diff.coffee

diffArray = (a, b, done) ->
  u = {};
  diff = 
    a: []
    b: []
  async.forEach b, (x, cb) ->
    u[x] = true
    cb()
  , (err) ->
    async.forEach a, (x, cb) ->
      if (!u[x])
        diff.a.push x
      else
        delete u[x]
      cb()
    , (err) ->
      diff.b = for item, b of u
        item
      done null, diff #a: items unique to a; b: items unique to be; u: union set of a and b