypes<Foo>;
// =>    
// type FooTypes = {
//     name: string;
//     fun1: number;
//     func2: {
//         foo: string;
//     };
// }

ReturnTypes.ts

export type ReturnTypes<T> = {
  [P in keyof T]: T[P] extends (...args: any[]) => any
    ? ReturnType<T[P]> extends PromiseLike<infer U>
      ? U
      : ReturnType<T[P]>
    : T[P];
};