TS-1类型声明

• 描述对象类型：

1. Object 范围太广
2. object ：Object除去基本类型之外的类型
3. 索引签名
4. Record
5. Class or Constructor

• 索引签名：

type A = {
[k:string] : number
}

• Record

type Record<K extends number | string | symbol, T> = {
  [k in K] : T
}

const person: Record<string,number> = {
  name: 'frank',
  age: 12
} 

• 带this的函数类型声明

type Person = {
  username: string
  age: number
  sayHi: FnWithThis
}

type FnWithThis = (this:Person,name) => void

const sayHi:FnWithThis = function(){
  console.log(this.username)
}

const person:Person = {
  username: 'frank',
  age: 12,
  sayHi: sayHi
}

// 调用必须指定this
person.sayHi()  // 'frank'
or
sayHi.call(person) // 'frank'

• never：侧重于类型推导，推到不可能出现的情况，就是never

type A = string | number | boolean

let a:A 

if(typeof a === 'string'){
  a.indexOf
}else if(typeof a === "number"){
 a.toFixed
}else if(typeof a === "boolean"){
  a.valueOf
}else{
  a // type is never
}