Mongoose 6.5.0 was released on July 26, 2022 and includes several features that we're excited to highlight. It has been a while since we've done a blog post for a Mongoose feature release, but castObject() and applyDefaults() represent an important shift that's worth breaking the hiatus for. These new functions represent a major step forward for Mongoose's portability: the ability to use bits and pieces of Mongoose functionality without needing to instantiate documents or even connections.

Here's the idea. Given a Mongoose schema, you can now cast a POJO to the schema, or apply schema-defined defaults to the POJO. No need to create a Mongoose document.

const User = mongoose.model('User', new mongoose.Schema({
  name: String,
  age: {
    type: Number,
    required: true,
    default: 0

// Returns `{ name: 'test', age: 42 }` as a POJO. Mongoose casts '42'
// to a number
User.castObject({ name: 'test', age: '42' });

// Returns `{ name: 'test', age: 0 }` as a POJO. Mongoose applies the
// default value of `age`
User.applyDefaults({ name: 'test' });


The applyDefaults() function takes in either a POJO or a Mongoose document, and applies all schema-applied defaults to the object. It modifies the object in place.

const TestModel = mongoose.model('Test', new mongoose.Schema({
  name: String,
  regularDefault: {
    type: Number,
    default: 0
  functionDefault: {
    type: String,
    function() {
      this === obj; // true

const obj = { name: 'test' };

// {
//   name: 'test',
//   regularDefault: 0,
//   functionDefault: 'test',
//   _id: new ObjectId("...")
// }

Without applyDefaults(), applying schema-defined defaults on a POJO is a potentially performance-intensive 2-step process. You would have to instantiate a new instance of TestModel() to apply defaults, then convert the document back to a POJO. Creating a new document instance can be slow, especially if your schema is complex.

applyDefaults() also works on documents. This can be handy in cases where you want to set a value on a document before applying defaults.

// Create a document, but skip applying defaults
const doc = new TestModel({}, undefined, { defaults: false });

doc.regularDefault; // undefined
doc.functionDefault; // undefined

// Set a value, and then apply defaults = 'test2';

doc.regularDefault; // 0
doc.functionDefault; // 'test2'


The castObject() function attempts to cast the given POJO to the model's schema. Unlike applyDefaults(), castObject() does not modify the object in place. castObject() returns a new object casted to the model's schema.

castObject() throws a Mongoose ValidationError if casting fails.

const TestModel = mongoose.model('Test', new mongoose.Schema({
  answer: {
    type: Number,
    validate: v => v === 42

let obj = { answer: '42' };
obj = TestModel.castObject(obj);

// `{ answer: 42 }` as a POJO. Mongoose casted '42' (string) to 42 (number)

// Throws a `ValidationError`
TestModel.castObject({ answer: 'not a number' });

Like applyDefaults(), castObject() is convenient because it lets you skip the potentially expensive step of hydrating a new document. castObject() can also be helpful with the lean() option. Mongoose doesn't do any casting on the results of lean queries. castObject() provides a neat way for you to handle that yourself.

const _id = new mongoose.Types.ObjectId();
await TestModel.collection.insertOne({ _id, answer: '42' });

let doc = await TestModel.findById(_id).lean();
typeof doc.answer; // 'string'

doc = TestModel.castObject(doc);
typeof doc.answer; // 'number'

Note that castObject() does not run schema-defined validation. Casting and validation are separate steps in Mongoose: castObject() will only make sure that answer is a number, not that answer is a valid number. Mongoose models also have a validate() function that runs both casting and validation. Use TestModel.validate() if you also want to run schema-defined validation.

TestModel.castObject({ answer: 32 }); // OK

TestModel.validate({ answer: 32 }); // throws a `ValidationError`

Moving On

castObject() and applyDefaults(), in combination with validate(), let you use your Mongoose schemas to work with POJOs. No need to hydrate a full document if you just want to cast a POJO against your schema, or apply default values. This makes Mongoose schemas more flexible, because you can use them in cases where it doesn't make sense for you to create a full Mongoose document. castObject() and applyDefaults() are just 2 of the 10 new features in Mongoose 6.5, so make sure you upgrade to take advantage of the new features!

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