• 描述對象類型：

1. Object 范圍太廣
2. object ：Object除去基本類型之外的類型
3. 索引簽名
4. Record
5. Class or Constructor

• 索引簽名：

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
} 

• 帶this的函數類型聲明

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
}

// 調用必須指定this
person.sayHi()  // 'frank'
or
sayHi.call(person) // 'frank'

• never：側重于類型推導，推到不可能出現的情況，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
}