Your schemas can define default values for certain paths. If you create a new document without that path set, the default will kick in.

Note: Mongoose only applies a default if the value of the path is strictly undefined.

[require:Declaring defaults in your schema]

You can also set the default schema option to a function. Mongoose will execute that function and use the return value as the default.

[require:Default functions]

Mongoose also sets defaults on update() and findOneAndUpdate() when the upsert option is set by adding your schema's defaults to a MongoDB $setOnInsert operator. You can disable this behavior by setting the setDefaultsOnInsert option to false.

[require:The `setDefaultsOnInsert` option]

You can also set setDefaultsOnInsert to false globally:

mongoose.set('setDefaultsOnInsert', false);

Unless it is running on a query with setDefaultsOnInsert, a default function's this refers to the document.

[require:Default functions and `this`]