콜백지옥

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

//promise를 반환하므로
function test() {
    a().then(() => {
        return b()
    }).then(() => {
        return c()
    }).then(() => {
        return d()
    }).then(() => {
        console.log('fin')
    })
}
 
test();