Promise

동

function a(){
    document.write("a");
    document.write("<br>");
}
function b(){
    document.write("b");
    document.write("<br>");
}
a();
b();

비동기

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

콜백지

// 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(callback){
    setTimeout(function(){
        console.log('d');
        callback();
    },1000);
}
a(function(){
    b(function(){
        c(function(){
            d();
        });
    });
});

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

Promise

// 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(() => {
    return b();
}).then(() => {
    return c();
}).then(() => {
    return d();
})

a()
.then(() => b())
.then(() => c())
.then(() => 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)
    })