Each key in the Mongoose schema defines a property that is associated with a SchemaType. For example, in our actors.js schema, the actor's name key is associated with a string SchemaType. String, number, date, buffer, boolean, mixed, objectId, and array are all valid schema types.
In addition to schema types, numbers have min and max validators and strings have enum and match validators. Validation occurs when a document is being saved (.save()) and will return an error object, containing type, path, and value properties, if the validation has failed.