Roleによって動的にメソッドを付け替える

user.ts

interface UserData {
  firstName: string
  lastName: string
  role: 'user' | 'admin'
  age: number
}

interface UserMethods {
  name: () => string
}

interface AdminMethod {
  removeAdmin: () => void,
}

type User = UserData & UserMethods
type Admin = UserData & UserMethods & AdminMethod

const fetchUser = ():UserData => {
  // APIコールを想定
  return {
    firstName: 'Ryota',
    lastName: 'Mochizuki',
    role: 'admin',
    age: 35
  }
}

const attachUserMethod = (userData: UserData):User => {
  return Object.assign<UserData, UserMethods>(userData, {
    name: function() {
      return `${userData.firstName} ${userData.lastName}`
    }
  })
}

const attachAdminMethod = (userData: UserData):Admin => {
  if (userData.role !== 'admin') {
    throw new Error('role error')
  }
 
  return Object.assign<User, AdminMethod>(attachUserMethod(userData), {
    removeAdmin() {
      return attachUserMethod({...userData, role: 'user'})
    }
  })
}

const attach = (userData: UserData) => {
  switch(userData.role) {
    case 'admin':
      return attachAdminMethod(userData)
    case 'user':
      return attachUserMethod(userData)
    default:
      throw new Error('unsupport role')
      break;
  }
}

const getUser = ():User|Admin => {
  return attach(fetchUser())
}


const admin = getUser() as Admin
admin.firstName = 'Taro'

const user = admin.removeAdmin()

console.log(user, admin)