callback - Promise

const a = () => {
  return new Promise(resolve => {
    setTimeout(() =>  {
      console.log(1)
      resolve()
    }, 1000)
  })
}

const b = () => console.log(2)

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

//f1,2,3는 new Promise return 하는 함수
f1()
.then(f2)
.then(f3)
.then(f4)
.then(() => console.log("ok"))