비동기처리 콜백

// JavaScript is synchronous.
// Execute the code block in oreder after hoisting.
// hoisting: var, function declaration
console.log('1');
setTimeout(() => console.log('2'), 1000 );
console.log('3');

// Synchronous callback
function printImmediately(print) {
    print();
}
printImmediately(() => console.log('hello'));

// Asynchronous callback
function printWithDelay(print, timeout) {
    setTimeout(print, timeout);
}
printWithDelay(() => console.log('async callbak'), 2000);

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

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

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