TypeScript 中你不知道的类型：Utility Types（实用程序类型），绝对有用！！！

概述

Utility Types在使用ts的实际开发过程中，偶尔是会遇到的，清楚这些类型的用法，可以加快我们的开发效率，避免写出一些冗余代码。

关于Utility Types，中文版的文档基本没有，有一些关于Utility Types的文章介绍的也不全面，本文参考typeScript英文文档，整理了一下目前英文文档中，所涉及的Utility Types，供大家开发的时过程中去参考。

TypeScript提供了几种实用工具类型（Utility Types）来促进常见的类型转换。 这些实用程序类型（Utility Types）是全局可用的。

Utility Types

Awaited<Type>

• This type is meant to model operations like await in async functions, or the .then() method on Promises - specifically, the way that they recursively unwrap Promises.
• 这种类型旨在模拟函数中的操作await，async或 s.then()上的方法Promise- 特别是它们递归解包Promises 的方式。

type A = Awaited<Promise<string>>; 
// type A = string
 
type B = Awaited<Promise<Promise<number>>>; 
// type B = number
 
type C = Awaited<boolean | Promise<number>>;
// type C = number | boolean

Partial<Type>

• Constructs a type with all properties of Type set to optional.
• This utility will return a type that represents all subsets of a given type.
• 构造一个将Type的所有属性设置为可选的类型。
• 此实用程序将返回表示给定类型的所有子集的类型。

interface Todo {
    title: string
    des: string
}

const a: Todo = {
    title: '',
    des: ''
}

// b中所有属性都为可选属性
const b: Partial<Todo> = {
    title: '',
}

Required<Type>

• Constructs a type consisting of all properties of Type set to required. The opposite of [Partial]
• 将Type所有的属性设置为必要属性，与Partial功能相反。

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>

• Constructs a type with all properties of Type set to readonly, meaning the properties of the constructed type cannot be reassigned.
• 将Type的所有属性设置为只读属性，意味着重新被构造的属性不能被修改

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>

• Constructs an object type whose property keys are Keys and whose property values are Type.
• This utility can be used to map the properties of a type to another type.
• 构造一个对象类型，其属性键为Keys，属性值为Type。
• 此实用程序可用于将类型的属性映射到另一类型。

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

// 对象键
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;
 
const cats: Record<CatName, CatInfo>

Pick<Type, Keys>

• Constructs a type by picking the set of properties Keys (string literal or union of string literals) from Type.
• 通过从Type中选择一组属性Keys(字符串字面量或字符串字面量的并集)来构造类型。

interface Todo {
  title: string;
  description: string;
  completed: boolean;
}
 
type TodoPreview = Pick<Todo, "title" | "completed">;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

Qmit<Type, Keys>

• Constructs a type by picking all properties from Type and then removing Keys (string literal or union of string literals).
• 通过从' Type '中选取所有属性，然后删除' Keys '(字符串字面量或字符串字面量的并集)来构造一个类型。

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,
};
 
const todo: TodoPreview
 
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
 
const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};

Exclude<UnionType, ExcludedMembers>

• Constructs a type by excluding from UnionType all union members that are assignable to ExcludedMembers.
• 通过从' UnionType '中排除所有可分配给' Excldedmembers '的联合成员来构造一个类型。

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>

• Constructs a type by extracting from Type all union members that are assignable to Union.
• 通过从type中提取可赋值给union的所有联合成员来构造一个类型。

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

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

NonNullable<Type>

• Constructs a type by excluding null and undefined from Type.
• 通过从Type中排除null和undefined来构造一个类型。

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

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

Parameters<Type>

• Constructs a tuple type from the types used in the parameters of a function type Type.
• 从函数类型Type的形参类型构造元组类型。

// 写法1
interface f2 {
    (a: number, b: string): void
}
type T0 = Parameters<f2>
const pa: T0 = [1, 'str']

//写法2
function f1(a: number, b: number): void {}
type T1 = Parameters<typeof f1>
const pa1: T1 = [2, 3]

