demo.coffee

# Polymorphism
class String
  add :(value)->
    alert "string add #{value}"

class Number
  add :(value)->
    alert "number add #{value}"

class Array
  add :(value)->
    alert "array add #{value}"

# create Demo
class Demo
  add : (value...) ->
    return undefined unless @isValid(value)

    # create instance
    instance = null
    switch typeof value[0]
      when 'string'
        instance = new String
      when 'number'
        instance = new Number
      when 'object'
        instance = new Array

    instance.add(value)

  isValid : (value) ->
    return if value[0]? then true else false


demo = new Demo
demo.add("0")   #=> string add 0
demo.add(0)     #=> number add 0
demo.add(["0"]) #=> array  add 0