Promise

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

a();
b();

//A
//B

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

a();
b();
//B
//A

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


a(function(){
    b(function(){
        c(function(){
            d()
        });
    });
});
//A
//B
//C
//D

//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())

//A
//B
//C
//D

//promise
function a(){
    return new Promise((resolve, reject) => {
        if(isError){
            reject()
        }
        setTimeout(() => {
            console.log("A");
            resolve();
        },1000);
    });
}
a()
    .then()
    .catch((error) => {
        console.log(error)
    });