映射类型

当您不想重复自己时，有时一种类型需要基于另一种类型。

映射类型建立在索引签名的语法之上，用于声明未提前声明的属性类型：

type Horse = {};
type OnlyBoolsAndHorses = {
  [key: string]: boolean | Horse;
};

const conforms: OnlyBoolsAndHorses = {
  del: true,
  rodney: false,
};

映射类型是一种泛型类型，它使用 PropertyKey（经常 通过 keyof 创建）的联合来遍历键以创建类型：

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

在此示例中，OptionsFlags 将获取 Type 类型的所有属性并将其值更改为布尔值。

type OptionsFlags<Type> = {
  [Property in keyof Type]: boolean;
};
type FeatureFlags = {
  darkMode: () => void;
  newUserProfile: () => void;
};

type FeatureOptions = OptionsFlags<FeatureFlags>;

映射修饰符

在映射过程中可以应用两个额外的修饰符：readonly 和 ?，它们分别影响可变性和可选性。

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// 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>;

// 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>;

通过 as 重新映射键

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

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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>;

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// 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>;

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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>

进一步探索

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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>;