TypeScriptエラーハンドリング

bad.ts

type Profile = {
  id: number;
  name: string;
  foo: string;
  bar: string;
};


const fetchProfile = () => {
  return fetch("/profile").then<Profile>((res) => res.json());
};

const main = async () => {
  let profile: Profile | undefined;
  try {
    profile = await fetchProfile();
  } catch (e) {
    // エラーハンドリング
  }

  if (profile) {
    console.log(profile.name, "さん");
  }
};

good.ts

type Profile = {
  id: number;
  name: string;
  foo: string;
  bar: string;
};

abstract class Data<T> {
  protected constructor(private _value: T) {}

  get value(): T {
    return { ...this._value } as const;
  }
}
class SuccessData<T> extends Data<T> {
  static of<V>(v: V): SuccessData<V> {
    return new SuccessData(v);
  }
}

class ExceptionData<T> extends Data<T> {
  static of<V>(v: V): ExceptionData<V> {
    return new ExceptionData(v);
  }
}

const fetchProfile = () => {
  return fetch("/profile")
    .then<Profile>((res) => res.json())
    .then((res) => SuccessData.of(res))
    .catch((e) => ExceptionData.of(e));
};

const main = async () => {
  const res = await fetchProfile().catch<never>((e) => e);
  if (res instanceof SuccessData) {
    const profile = res.value;
    console.log(profile.name, "さん");
    return;
  }

  if (res instanceof ExceptionData) {
    // エラーハンドリング
    return;
  }

  throw new Error("想定外");
};