How to use Node.js nested Async forEachSeries loop

Using coffeescript to code One solution | see this post

I have two loops. I want to take each value from array 'a' and then loop through all the values of 'b' process them and move to next value in array 'a'

output expected:

1 a b c
2 a b c
3 a b c

Error I see: 

 [ 1, 2, 3 ]

 [ 'a', 'b', 'c' ]

 1
 2
 3

 TypeError: Cannot read property 'length' of undefined
   at Object.forEachSeries(~/src/node_modules/async/lib/async.js:103:17)




  Async = require('async')

  @a = [1,2,3]
  @b = ['a','b','c']

  console.dir @a
  console.dir @b

  Async.forEachSeries @a, (aa , cbLoop1) ->
    console.log aa
    cbLoop1()
    Async.forEachSeries @b, (bb , cbLoop2) ->
      #here will be some callback that I need to process before moving to next value in
      #b array
      console.log bb
      cbLoop2()

Your first Async.forEachSeries call takes a callback, which changes the value of this

Async.forEachSeries @a, (aa , cbLoop1) ->
  # inside the callback, the value of `this` has changed,
  # so @b is undefined!

Use the => syntax to keep the value of this inside the callback:

Async.forEachSeries @a, (aa , cbLoop1) =>
  console.log aa
  cbLoop1()
  Async.forEachSeries @b, (bb , cbLoop2) ->
    # use `=>` again for this callback if you need to access this (@) inside
    # this callback as well