Eamonn O'Brien-Strain

http://javascript.crockford.com/private.html

Here are two different ways of defining a class in Coffeescript:

    class Container

      constructor: (@member) ->

      secret = 3

      dec = ->
        if secret > 0
          secret -= 1
          true
        else
          false

      service : ->
        if dec() then @member else null

    Container::stamp = (string) ->
      @member + string

    Container = (param) ->

      @member = param
      secret = 3

      dec = ->
        if secret > 0
          secret -= 1
          true
        else
          false

      @service = ->
        if dec() then @member else null

      null

    Container::stamp = (string) ->
      @member + string

Both of these define a class called Container with

• a public field member initialized by a constructor parameter
• a private field secret
• a private method dec
• a privileged method service
• a public method stamp

We can use both in the same way:

    myContainer = new Container 'abc'

    console.log  myContainer.member      # abc
    console.log  myContainer.stamp 'def' # abcdef
    console.log  myContainer.service()   # abc
    console.log  myContainer.service()   # abc
    console.log  myContainer.service()   # abc
    console.log  myContainer.service()   # null

The version above on the left is the special class syntax that Coffeescript provides. However, to me, this extra layer of syntax seems to depart a bit from the “Coffeescript is just JavaScript” philosophy.

The version above on the right is a translation of Douglas Crockford's pattern, using his example.

Which is better? The left is a bit easier to read for a newcomer to the language, but I find the right more elegant because there is less “magic”.

And there is another advantage to the Crockford style. Consider this small modification:

    Container = (param, decrementBy) ->

      @member = param
      secret = 3

      dec = ->
        if secret > 0
          secret -= decrementBy
          true
        else
          false

      @service = ->
        if dec() then @member else null

      null

    Container::stamp = (string) ->
      @member + string

Here we have generalized the class by adding a decrementBy parameter to the constructor. We do not copy this to a property, but any of the private or privileged methods in the class can use it. (Don't you love closures!). There is no way to do this using the Coffeescript class syntax in a way that would prevent the decrementBy value being modified from outside the object.

I think I might switch to using the Crockford-style classes.

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