Mixins in CoffeeScript (does not override existing properties, but makes them into a function or an array)

mixins.coffee

mixin = (target, source...) ->
  return unless target or source

  for s in source
    for key, value of s
      unless Object.hasOwnProperty.call target, key
        console.log key
        console.log value
        target[key] = value
      else
        oldRef = target[key]
        target[key] = do ->
          if typeof oldRef is 'function' and typeof value is 'function'
            return ->
              oldRef.apply @, arguments
              value.apply @, arguments
          else
            [oldRef, value]
  null

usage.coffee

class MyNewClass
  mixin @::, {a: 5}, {a: 6}

  constructor: ->

  c: -> alert 'c'

mnc = new MyNewClass()
console.log mnc.a # [5, 6]