Result type in TypeScript

Result.ts

type Success<T> = { isSuccess: true; data: T};
type Failure<E> = { isSuccess: false; error: E };

export type Result<T, E> = Success<T> | Failure<E>;

export const result = {
  succeed<T>(data: T) {
    return { isSuccess: true, data } as Success<T>;
  },
  fail<E>(error: E) {
    return { isSuccess: false, error } as Failure<E>;
  }
};

// DEMO

function doSomething(fail: boolean): Result<{message: string}, string> {
  if (fail) return result.fail('Something went wrong');

  return result.succeed({ message: 'Success' });
}

(function main() {
  const result = doSomething(false);

  // isSuccess determines whether it's a failure or not
  if (!result.isSuccess) {
    const error: string = result.error;
    console.log(result.error.toString());

    console.log(error);
    return;
  }

  // result.data is not undefined because isSuccess is guaranteed to be true
  const data: {message: string} = result.data; // no ts error

  console.log(data);
})();