Structural Type System vs Nominal Type System

Structural Type System - 구조가 같으면, 같은 타입이다

interface Iperson {
  name: string
  age: number
  speak(): string;
}

type PersonType = {
  name: string
  age: number
  speak(): string;
}
// Iperson 과 PersonType은 똑같은 구조다

let personInterFace: IPerson = {} as any
let personType: PersonType = {} as any

personInterface = personType;
personType = personInterface

nominal type system - 구조가 같아도 이름이 다르면, 다른 타입이다

• nominal type system은 타입스크립트는 따르지 않는다

  type PersonID = string  & { readonly brand: unique symbol}
  
  function PersonID(id: string): PersinID{
    return id as PersonID
  }
  
  function getPersonById(id: PersonID){}
  
  getPersonbyId(personId('id-aaaa'))
  getPersonbyId(('id-aaaa')) // error TS2345: Argument of type 'string' is not assignable to parameter of type 'PersonID'. Type 'string' is not assignable to type '{ readonly brand: unique symbol}'