Fixing the problem

Let's open up animal.coffee in our text editor. First, we should add logic for birds if we're going to have them in our pet shop:

  behaviors: ->
    switch @type
      when "cat" then ["meow", null]
      when "dog" then ["bark", "wag"]
      when "rabbit" then [null, "hop hop"]
      when "bird" then ["chirp", "flap"]
      when "horse", "donkey"
        ["neigh", null]
      else
        [null, null]

Second, this bug occurred because the default return value from this function didn't play well with the caller's expectation that it would always be a string. Let's revamp our code in pet_view.coffee to properly handle the case in which all nulls are returned:

    if options.showBehavior
      [sound, action] = @pet.behaviors()
      [behavior, cssClass] = if sound?
        ["#{sound}!", "sound"]
      else if action?
        [action, "action"]
      else
        ["", ""]

This ensures that adding another unknown category in the future won't cause an error in our application...