关于前端:ts映射类型

在映射类型里，新类型以雷同的模式去转换旧类型的每个属性。

将每个属性转换为可选属性

type Partial<T> = {[P in keyof T]?: T[P];
}

例子：

type PersonPartial = Partial<Person>;
//   ^ = type PersonPartial = {
//       name?: string | undefined;
//       age?: number | undefined;
//   }

将每个属性转换为只读属性

type Readonly<T> = {readonly [P in keyof T]: T[P];
}

例子：

type ReadonlyPerson = Readonly<Person>;
//   ^ = type ReadonlyPerson = {
//       readonly name: string;
//       readonly age: number;
//   }

转换为旧类型和 null 的联结类型

type Nullable<T> = {[P in keyof T]: T[P] | null 
}

例子：

type NullablePerson = Nullable<Person>;
//   ^ = type NullablePerson = {
//       name: string | null;
//       age: number | null;
//   }

选取一组属性指定新类型

type Pick<T, K extends keyof T> = {[P in K]: T[P];
}

例子：

interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = Pick<Todo, "title" | "completed">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

todo;
// ^ = const todo: TodoPreview

创立一组属性指定新类型，罕用来申明一般 Object 对象

type Record<K extends keyof any, T> = {[P in K]: T;
}

Record 属于非同态，实质上会创立新属性，不会拷贝属性修饰符。

例子：

interface PageInfo {title: string;}

type Page = "home" | "about" | "contact";

const nav: Record<Page, PageInfo> = {about: { title: "about"},
  contact: {title: "contact"},
  home: {title: "home"},
};

nav.about;
// ^ = const nav: Record

去除交加，返回残余的局部

type Exclude<T, U> = T extends U ? never : T

例子：

interface Props {
  a?: number;
  b?: string;
}

const obj: Props = {a: 5};

const obj2: Required<Props> = {a: 5};
Property 'b' is missing in type '{a: number;}' but required in type 'Required<Props>'.

实用于键值对对象的 Exclude，去除类型中蕴含的键值对

type Omit = Pick<T, Exclude<keyof T, K>>

例子：

interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = Omit<Todo, "description">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

获取返回值类型，个别为函数

type ReturnType<T extends (...args: any) => any>
  = T extends (...args: any) => infer R ? R : any;

例子：

declare function f1(): { a: number; b: string};
type T4 = ReturnType<typeof f1>;
//    ^ = type T4 = {
//        a: number;
//        b: string;
//    }

将每个属性转换为必选属性

type Required<T> = {[P in keyof T]-?: T[P]
}

例子：

interface Props {
  a?: number;
  b?: string;
}

const obj: Props = {a: 5};

const obj2: Required<Props> = {a: 5};
// Property 'b' is missing in type '{a: number;}' but required in type 'Required<Props>'.

除此以外，还有很多相似的映射类型，能够参考 TypeScript: Documentation 取得更具体的信息。