Response envelope

responseEnvelope.ts

type ResponseEnvelopeYuki<T> = ResponseEnvelopeSuccess<T> | ResponseEnvelopeFailure;

type ResponseEnvelopeSuccess<T> = {
  success: true;
} & T;

type ResponseEnvelopeFailure = {
  success: false;
  errorMessage: string;
};

async function myFetch<T>(userId: number): Promise<ResponseEnvelopeYuki<T>> {
  try {
    const responseBody = await ((await fetch('url')).json() as Promise<T>);
    return {
      ...responseBody,
      success: true,
    };
  } catch (e) {
    // error handling.
    throw e;
  }
}

myFetch();

type MyCardItemResponse = ResponseEnvelopeYuki<{
  items: string[];
}>;

async function addCartItem() {
  return myFetch<MyCardItemResponse>(3434);
}

async function businessLogic() {
  const result = await addCartItem();
  if (result.success) {
    console.info(result.items);
  }
}