CoffeeScript's awesome inheritance model lets you strip boilerplate type checking.

types.coffee

# WARNING: This will no longer work with CoffeeScript 1.3.2 in IE:
# http://coffeescript.org/#changelog

class Animal
  @is: (node) ->
    if not @TYPE then return true
    node.type is @TYPE
  @isnt: (node) ->
    not @is node

class Cat extends Animal
  @TYPE: @name

class Dog extends Animal
  @TYPE: @name
  constructor: ->
    super
    @kind = @constructor.KIND
  @is: (node) ->
    node.type is @TYPE and (
      if not @KIND then true else node.kind is @KIND
    )

class Corgi extends Dog
  @KIND: @name

class Poodle extends Dog
  @KIND: @name

test = (msg, cond) ->
  alert "#{msg}: #{if cond then 'pass' else 'FAIL'}"

bob = new Cat()
test 'bob SHOULD be an animal', Animal.is bob
test 'bob SHOULD be a cat', Cat.is bob
test 'bob should NOT be a dog', Dog.isnt bob

ein = new Corgi()
test 'ein SHOULD be an animal', Animal.is ein
test 'ein should NOT be a cat', Cat.isnt ein
test 'ein SHOULD be a dog', Dog.is ein
test 'ein SHOULD be a corgi', Corgi.is ein
test 'ein should NOT be a poodle', Poodle.isnt ein

what's that have to do with CS? Ideally JS wouldn't have instanceof or typeof, but I dont think CS has anything to do with that. protos > classes