Partially update a session variable in meteor? - meteor

Question

I think that I might be missing something. I've got the entire contents of a user's product order form stored in a session variable.
var orderFormContents = {
numDoors: 4
numWheels: 4
numSeats: 5
};
Session.set("orderFormContentsSessionVar", orderFormContents);
How do I update the value of just one key in orderFormContentsSessionVar, for instance, just numDoors?
I don't want to overwrite the entire existing contents of the session var.
I would love to be able to do something like:
Session.set("orderFormContentsSessionVar.numDoors", 2);
Something equivalent to _.extend
UPDATE
Following the example of the answer below, I just wrote a function for it:
var updateSession = function(sessionVarName, updateParams){
var obj = Session.get(sessionVarName);
_.extend(obj, updateParams);
Session.set(sessionVarName, obj);
console.log("updated session name: ", sessionVarName, "new session contents: ", Session.get(sessionVarName));
};

Answer 1

Using like you proposed:
var obj = Session.get("orderFormContentSessionVar");
Session.set("orderFormContentsSessionVar", _.extend(obj, {numDoors: 2}));

Answer 2

_.extend should work just fine.
Session.set( "orderFormContentsSessionVar",
_.extend(Session.get("orderFormContentsSessionVar"), { numSeats: 10 }) )
I doubt there is any other way.