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\';
  }
});         


        

Answer 1

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.

Answer 2

Now you can use lean() method to return a plain js object:

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

Why can't you modify the data returned by a Mongoose Query (ex: findById)

Answer 3

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.