ts 关键字 keyof 和 typeof、in

1. typeof 提取类型

• 提取对象的类型

const person = {
    name:'Rock',
    age:22,
}

type perSonType = typeof person
// 等价于
type personType = {name: string, age: number}

• 提取数组的类型

const arr = ['rock',22,true]  
type arrType = typeof arr  
// 等价于 type arrType = (atring | number | boolean)[]
let arr1:arrType = ['123','234',4234,false]

• typeof 提取函数类型

const func = (a:number)=>{  
    if(a===1) return false  
    else return 666  
}  
type funcType = typeof func; 
// 等价于  type funcType = (a: number) => (false | 666)
 
 // 下面会报错，123 不等于666
const func1:funcType = (b)=>{  
    return 123  
}

2. keyof 获取 "键" 的联合类型

1. keyof 后面可以跟class 定义的对象

class Person{  
    name:123  
    age:18  
    constructor(name,age) {  
        this.name = name  
        this.age = age  
    }  
    hello(){  
        `${this.name},say hello`  
    }  
}  
// 相当于 type k1 = "name" | "age" | "hello" ,提取到的对象没有继承的属性  
type k1 = keyof Person

3. 跟一个数组[]

// 相当于type k = never  
type k2 = {}

// 可以提取到数组的继承属性
// 相当于 type k3 = "length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | ...  
type k3 = keyof []

4. 跟字符串 ""

// 试试字符串
// 相当于 type k4 = "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | ...  
type k4 = keyof ''

5. 跟类型

// keyof  
type PersonType = {  
    name:string,  
    age:number  
}  
// 等价于 type keyofPerson = "name" | "age"  
type keyofPerson = keyof PersonType  

注意： keyof 后面不能直接跟 {} (字面量) 定义的非空对象,空对象的返回值是never

const obj = {
    name:'Rock,
    age:18
}
type k = keyof obj //  报错

// never
type k1 = keyof {}

3. in 关键字

in的右侧一般会跟一个联合类型，使用in操作符可以对该联合类型进行迭代。 其作用类似JS中的for...in或者for...of

type roles = 'admin'| 'user' | 'student' | 'teacher'
type roleAgeType = {
    [a in roles]: number
}
// 相当于  
type roleType1 = {  
    admin:string  
    user:string  
    student:string  
    teacher:string  
}

4. typeof 和 keyof 连用

typeof 提取类型，keyof提取类型的的键组合为联合类型。

const person = {  
    name:'Rock',  
    age:19,  
    gender: false,  
    action:()=>{  
        console.log('hello');  
    }  
}  
// 相当于 type propertyType = "gender" | "name" | "action" | "age"  
type propertyType = keyof typeof person

5. typeof、keyof 、in 连用

const obj = {
    name:'Rock',
    age: 20
}

//等价于
type myType = {
    name:string
    age:string
}

type myType = {
    [key in keyof typeof obj]: string
}