No idea what the folks at MongoDB were smoking when they defined this API. As you have rightly pointed out, the JavaScript object only represents the unordered collection of key-value pairs both by design and spirit. For ordered collection, one could use array of objects.

The recent versions of JavaScript specifications encourage preserving the creation order of keys in the object for Object.keys and such methods. As others pointed out in the comments, relying on creation order of properties for the Object parameter for an API is plain stupid. It becomes even muddier if the object is cloned, the cloning library maintains the order or not. But probably it works since recent implementations now a days honors creation order and they pass the constant object inline.