Typescriptの列挙型のサブセット 質問する

別の列挙型のサブセットである列挙型のように動作するtypeand の組み合わせを作成できます。const

enum欠点: このソリューションでは、任意のキーがその値と一致するようにする必要があります。（または、そのケースを処理する解決策をまだ見つけていません）

// Given an enum
export enum CarBrand {
  TOYOTA = 'TOYOTA',
  FORD = 'FORD',
  PEUGEOT = 'PEUGEOT',
  RENAULT = 'RENAULT'
}

// We can create the type of the subset
export type FrenchCarBrand = Extract<CarBrand, CarBrand.PEUGEOT | CarBrand.RENAULT>;

// And the const that goes with it
export const FrenchCarBrand = {
  [CarBrand.PEUGEOT]: CarBrand.PEUGEOT,
  [CarBrand.RENAULT]: CarBrand.RENAULT
} as const;

// With that you can :

// assign a CarBrand to a FrenchCarBrand ...
const a: FrenchCarBrand = CarBrand.RENAULT;

// ... as long as the CarBrand is in the type
const a_invalid: FrenchCarBrand = CarBrand.TOYOTA; // Type 'CarBrand.TOYOTA' is not assignable to type 'FrenchCarBrand'.

// assign a FrenchCarBrand to a CarBrand ...
const b: CarBrand = FrenchCarBrand.PEUGEOT;

// ... as long as the key exists
const b_invalid: CarBrand = FrenchCarBrand.TOYOTA; // Property 'TOYOTA' does not exist on type 'Readonly<Record<FrenchCarBrand, FrenchCarBrand>>'

// You cannot reassign the value of a FrenchCarBrand ...
FrenchCarBrand.PEUGEOT = CarBrand.RENAULT; // Cannot assign to 'PEUGEOT' because it is a read-only property.

// ... just like you cannot reassign the value of an enum
CarBrand.TOYOTA = 'MAZDA'; // Cannot assign to 'TOYOTA' because it is a read-only property.

declare const brand: CarBrand;
const frenchBrand: FrenchCarBrand = brand; // Type 'CarBrand' is not assignable to type 'FrenchCarBrand'

// You can use the `as` keyword but it will hide bugs, so I do not recommend at all.
const frenchBrandNoError: FrenchCarBrand = brand as FrenchCarBrand; // No error, but a possible bug in your application.

export const isFrenchCarBrand = (brand: CarBrand): brand is FrenchCarBrand => {
  return brand in FrenchCarBrand;
}

if (isFrenchCarBrand(brand)) {
  // You can safely assume that `brand` is a `FrenchCarBrand`
} else {
  // `brand` is definitely not a `FrenchCarBrand`
}