TS文档学习 --- Utility Types在 TypeScript 中提供了许多自带的工具类型，因为这些类型都是全

本篇翻译整理自 TypeScript Handbook 中「utility-types」章节。

TypeScript 提供了几种工具类型来帮助我们进行常见的类型转换

`Partial<Type>`

将Type的所有属性都设置为可选的类型。

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

// 工具类型定义的是类型，所以需要使用type来进行声明
type PartialTodo = Partial<Todo>

/*
  type PartialTodo = {
    title?: string | undefined;
    description?: string | undefined;
  }
 */

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>`

Type的所有属性都设置为必填项

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>`

Type的所有属性都设置为只读

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.

`Record<Keys, Type>`

以keys中的每一项为key，vakue为Type构建一个新对象

此工具类型可用于将一种类型的属性映射到另一种类型。

type CatName = "Klaus" | 'Steven' | 'Alex';
 
const cats: Record<CatName, { age: number }> = {
  Klaus: { age: 10},
  Steven: { age: 5},
  Alex: { age: 5 }
}

interface CatInfo {
  age: number;
  breed: string;
}
 
 const s1 = Symbol()

// 因为CatName会作为对象的key使用
// 所以key可以取值的类型有 string | number | symbol
type CatName = "miffy" | 111 | typeof s1;
 
const cats: Record<CatName, CatInfo> = {
  miffy: { age: 10, breed: "Persian" },
  111: { age: 5, breed: "Maine Coon" },
  [s1]: { age: 5, breed: "Maine Coon" }
}

`Pick<Type, Keys>`

从Type中提起Keys中对应的value组成一个新的类型

interface Todo {
  title: string;
  description: string;
  completed: boolean;
}
 
// 设置的key值必须是Todo接口上存在的值 
type TodoPreview = Pick<Todo, "title" | "completed">
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

`Omit<Type, Keys>`

在Type（此时的Type的类型一般是对象）中移除对应的Keys

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,
}

`Exclude<Type, ExcludedUnion>`

从一个Type(此时的Type的类型一般为联合类型)中移除对应的类型

type T0 = Exclude<"a" | "b" | "c", "a">;
     // type T0 = "b" | "c"

type T1 = Exclude<"a" | "b" | "c", "a" | "b">;
     // type T1 = "c"

type T2 = Exclude<string | number | (() => void), Function>;
     // type T2 = string | number

`Extract<Type, Union>`

从Type中提取符合Union的类型

type T0 = Extract<"a" | "b" | "c", "a" | "f">;
     // type T0 = "a"

type T1 = Extract<string | number | (() => void), Function>;
     // type T1 = () => void

`NonNullable<Type>`

通过排除null和undefined来自构造一个类型Type

type T0 = NonNullable<string | number | undefined>;
     // type T0 = string | number

type T1 = NonNullable<string[] | null | undefined>;
     // type T1 = string[]

`Parameters<Type>`

Type为函数时候，提取函数的参数组成一个新的元组

declare function f1(arg: { a: number; b: string }): void;
 
type T0 = Parameters<() => string>;
     // type T0 = []

type T1 = Parameters<(s: string) => void>;
     // type T1 = [s: string]

type T2 = Parameters<<T>(arg: T) => T>;
     // type T2 = [arg: unknown]

type T3 = Parameters<typeof f1>;
    /*     
      type T3 = [arg: {
          a: number;
          b: string;
      }]
    */
                     
type T4 = Parameters<any>;
     // type T4 = unknown[]

type T5 = Parameters<never>;
     // type T5 = never
                     
type T6 = Parameters<string>; // error
type T7 = Parameters<Function>; // error

`ConstructorParameters<Type>`

Type为构造函数

通过构造函数的参数组成元组或数组类型

如果Type不是函数的类型，会返回never

// ErrorConstructor, FunctionConstructor都是TS内置的构造函数的类型
type T0 = ConstructorParameters<ErrorConstructor>;
     
type T1 = ConstructorParameters<FunctionConstructor>;
     
type T2 = ConstructorParameters<RegExpConstructor>;
     
type T3 = ConstructorParameters<any>;

`ReturnType<Type>`

Type为函数，获取函数返回值的类型

 
type T0 = ReturnType<() => string>;
     // type T0 = string

type T1 = ReturnType<(s: string) => void>;
     // type T1 = void

type T2 = ReturnType<<T>() => T>;
     // type T2 = unknown

type T3 = ReturnType<<T extends U, U extends number[]>() => T>;
     // type T3 = number[]

`InstanceType<Type>`

Type为类，获取Type的实例类型

class C {
  x = 0;
  y = 0;
}
 
type T0 = InstanceType<typeof C>;
     // type T0 = C

type T1 = InstanceType<any>;
     // type T1 = any

type T2 = InstanceType<never>;
    //  type T2 = never

`ThisParameterType<Type>`

Type为函数，获取函数调用时候的this参数的类型

function toHex(this: Number) {
  return this.toString(16);
}
 
// type n = number
function numberToString(n: ThisParameterType<typeof toHex>) {
  return toHex.apply(n);
}

function toHex() {
  return 16
}
 
// 如果函数没有this参数，那么类型就是unknown
// type n = unknown
function numberToString(n: ThisParameterType<typeof toHex>) {
  return toHex.apply(n);
}

`OmitThisParameter<Type>`

从Type中剔除this参数

function toHex(this: Number) {
  return this.toString(16);
}
 
const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);
         // const fiveToHex: () => string

`ThisType<Type>`

用于指定函数调用的时候，函数内部this的类型

type ObjectDescriptor<D, M> = {
  data?: D;
  methods?: M & ThisType<D & M>; // Type of 'this' in methods is D & M
};
 
function makeObject<D, M>(desc: ObjectDescriptor<D, M>): D & M {
  let data: object = desc.data || {};
  let methods: object = desc.methods || {};
  return { ...data, ...methods } as D & M;
}
 
let obj = makeObject({
  data: { x: 0, y: 0 },
  methods: {
    moveBy(dx: number, dy: number) {
      this.x += dx; // Strongly typed this
      this.y += dy; // Strongly typed this
    },
  },
});
 
obj.x = 10;
obj.y = 20;
obj.moveBy(5, 5);

内置字符串操作类型

内置字符串操作类型的使用可以在模板字面量类型中查看

TS文档学习 --- Utility Types