ConstructorParameters<Type>

• Constructs a tuple or array type from the types of a constructor function type.
• It produces a tuple type with all the parameter types (or the type never if Type is not a function).
• 从构造函数类型的类型构造元组或数组类型。
• 它生成一个包含所有参数类型的元组类型(如果' Type '不是函数，则类型为' never ')。

type T0 = ConstructorParameters<ErrorConstructor>;
// type T0 = [message?: string]

type T1 = ConstructorParameters<FunctionConstructor>;
// type T1 = string[]

type T2 = ConstructorParameters<RegExpConstructor>;
// type T2 = [pattern: string | RegExp, flags?: string]

type T3 = ConstructorParameters<any>;
// type T3 = unknown[]

ReturnType<Type>

• Constructs a type consisting of the return type of function Type.
• 构造由函数type的返回类型组成的类型。

declare function f1(): { a: number; b: string };
 
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[]

type T4 = ReturnType<typeof f1>;
// type T4 = {
//    a: number;
//    b: string;
// }

InstanceType<Type>

• Constructs a type consisting of the instance type of a constructor function in Type.
• 构造由' Type '中的构造函数的实例类型组成的类型。

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>

• Extracts the type of the this parameter for a function type, or unknown if the function type has no this parameter.
• 提取函数类型的this参数的类型，如果函数类型没有this参数则为未知。

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

OmitThisParameter<Type>

• Removes the this parameter from Type.
• If Type has no explicitly declared this parameter, the result is simply Type.
• Otherwise, a new function type with no this parameter is created from Type.
• Generics are erased and only the last overload signature is propagated into the new function type.
• 从Type中移除this参数。
• 如果Type没有显式声明此参数，则结果只是Type。
• 否则，将从type创建一个不带此参数的新函数类型。
• 泛型被擦除，只有最后一个重载签名被传播到新函数类型中。

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

ThisType<Type>

• This utility does not return a transformed type.
• Instead, it serves as a marker for a contextual this type.
• Note that the noImplicitThis flag must be enabled to use this utility.
• 此实用程序不返回转换后的类型。
• 相反，它作为上下文这类类型的标记。
• 注意，必须启用noImplicitThis标志才能使用这个实用程序。

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);

Intrinsic String Manipulation Types

Uppercase<StringType>

• Converts each character in the string to the uppercase version.
• 将字符串中的每个字符转换为大写版本。

type Greeting = "Hello, world"
type ShoutyGreeting = Uppercase<Greeting>
// type ShoutyGreeting = "HELLO, WORLD"
 
type ASCIICacheKey<Str extends string> = `ID-${Uppercase<Str>}`
type MainID = ASCIICacheKey<"my_app">  
// type MainID = "ID-MY_APP"

Lowercase<StringType>

• Converts each character in the string to the lowercase equivalent.
• 将字符串中的每个字符转换为等效的小写字符。

type Greeting = "Hello, world"
type QuietGreeting = Lowercase<Greeting>    
// type QuietGreeting = "hello, world"
 
type ASCIICacheKey<Str extends string> = `id-${Lowercase<Str>}`
type MainID = ASCIICacheKey<"MY_APP">    
// type MainID = "id-my_app"

Capitalize<StringType>

• Converts the first character in the string to an uppercase equivalent.
• 将字符串中的第一个字符转换为等效的大写字母。

type LowercaseGreeting = "hello, world";
type Greeting = Capitalize<LowercaseGreeting>;
// type Greeting = "Hello, world"

Uncapitalize<StringType>

• Converts the first character in the string to a lowercase equivalent.
• 将字符串中的第一个字符转换为等效的小写字符。

type UppercaseGreeting = "HELLO WORLD";
type UncomfortableGreeting = Uncapitalize<UppercaseGreeting>;
// type UncomfortableGreeting = "hELLO WORLD"

总结

以上是目前关于Utility Types的一些内容，有不理解的地方，可以一起探讨。 关于本文中的一些细节，可以参考网址：www.typescriptlang.org/docs/handbo…