對應類型

當您不希望重複時，有時一個類型需要建立在另一個類型之上。

對映類型建立在索引簽章的語法上，用於宣告尚未預先宣告的屬性類型

ts
type OnlyBoolsAndHorses = {
  [key: string]: boolean | Horse;
};
 
const conforms: OnlyBoolsAndHorses = {
  del: true,
  rodney: false,
};Try

對映類型是一種泛型類型，它使用 PropertyKey 的聯集（通常透過 keyof 建立）來反覆處理鍵以建立類型

ts
type OptionsFlags<Type> = {
  [Property in keyof Type]: boolean;
};Try

在此範例中，OptionsFlags 會採用類型 Type 中的所有屬性，並將其值變更為布林值。

ts
type Features = {
  darkMode: () => void;
  newUserProfile: () => void;
};
 
type FeatureOptions = OptionsFlags<Features>;
           
type FeatureOptions = {
    darkMode: boolean;
    newUserProfile: boolean;
}Try

對映修改器

在對映期間可以套用兩個額外的修改器：readonly 和 ?，分別影響可變性和可選性。

您可以透過加上前綴 - 或 + 來移除或新增這些修改器。如果您未新增前綴，則假設為 +。

ts
// Removes 'readonly' attributes from a type's properties
type CreateMutable<Type> = {
  -readonly [Property in keyof Type]: Type[Property];
};
 
type LockedAccount = {
  readonly id: string;
  readonly name: string;
};
 
type UnlockedAccount = CreateMutable<LockedAccount>;
           
type UnlockedAccount = {
    id: string;
    name: string;
}Try

ts
// Removes 'optional' attributes from a type's properties
type Concrete<Type> = {
  [Property in keyof Type]-?: Type[Property];
};
 
type MaybeUser = {
  id: string;
  name?: string;
  age?: number;
};
 
type User = Concrete<MaybeUser>;
      
type User = {
    id: string;
    name: string;
    age: number;
}Try

透過 as 重新對應鍵

在 TypeScript 4.1 及後續版本中，您可以使用對映類型中的 as 子句重新對應對映類型中的鍵

ts
type MappedTypeWithNewProperties<Type> = {
    [Properties in keyof Type as NewKeyType]: Type[Properties]
}

您可以利用 範本字串類型 等功能，從先前的屬性名稱建立新的屬性名稱

ts
type Getters<Type> = {
    [Property in keyof Type as `get${Capitalize<string & Property>}`]: () => Type[Property]
};
 
interface Person {
    name: string;
    age: number;
    location: string;
}
 
type LazyPerson = Getters<Person>;
         
type LazyPerson = {
    getName: () => string;
    getAge: () => number;
    getLocation: () => string;
}Try

您可以透過條件類型產生 never 來篩選鍵

ts
// Remove the 'kind' property
type RemoveKindField<Type> = {
    [Property in keyof Type as Exclude<Property, "kind">]: Type[Property]
};
 
interface Circle {
    kind: "circle";
    radius: number;
}
 
type KindlessCircle = RemoveKindField<Circle>;
           
type KindlessCircle = {
    radius: number;
}Try

您可以對映任意聯集，不只是 string | number | symbol 的聯集，而是任何類型的聯集

ts
type EventConfig<Events extends { kind: string }> = {
    [E in Events as E["kind"]]: (event: E) => void;
}
 
type SquareEvent = { kind: "square", x: number, y: number };
type CircleEvent = { kind: "circle", radius: number };
 
type Config = EventConfig<SquareEvent | CircleEvent>
       
type Config = {
    square: (event: SquareEvent) => void;
    circle: (event: CircleEvent) => void;
}Try

進一步探索

映射類型適用於此類型操作區段中的其他功能，例如這裡是 使用條件類型的映射類型，它會傳回 true 或 false，視物件是否將屬性 pii 設定為文字 true

ts
type ExtractPII<Type> = {
  [Property in keyof Type]: Type[Property] extends { pii: true } ? true : false;
};
 
type DBFields = {
  id: { format: "incrementing" };
  name: { type: string; pii: true };
};
 
type ObjectsNeedingGDPRDeletion = ExtractPII<DBFields>;
                 
type ObjectsNeedingGDPRDeletion = {
    id: false;
    name: true;
}Try