object without properties

undermore.coffee

_.mixin
  withoutProperties: (object,failTest) ->
    newObj = _(object).clone()
    _(object).each (val,key) ->
      delete newObj[key] if failTest(val,key) == true
    newObj

undermore_spec.coffee

describe "foo", ->  
  Given -> @data = a: 0, b: "foo"
  When -> @result = _(@data).withoutProperties (v,k) -> k == 'b'
  Then -> expect(@result).toEqual(a:0)

I'm curious, why is the failTest required?

As opposed to optional?

The failTest serves the opposite of a truthTest in, say, a select function. te user passes in the test to the function an if it returns true, then that property will be removed from the cloned object.

It seems unnecessary to me is all, because you can blindly issue "delete newObj[key]" without any test and whether it succeeded or failed it won't throw an error. The api for your function could then be simplified such that you can just pass in a list of keys you want deleted, ie

_(@data).withoutProperties(["a", "b", "c"]) ...