parallel_waterfall.coffee

async = require 'async'

arr = [1, 2, 3, 4, 5, 6]

async.reduce(
  [
    (i, cb) -> cb(null, i * 2)
    (i, cb) -> cb(null, i + 1)
  ]
  (i, cb) -> cb(null, i)
  (f, g, cb) ->
    cb(
      null
      (i, kb) ->
        f i, (err, j) ->
          g j, kb
    )
  (err, fn) ->
    async.map(
      arr
      fn
      (err, results) ->
        console.log results
    )
)

# => [ 3, 5, 7, 9, 11, 13 ]