TypeScript系列教程九《高级类型》

TS 原文是叫做 Utility Types，可以翻译成实用类型，玩转技巧的类型，暂定高级类型吧，感觉这些是玩转类型的灵魂方法。

TypeScript 提供一些全局通用方法，可以让你玩转类型转换，下面一起看看。

Partial<Type>

作用：将类型属性都设置为可选

代码语言：javascript复制

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

function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
  return { ...todo, ...fieldsToUpdate };
}

const todo1 = {
  title: "organize desk",
  description: "clear clutter",
};

const todo2 = updateTodo(todo1, {
  description: "throw out trash",
});

Required<Type>

作用：和Partial 相反，将类型属性都设置为必须

代码语言：javascript复制

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>'.

Readonly<Type>

作用：将类型属性设置成Readonly

代码语言：javascript复制

interface Todo {
  title: string;
}

const todo: Readonly<Todo> = {
  title: "Delete inactive users",
};

todo.title = "Hello";
//Cannot assign to 'title' because it is a read-only property.

这个技巧在运行时赋值表达式十分有用，例如冷冻一个对象（ frozen object)）：

Object.freeze

代码语言：javascript复制

function freeze<Type>(obj: Type): Readonly<Type>;

Record<Keys,Type>

作用： Record<K,T>构造具有给定类型T的一组属性K的类型。在将一个类型的属性映射到另一个类型的属性时，Record非常方便。

源码：

代码语言：javascript复制

/**
 * Construct a type with a set of properties K of type T
 */
type Record<K extends keyof any, T> = {
    [P in K]: T;
};

例子：

代码语言：javascript复制

interface CatInfo {
  age: number;
  breed: string;
}

type CatName = "miffy" | "boris" | "mordred";

const cats: Record<CatName, CatInfo> = {
  miffy: { age: 10, breed: "Persian" },
  boris: { age: 5, breed: "Maine Coon" },
  mordred: { age: 16, breed: "British Shorthair" },
};

cats.boris;

更多实战案例：https://zhuanlan.zhihu.com/p/356662885

Pick<Type, Keys>

作用: 构造一个类型从另一个类型的属性中挑选出几个

源码：

代码语言：javascript复制

/**
 * From T, pick a set of properties whose keys are in the union K
 */
type Pick<T, K extends keyof T> = {
    [P in K]: T[P];
};

例子：

代码语言：javascript复制

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

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

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

todo;

Exclude<Type, ExcludedUnion>

作用：构造一个类型从另一个类型中剔除部分属性key

Omit<Type, Keys>

作用：构造一个类型从另一个类型中剔除部分属性key

源码：

代码语言：javascript复制

/**
 * Construct a type with the properties of T except for those in type K.
 */
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

例子：

代码语言：javascript复制

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

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

const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
  createdAt: 1615544252770,
};

todo;
 
const todo: TodoPreview

type TodoInfo = Omit<Todo, "completed" | "createdAt">;

const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};

todoInfo;

freeze key object partial types

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