Blog post - https://rakit.my/blog/simplify-asynchronous-control-flow-javascript-part-2/

movie_app.js

// Stub Implementation
// ===================

function notify (msg) { console.log(msg) }
function renderListItem () { /* UI update goes here */ }

var movieApi = {
  titles: [
    'IV – A New Hope (1977)',
    'V – The Empire Strikes Back (1980)',
    'VI – Return of the Jedi (1983)',
    'I – The Phantom Menace (1999)',
    'II – Attack of the Clones (2002)',
    'III – Revenge of the Sith (2005)',
    'VII – The Force Awakens (2015)',
    'VIII – The Last Jedi (2017)'
  ],

  fetchTitles: function (cb) {
    // Pretending to download titles...
    var titles = this.titles
    setTimeout(function () {
      // Remove the last one 80% of the time, then shuffle the list a bit
      titles = titles.slice(0, titles.length - (Math.ceil(Math.random() * 9) > 7 ? 0 : 1))
      titles.sort(function () { return 1 - (Math.ceil(Math.random() * 3) - 1) })
      cb(null, titles)
    }, 1000)
  },

  fetchSynopsis: function (title, cb) {
    // Pretending to download the synopsis
    var titles = this.titles
    setTimeout(function () {
      if (title === titles[titles.length - 1]) {
        cb(new Error(title + ' is in the making!'))
      } else {
        cb(null, title + ' jeng'.repeat(4))
      }
    }, Math.random() * 200)
  }
}

// The application
// ===============

function createQueue (taskCb, drainCb, concurrency) {
  var processing = 0
  var queue = []
  concurrency = concurrency || 1

  function log (msg) { notify('[queue] ' + msg) }
  function drained () { return !processing && !queue.length }

  function process () {
    if (drained()) {
      log('drained')
      return drainCb()
    }

    if (processing < concurrency) {
      var data = queue.pop()
      if (!data) return

      processing++
      log('processing: ' + processing + ', queued: ' + queue.length)
      taskCb(data, function () {
        setTimeout(function () {
          processing--
          process()
        }, 0)
      })
    }
  }

  this.push = function (data) {
    queue.push(data)
    process()
  }

  return this
}

function fetchEachSynopsis (titles, synopsisCb, doneCb) {
  function task (title, taskDoneCb) {
    movieApi.fetchSynopsis(title, function (err, synopsis) {
      synopsisCb(err, title, synopsis)
      taskDoneCb()
    })
  }
  var queue = createQueue(task, doneCb, 2)

  titles.forEach(function (title) { queue.push(title) })
}

function fetchEachMovie (movieCb, doneCb) {
  movieApi.fetchTitles(function (err, titles) {
    if (err) return doneCb(err)
    fetchEachSynopsis(titles, movieCb, doneCb)
  })
}

function updateUIAtInterval (interval) {
  notify('Fetching latest titles')
  fetchEachMovie(
    function each (err, title, synopsis) {
      if (err) return notify('Error fetching synopsis!. Reason: ' + err.message)
      notify('Fetched synopsis ' + synopsis)
      // Update UI
      renderListItem(title, synopsis)
    },
    function done (err) {
      notify('Done fetching all synopsis, error: ' + (err || 'none'))
      setTimeout(updateUIAtInterval.bind(null, interval), interval)
    }
  )
}

// Start the program
var interval = 60 * 60 * 1000;
updateUIAtInterval(interval)