Another point to add is Meteor's EJSON, which is a simple spec for defining how any complex, constructed or factory-made JavaScript objects should be serialized to JSON, and then deserialized back into a full JS object with behavior. Objects inserted/retrieved to/from the MongoDB or Session variables are automatically serialized/deserialized if you have made them EJSON compatible (quite easy to do, and built-in for Date and binary objects). By sharing EJSON definitions on the client and server, you have a flexible basis on which you can build an isomorphic model layer to your liking. The constructed model objects you're working with can easily be made identical (truly or superficially, depending on your needs) on both the client and server code.