The simplest promise implementation.

naivePromise.js

'use strict'
function promisify (fakeAjax){
  let content = null
  let func = null

  return function (url) {
    fakeAjax(url, (err, data) => {
      if (err) {
        throw Error('unhandled error')
      }
      
      if (func) {
        return func(data)
      }
      
      return content = data
    })

    return function (then) {
      if (content) {
        return then(content)
      }
      
      return func = then
    }
  }
}

function fakeAjax (url, cb) {
  setTimeout(
    () => {cb(null, 'all done')},
    2000
  )
}

function log (content) {
  console.log(content)
}

// called after 2s
promisify(fakeAjax)('fake url')(log)

const p = promisify(fakeAjax)('fake url')
setTimeout(
    () => {
      // called after 2s
      log('calling p now')
      // called immediately after log 'calling p now'
      p(log)
    },
    2000
  )