Promise

1. 동기화

function a(){
    console.log('A')
}
function b(){
    console.log('B')
}
a();
b();
// A B

2. 동기화

function a(){
    setTimeout(function(){
        console.log('A')
    }, 1000)
}
function b(){
    console.log('B')
}
a();
b();
// B A

3. 콜백 지옥

function a(callback){
    setTimeout(function(){
        console.log('A')
        callback();
    }, 1000);
}
function b(){
    setTimeout(function(){
        console.log('B')
        callback();
    }, 1000);
}
function c(){
    setTimeout(function(){
        console.log('C')
        callback();
    }, 1000);
}
function d(){
    setTimeout(function(){
        console.log('D')
    }, 1000);
}
a(function(){
    b(function(){
        c(function(){
            d();
        });
    }); 
});

4. promise

function a(){
    return new Promise((resolve, reject)=>{
        setTimeout(()=>{
            console.log('A');
            resolve();
        }, 1000);
    });
}
function b(){
    return new Promise((resolve, reject)=>{
        setTimeout(()=>{
            console.log('B');
            resolve();
        }, 1000);
    });
}
function c(){
    return new Promise((resolve, reject)=>{
        setTimeout(()=>{
            console.log('C');
            resolve();
        }, 1000);
    });
}
function d(){
    return new Promise((resolve, reject)=>{
        setTimeout(()=>{
            console.log('D');
            resolve();
        }, 1000);
    });
}
a()
.then(() => b())
.then(() => c())
.then(() => d())

function a(){
    return new Promise((resolve, reject)=>{
        if(isError){
            reject()
        }
        setTimeout(()=>{
            console.log('A');
            resolve();
        }, 1000);
    });
}
a().then()
    .catch((error) => {
        console.log(error);
    })
    
// 데이터를 불러오면 reject, 실패하면 콘솔로그로 확인