scala groupBy in Coffeescript

groupBy.coffee

a = [
  {'id': 1, 'ca': 123}, {'id': 2, 'ca': 321}, {'id': 3, 'ca': 213}
]

Array.prototype.groupBy = (key, callback) ->
  res = {}
  if key is undefined
    throw "key cannot be undefined. you MUST define key function."
  for i in this
    do (i) ->
      k = key(i)
      if typeof(k) isnt "string"
        throw "key must return `string`"
      if !res.hasOwnProperty(k)
        res[k] = []
      res[k].push(i)

  callback(res)


keyFunc = (i) ->
  r = i.ca % 100
  parseInt(r / 10).toString()

a.groupBy keyFunc, (r) ->
  console.log(r) # { '1': [ { id: 3, ca: 213 } ], '2': [ { id: 1, ca: 123 }, { id: 2, ca: 321 } ] }