Use Promise

Promise 사용전

class UserStorage {
  loginUser(id, password, onSuccess ,onError){
    setTimeout(()=>{
      if(
        (id === 'ellie' && password === 'dream') ||
        (id === 'coder' && password === 'academy')
      ) {
        onSuccess(id);
      } else {
        onError(new Error('not found'))
      }
    },2000)
  }

  getRoles(user, onSuccess, onError){
    setTimeout(()=>{
      if(user === 'ellie'){
        onSuccess({ name:'ellie', role:'admin'});
      } else{
        onError(new Error('no access'))
      }
    },1000)
  }
}

const userStorage = new UserStorage();
const id = prompt('enter your id');
const password = prompt('enter your password');

userStorage.loginUser(
  id,
  password,
  user => {
    userStorage.getRoles(
      user,
      userWithRole => {
        alert(`Hello ${userWithRole.name}, you have a ${ userWithRole.role} role`)
      },
      error =>{
        console.log(error);
      }
    )
  },
  error =>{
    console.log(error);
  }
)

Promise 사용후

class UserStorage {
  loginUser(id, password){
    return new Promise((resolve, reject) =>{
       setTimeout(()=>{
      if(
        (id === 'ellie' && password === 'dream') ||
        (id === 'coder' && password === 'academy')
      ) {
        resolve(id);
      } else {
        reject(new Error('not found'))
      }
    },2000);      
    })   
  }

  getRoles(user){
    return new Promise((resolve, reject) =>{
       setTimeout(()=>{
      if(user === 'ellie'){
        resolve({ name:'ellie', role:'admin'});
      } else{
        reject(new Error('no access'))
      }
    },1000)
    })
  }
}

const userStorage = new UserStorage();
const id = prompt('enter your id');
const password = prompt('enter your password');
userStorage
  .loginUser(id, password)
  .then(userStorage.getRole)
  .then(user => alert(`Hello ${user.name}, you have a ${user.role} role`))
  .catch(console.log);