PartialとRequired：オプション性の切り替え

概要

Partial<T>

interface User { id: number; name: string; email: string; }
type PartialUser = Partial<User>;
// { id?: number; name?: string; email?: string; }

更新関数での Partial

function updateUser(id: number, changes: Partial<User>): User {
  const existing = findUser(id)!;
  return { ...existing, ...changes };
}

Required<T>

type RequiredUser = Required<User>;
// All properties are now required — including those that were optional

設定の検証に使う Required

interface Config { host?: string; port?: number; }
const defaults: Required<Config> = { host: 'localhost', port: 3000 };
function createConfig(opts: Config): Required<Config> {
  return { ...defaults, ...opts };
}

Partial の実装方法

type Partial<T> = { [K in keyof T]?: T[K] };
// Equivalent to: { [K in keyof T]+?: T[K] }

Required の実装方法

type Required<T> = { [K in keyof T]-?: T[K] };

Deep Partial

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

Partial と Required の組み合わせ

// id required, everything else optional
type UpsertUser = Required<Pick<User, 'id'>> & Partial<Omit<User, 'id'>>;

Partial とインターフェースのオプショナルプロパティの違い

Partial による型推論

function show(u: Partial<User>) {
  if (u.name !== undefined) {
    console.log(u.name.toUpperCase()); // safe
  }
}

確認問題

まとめ