Is there a way to add/subtract all fields inside a JSON with another object with the same fields?

I have a Mongoose.js schema (but you probably don't need to know that), which looks like this:

{
  "City": String,
  "Year": String,
  "Population": Number,
  "Blah": Number,
  "Nested": {
    "Something": Number,
    "More stuff": Number 
  }
}

Is there a convenient/best method to take another object with the same fields and add its number fields with this one's or do I need to write a function to actually go in and do that?

I'm dealing with a very large schema here. If I write a function to do this, I'll have to manually maintain it; meaning if I change the schema, I'll have to manually change the member function I define to add stuff.