타입 가드 함수와 오버로드

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1interface Coffee {
2  type: 'coffee';
3}
4
5interface CoffeeInfo extends Coffee {
6  price: number;
7  caffeine: number;
8}
9
10interface CoffeeDetailInfo extends CoffeeInfo {
11  coffeeBean: string;
12}
13
14interface Ade {
15  type: 'ade';
16}
17
18interface AdeInfo extends Ade {
19  price: number;
20  sugar: number;
21}
22
23interface AdeDetailInfo extends AdeInfo {
24  fruit: string;
25}
26
27type Drink = Coffee | Ade;
28type DrinkInfo = CoffeeInfo | AdeInfo;
29type DrinkDetailInfo = CoffeeDetailInfo | AdeDetailInfo;

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1function isCoffeeInfo(value: DrinkInfo): value is CoffeeInfo {
2  return value.type === 'coffee';
3}
4
5function isCoffeeDetailInfo(value: DrinkDetailInfo): value is CoffeeDetailInfo {
6  return value.type === 'coffee';
7}

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1function getCaffeine(value: DrinkInfo): number {
2  if (isCoffeeInfo(value)) {
3    // (parameter) value: CoffeeInfo
4    return value.caffeine;
5  }
6
7  return 0;
8}
9
10function getCoffeeBean(value: DrinkDetailInfo): string | null {
11  if (isCoffeeDetailInfo(value)) {
12    // (parameter) value: CoffeeDetailInfo
13    return value.coffeeBean;
14  }
15
16  return null;
17}

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1function isCoffee<U extends T, T extends Drink = Drink>(value: T): value is U {
2  return value.type === 'coffee';
3}
4
5function getCaffeine(value: DrinkInfo): number {
6  if (isCoffee<CoffeeInfo>(value)) {
7    // (parameter) value: CoffeeInfo
8    return value.caffeine;
9  }
10
11  return 0;
12}
13function getCoffeeBean(value: DrinkDetailInfo): string | null {
14  if (isCoffee<CoffeeDetailInfo>(value)) {
15    // (parameter) value: CoffeeDetailInfo
16    return value.coffeeBean;
17  }
18
19  return null;
20}

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1type CoffeeOf<T> = T extends DrinkDetailInfo
2  ? CoffeeDetailInfo
3  : T extends DrinkInfo
4  ? CoffeeInfo
5  : T extends Drink
6  ? Coffee
7  : never;
8
9// Coffee
10type A = CoffeeOf<Drink>;
11// CoffeeInfo
12type B = CoffeeOf<DrinkInfo>;
13// CoffeeDetailInfo
14type C = CoffeeOf<DrinkDetailInfo>;

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1type CoffeeOf<T> = T extends Drink
2  ? Coffee
3  : T extends DrinkInfo
4  ? CoffeeInfo
5  : T extends DrinkDetailInfo
6  ? CoffeeDetailInfo
7  : never;
8
9// Coffee
10type A = CoffeeOf<Drink>;
11// Coffee
12type B = CoffeeOf<DrinkInfo>;
13// Coffee
14type C = CoffeeOf<DrinkDetailInfo>;

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1function isCoffee<T extends Drink>(value: T): value is CoffeeOf<T> {
2  return value.type === 'coffee';
3}

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1// A type predicate's type must be assignable to its parameter's type.
2//   Type 'CoffeeOf<T>' is not assignable to type 'T'.
3//     'CoffeeOf<T>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Drink'.

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1// Type 'string' is not assignable to type 'T'.
2//   'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
3function fn1<T extends string>(x: T): T {
4  return 'hello world!';
5}

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1function isCoffee(value: DrinkDetailInfo): value is CoffeeDetailInfo;
2function isCoffee(value: DrinkInfo): value is CoffeeInfo;
3function isCoffee(value: Drink): value is Coffee {
4  return value.type === 'coffee';
5}

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1function getCaffeine(value: DrinkInfo): number {
2  if (isCoffee(value)) {
3    // (parameter) value: CoffeeInfo
4    return value.caffeine;
5  }
6
7  return 0;
8}
9
10function getCoffeeBean(value: DrinkDetailInfo): string | null {
11  if (isCoffee(value)) {
12    // (parameter) value: CoffeeDetailInfo
13    return value.coffeeBean;
14  }
15
16  return null;
17}

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1// 시그니쳐 (1)
2function isCoffee(value: DrinkDetailInfo): value is CoffeeDetailInfo;
3// 시그니쳐 (2)
4function isCoffee(value: DrinkInfo): value is CoffeeInfo;
5function isCoffee(value: Drink): value is Coffee {
6  return value.type === 'coffee';
7}
8
9declare const drinkInfos: DrinkInfo[];
10// 시그니쳐 (1)이 뒤에 올 때 에러가 발생함.
11drinkInfos.filter(isCoffee).map((value) => value.caffeine);
12
13declare const drinkDetailInfos: DrinkDetailInfo[];
14// 시그니쳐 (2)가 뒤에 올 때 에러가 발생함.
15drinkDetailInfos.filter(isCoffee).map((value) => value.coffeeBean);

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1declare const drinkInfos: DrinkInfo[];
2drinkInfos.filter<CoffeeInfo>(isCoffee).map((value) => value.caffeine);
3
4declare const drinkDetailInfos: DrinkDetailInfo[];
5drinkDetailInfos
6  .filter<CoffeeDetailInfo>(isCoffee)
7  .map((value) => value.coffeeBean);

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1filter<S extends T>(predicate: (value: T, index: number, array: T[]) => value is S, thisArg?: any): S[];

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1declare const drinkDetailInfos: DrinkDetailInfo[];
2// value: DrinkDetailInfo
3drinkDetailInfos.filter(isCoffee).map((value) => value.coffeeBean);

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1function isCoffee<T extends Drink>(value: T): value is T & {type: 'coffee'} {
2  return value.type === 'coffee';
3}