Getting private member functions working in CoffeeScript by defining them in the prototype breaks inheritance

gistfile1.coffee

# This is an example in a post about private members in CoffeeScript.
# Read more: http://evanhahn.com/?p=1126

class Sorcerer
  conjureSpell = null
  constructor: (@spell) ->
    conjureSpell = =>   # private
      @spell.conjure()
  useSpell: ->
    conjureSpell()
    @spell.use()

class Child extends Sorcerer
  constructor: (@spell) ->
    super()

c = new Child
  conjure: -> console.log "Brewing potion..."
  use: -> console.log "Now I have superpowers!"

c.useSpell()  # Cannot read property 'spell' of null

# This code by Evan Hahn (evanhahn.com) is licensed under the Unlicense.
# http://unlicense.org/

Where is @spell defined? You're not passing anything so you can't call methods on it. You'd need to call it like so:

class Child extends Sorcerer constructor: () -> super(this)