# 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 PropertyKeys(经常创建的 通过 keyof )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?,它们分别影响可变性和可选性。

您可以通过添加前缀 -+来移除或添加这些修饰符。如果您不添加前缀,则假定为 +

// 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

在 TypeScript 4.1 及更高版本中,您可以使用映射类型中的 as子句重新映射映射类型中的键:

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

您可以利用 模板字面类型 等功能从以前的属性名称中创建新的属性名称:

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>;

您可以通过条件类型生成 never来过滤掉键:

// 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>;

您可以映射任意联合,不仅是 string | number | symbol的联合,还可以映射任何类型的联合:

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

映射类型与此类型操作部分中的其他功能配合得很好,例如这里是 使用条件类型的映射类型 ,它返回 truefalse,具体取决于对象是否将属性 pii设置为字面 true

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>;
Last Updated: 5/25/2023, 2:35:11 PM