Run n async operations concurrently

scalePhotos.coffee

Q = require 'q'
_ = require 'underscore'

photos = [1..100]

partition = (n, coll) ->
  _.values(_.groupBy(coll, (x, i) -> Math.floor i/n))

scaleImage = (img, i) ->
  Q.delay(200).then -> console.log "processed image #{i}"

processGroup = (task) -> (g) -> ->
  Q.allSettled g.map task

runTasks = (concurrency, task, coll) ->
  partition(concurrency, coll)
    .map(processGroup task)
    .reduce(Q.when, Q())

runTasks 8, scaleImage, photos