Is data returned from Mongoose immutable?

Question

I want to add to the return data from a mongoose query:

User.findById(userId, function(err, data) {
  if (!err) {
    data.newvar = 'Hello, world';
  }
});

However, when I console log the output, the newvar does not exist. I've also tried this using Underscore's extend:

_.extend(data, {'newvar': 'Hello, world'});

With no luck either. Since I have nested documents, using a shallow copy won't work. Is there any way to append data here?

Answer 1

One way to handle this is to convert your mongoose model instance into a plain object that you have full control over by calling toObject() on it:

User.findById(userId, function(err, data) {
  if (!err) {
    data = data.toObject();
    data.newvar = 'Hello, world';
  }
});

If you want a more structured solution, you can add virtual attributes to your schema as described here.

Answer 2

As it turns out, Mongoose documents are their own special class and not standard Javascript objects. In order to get a javascript option that can be extended, you must use the toObject() method.