How to create coffeescript singleton subclass - class

Question

I've created a singleton class which I want to extend. It (half) works in that it only creates a single instance of the class, but properties added to the subclass are undefined. Here is the original singleton:
class Singleton
_instance = undefined
#getInstance: ->
if _instance is undefined
console.log 'no instance exists, so create one'
_instance = new _Singleton()
else
console.log 'an instance already exists.'
class _Singleton
constructor: ->
console.log 'new singelton'
module.exports = Singleton
And here is the subclass:
Singleton = require('./singleton')
class Stinky extends Singleton
constructor: ->
var1 : 'var1'
module.exports = Stinky
Now if I use the following in my node app:
Stinky = require './stinky'
thing1 = Stinky.getInstance()
thing2 = Stinky.getInstance()
console.log "Thing var1: #{thing1.var1}"
the getInstance() method behaves as expected, but var1 is undefined. If I do this same thing on non singleton classes they work fine. Thanks.

Answer 1

I trimmed your code down a bit. Here are the 2 remaining classes:
class Singleton
#_instance: null
#getInstance: ->
#_instance or= new #( arguments... )
class Stinky extends Singleton
constructor: ( #num ) ->
thing1 = Stinky.getInstance( 1 )
thing2 = Stinky.getInstance( 2 )
console.log( thing1.num, thing2.num )
I made the following changes:
Merged Singleton and _Singleton
Changed _instance to #_instance so that it would be attached to Singleton rather than its prototype
Added arguments splat in getInstance (in case arguments are needed)
Pointing getInstance() to extended object rather than Singleton
In this example, I used 2 different numbers to ensure that the 2nd constructor was never called.

Answer 2

I see how you're using the _Singleton class to try to simulate a private class, but unfortunately I don't think you can use it in this case.
Here is some code that works:
class Singleton
_instance = undefined
constructor: ->
console.log 'new singleton'
#getInstance: ->
if _instance is undefined
console.log 'no instance exists, so create one'
_instance = new #()
else
console.log 'an instance already exists.'
_instance
class Stinky extends Singleton
constructor: ->
console.log 'Stinky constructor'
#var1 = 'var1'
thing1 = Stinky.getInstance()
thing2 = Stinky.getInstance()
console.log "Thing var1: #{thing1.var1}"​​​​​​​​​​​​​​​​​​, thing1, thing2​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​
I removed the Node.js (require) code, but adding that in should be straightforward. The main difference is that the instance my code is creating is an instance of # or this. Doing so will make sure your constructor is called first then continue up the parent chain. Your code was explicitly creating an instance of _Singleton so your Stinky constructor was never being called. Another minor issue that you would have eventually noticed was that your getInstance method was not actually returning a instance of _instance.
I hope this helps,
Sandro

Answer 3

I'm not sure what the goal is, but you can achieve the same result by making Singleton a real singleton (a plain object):
Singleton =
doNothing: ->
# ...
doMoreNothing: ->
# ...
class Stinky
constructor: ->
#var1: 'var1'
getInstance: ->
return Singleton
It doesn't make much sense for Singleton to have a method that returns itself.