[Izkata]

> In JavaScript because there is both null and undefined it is easy to assume that undefined means we don't know the value and null means we do know it has no value.

Javascript objects have two kinds of undefined that are both represented by the same value. You have to use another method to see which it is, and I've seen "foo" in this example used for the same thing as "null" in your example:

  >> z = {foo: undefined}
  Object { foo: undefined }

  >> z.foo
  undefined

  >> z.bar
  undefined

  >> z.hasOwnProperty('foo')
  true

  >> z.hasOwnProperty('bar')
  false

This is something you have to account for because the key is still there if you try to remove a value by just setting it to undefined:

  >> Object.keys(z)
  Array [ "foo" ]

  >> for (let k in z) { console.info(k); }
  foo

This is the right way to remove the key:

  >> delete z.foo
  true

  >> z
  Object {  }

  >> Object.keys(z)