TSDoc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：
ts
type Horse = {};
// ---cut---
type OnlyBoolsAndHorses = {
[key: string]: boolean | Horse;
};
const conforms: OnlyBoolsAndHorses = {
del: true,
rodney: false,
};type Horse = {};
// ---cut---
type OnlyBoolsAndHorses = {
[key: string]: boolean | Horse;
};
const conforms: OnlyBoolsAndHorses = {
del: true,
rodney: false,
};映射类型是一种通用类型，它使用 PropertyKey 的联合（经常通过 keyof 创建）来遍历键以创建 类型：
ts
type OptionsFlags<Type> = {
[Property in keyof Type]: boolean;
};type OptionsFlags<Type> = {
[Property in keyof Type]: boolean;
};在此示例中，OptionsFlags 将从类型Type中获取所有属性，并将它们的值更改为布尔值。
ts
type OptionsFlags<Type> = {
[Property in keyof Type]: boolean;
};
// ---cut---
type FeatureFlags = {
darkMode: () => void;
newUserProfile: () => void;
};
type FeatureOptions = OptionsFlags<FeatureFlags>;
// ^?
/*
type FeatureOptions = {
darkMode: boolean;
newUserProfile: boolean;
}
*/type OptionsFlags<Type> = {
[Property in keyof Type]: boolean;
};
// ---cut---
type FeatureFlags = {
darkMode: () => void;
newUserProfile: () => void;
};
type FeatureOptions = OptionsFlags<FeatureFlags>;
// ^?
/*
type FeatureOptions = {
darkMode: boolean;
newUserProfile: boolean;
}
*/映射修饰符
在映射期间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。
您可以通过在 - 或 + 前加上前缀来删除或添加这些修饰符。 如果您不添加前缀，则假定为+。
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>;
// ^?// 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>;
// ^?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>;
// ^?// 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 映射重新映射
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：
ts
type MappedTypeWithNewProperties<Type> = {
[Properties in keyof Type as NewKeyType]: Type[Properties]
}type MappedTypeWithNewProperties<Type> = {
[Properties in keyof Type as NewKeyType]: Type[Properties]
}您可以利用 模版字面量 等功能从先前的属性名称创建新的属性名称：
ts
type Getters<Type> = {
[Property in keyof Type as `get${Capitalize<string & Property>}`]: () => Type[Property]
};
interface Person {
name: string;
age: number;
location: string;
}
type LazyPerson = Getters<Person>;
// ^?type Getters<Type> = {
[Property in keyof Type as `get${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：
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>;
// ^?// 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，而不仅仅是 string | number | symbol，但任何类型的联合：
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 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 true or false 取决于对象是否将属性 pii 设置为字面量 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 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>;
// ^?