Add Property to Object that is returned by Sequelize FindOne

The Sequelize Model class (of which your cats are instances) has a toJSON() method which res.json will presumably use to serialise your cats. The method returns the result of Model#get() (https://github.com/sequelize/sequelize/blob/95adb78a03c16ebdc1e62e80983d1d6a204eed80/lib/model.js#L3610-L3613), which only uses attributes defined on the model. If you want to be able to set the cats name, but not store names in the DB, you can use a virtual column when defining your cat model:

sequelize.define('Cat', {
  // [other columns here...]
  name: Sequelize.VIRTUAL
});

Alternatively, if you don't want to add properties to the model definition:

cat = cat.toJSON(); // actually returns a plain object, not a JSON string
cat.name = 'Macavity';
res.json(cat);

The following works for sequelize v4.

...
const order = Order.findOne(criteria);
order.setDataValue('additionalProperty', 'some value');
...

Hope this helps. It's a bit late but in case people are still looking for answers.