How to validate only given properties exist?

[trusktr]

How do we specify that any fields other than the ones specified are not allowed? SO, f.e., I want this:

var Thing = schema({
  foo: Number
})

Thing({foo: 5}) // true
Thing({foo: 5, bar: "blah"}) // false, because `blah` isn't a specified property.

[matsuki820]

This might be way to late for an answer, but I found myself looking for the same answer and ended up using something like this:
Thing:{ "*":Object.like(undefined), foo:5 }