typeof

1. 对于对象
   如果对象嵌套层级比较深，我们手动定义ts类型会很复杂

const lo = {
  name: 'ff',
  age: 18,
  address: {
    province: '中国'
  }
}

type Person = typeof lo
等价于 
type Person = {
  name: string
  age: number
  address: {
    province: string
  }
}

2. 配合 keyof 获取所有枚举属性名

enum HttpMethod {
     Get,
     Post
 }
 const method: typeof HttpMethod = {
     Get: 0,
     Post: 1,
 }
 type Method = keyof typeof HttpMethod
 // "Get"  | "Post"

3. 获取函数的类型

function add(a: number, b: number) {
  return a + b
}
type AddType = typeof add // (a: number, b: number) => number

// 获取函数返回值类型
type AddReturnType = ReturnType<AddType> // number

// 获取函数参数类型
type AddParamsType = Parameters<AddType> // [a: number, b: number]

4. 配合 as const 获取更精确的类型

let user = {
  id: 666,
  name: "发发"
} as const
type User = typeof user 
// {readonly id: 666, readonly name: '发发'}