promise-2.js

function printLater (number, fn) {
  setTimeout(
    function () {
      console.log(number)
      fn && fn()
    },
    1000
  )
}

printLater(1, function () {
  printLater(2, function () {
    printLater(3, function () {
      printLater(4)
    })
  })
})