How to get data passed to mongoose schema constructor

I am testing my application and need to verify that mongoose schema constructor is called with correct data.

let's say I do this:

const UserData = new User(user)
console.log(UserData.contructor.args)

I would expect log of the user object. