constアサーション「as const」 (const assertion)

オブジェクトリテラルの末尾にas constを記述すればプロパティがreadonlyでリテラルタイプで指定した物と同等の扱いになります。

ts
const pikachu = {
  name: "pikachu",
  no: 25,
  genre: "mouse pokémon",
  height: 0.4,
  weight: 6.0,
} as const;

ts
const pikachu = {
  name: "pikachu",
  no: 25,
  genre: "mouse pokémon",
  height: 0.4,
  weight: 6.0,
} as const;

代入はもちろんできません。

ts
pikachu.name = "raichu";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.

ts
pikachu.name = "raichu";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.

`readonly`と`const assertion`の違い

どちらもオブジェクトのプロパティをreadonlyにする機能は同じですが、以下が異なります。

`readonly`はプロパティごとにつけられる

const assertionはオブジェクト全体に対する宣言なので、すべてのプロパティが対象になりますが、readonlyは必要なプロパティのみにつけることができます。

`const assertion`は再帰的に`readonly`にできる

オブジェクトの中にオブジェクトがあるときの挙動が異なります。たとえば次のようなオブジェクトがあるとします。

ts
type Country = {
  name: string;
  capitalCity: string;
};
 
type Continent = {
  readonly name: string;
  readonly canada: Country;
  readonly us: Country;
  readonly mexico: Country;
};
 
const america: Continent = {
  name: "North American Continent",
  canada: {
    name: "Republic of Canada",
    capitalCity: "Ottawa",
  },
  us: {
    name: "United States of America",
    capitalCity: "Washington, D.C.",
  },
  mexico: {
    name: "United Mexican States",
    capitalCity: "Mexico City",
  },
};

ts
type Country = {
  name: string;
  capitalCity: string;
};
 
type Continent = {
  readonly name: string;
  readonly canada: Country;
  readonly us: Country;
  readonly mexico: Country;
};
 
const america: Continent = {
  name: "North American Continent",
  canada: {
    name: "Republic of Canada",
    capitalCity: "Ottawa",
  },
  us: {
    name: "United States of America",
    capitalCity: "Washington, D.C.",
  },
  mexico: {
    name: "United Mexican States",
    capitalCity: "Mexico City",
  },
};

ここでContinentのタイプエイリアスがもつプロパティはすべてreadonlyです。よって次のようなことはできません。

ts
america.name = "African Continent";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.america.canada = {
Cannot assign to 'canada' because it is a read-only property.2540Cannot assign to 'canada' because it is a read-only property.  name: "Republic of Côte d'Ivoire",
  capitalCity: "Yamoussoukro",
};

ts
america.name = "African Continent";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.america.canada = {
Cannot assign to 'canada' because it is a read-only property.2540Cannot assign to 'canada' because it is a read-only property.  name: "Republic of Côte d'Ivoire",
  capitalCity: "Yamoussoukro",
};

しかしながら、次のようなことは問題なくできてしまいます。

ts
america.canada.name = "Republic of Côte d'Ivoire";
america.canada.capitalCity = "Yamoussoukro";

ts
america.canada.name = "Republic of Côte d'Ivoire";
america.canada.capitalCity = "Yamoussoukro";

これはreadonlyをつけたプロパティがオブジェクトである場合に、そのオブジェクトのプロパティまでreadonlyにはしないことに起因します。

`const assertion`はすべてのプロパティを固定する

as constを付けます。

ts
const america = {
  name: "North American Continent",
  canada: {
    name: "Republic of Canada",
    capitalCity: "Ottawa",
  },
  us: {
    name: "United States of America",
    capitalCity: "Washington, D.C.",
  },
  mexico: {
    name: "United Mexican States",
    capitalCity: "Mexico City",
  },
} as const;

ts
const america = {
  name: "North American Continent",
  canada: {
    name: "Republic of Canada",
    capitalCity: "Ottawa",
  },
  us: {
    name: "United States of America",
    capitalCity: "Washington, D.C.",
  },
  mexico: {
    name: "United Mexican States",
    capitalCity: "Mexico City",
  },
} as const;

readonlyと同様にトップレベルのプロパティへの代入はできません。

ts
america.name = "African Continent";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.america.canada = {
Cannot assign to 'canada' because it is a read-only property.2540Cannot assign to 'canada' because it is a read-only property.  name: "Republic of Côte d'Ivoire",
  capitalCity: "Yamoussoukro",
};

ts
america.name = "African Continent";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.america.canada = {
Cannot assign to 'canada' because it is a read-only property.2540Cannot assign to 'canada' because it is a read-only property.  name: "Republic of Côte d'Ivoire",
  capitalCity: "Yamoussoukro",
};

これだけではなくオブジェクトが持つプロパティも同様にreadonlyにしてくれます。

ts
america.canada.name = "Republic of Côte d'Ivoire";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.america.canada.capitalCity = "Yamoussoukro";
Cannot assign to 'capitalCity' because it is a read-only property.2540Cannot assign to 'capitalCity' because it is a read-only property.

ts
america.canada.name = "Republic of Côte d'Ivoire";
Cannot assign to 'name' because it is a read-only property.2540Cannot assign to 'name' because it is a read-only property.america.canada.capitalCity = "Yamoussoukro";
Cannot assign to 'capitalCity' because it is a read-only property.2540Cannot assign to 'capitalCity' because it is a read-only property.