Mongoose - required field

Hello, when I’m setting one of the fields of newly created document to be required, when does the mongoose check if it not undefined? Here is my User schema:

const userSchema = new mongoose.Schema({
    username: String,
    password: String,
    tokens: [{
        token: {
            type: String,
            required: true

Firstly I have created the user instance only with username and password and then i saved him to the database using .save() method. Only after that I have attached the token to this instace. It didn’t showed any error and everything worked fine, why is that since I have save user instance without required field at the beginning?