Recursively merge objects with corresponding property names

Question

I have such objects that I need to merge:

defaultConditions = 
  where: 
    enabled: true
    acc_id: 
      ne: null

conditions = 
  where:
    id: 2
    acc_id: 
      ne: 5

This helper recipe, which replaces where from defaultConditions with where from conditions, is not quite right for this purpose. So I wrote this:

extend = (object, properties) ->
  for key, val of properties
    if val?.constructor is Object and object[key]?.constructor is Object
      object[key] = extend object[key], val
    else
      object[key] = val
  object

How can it be improved?