Documents have their own built-in instance methods such as save and remove. However, we can write our own instance methods as well.
Documents are instances of models. They can be explicitly created:
const instance = new Model()
Or they can be the result of a query:
Model.findOne([conditions]).then((instance) => {})
Document instance methods are defined in the schema. All schemas have a method called method which allows you to define custom instance methods.