An easy thing to overlook with classes and instance variables in Javascript

class-gotcha.coffee

# =======================================================================
# INCORRECT
# =======================================================================

class Foo
  # don't expect this to be an instance variable. it is not!! it is the
  # java equivalent of: public static colours = new Array()
  colours: []

  addColour: (colour) =>
    # i believe java wouldn't even compile this with this.colours, but
    # Javascript is such a whore sometimes, it lets it pass without
    # complaining
    @colours.push colour

bar = new Foo()
bar.addColour "red"

console.log bar.colours

# [ "red" ]

bat = new Foo()
bat.addColour "orange"

console.log bat.colours

# [ "red", "orange" ]

# ^^^ ack!

# =======================================================================
# CORRECT
# =======================================================================

class Foo
  constructor: =>
    # initialize instance variables in your object's constructor
    @colours = []
  
  addColour: (colour) =>
    @colours.push colour

bar = new Foo()
bar.addColour "red"

console.log bar.colours

# [ "red" ]

bat = new Foo()
bat.addColour "orange"

console.log bat.colours

# [ "orange" ]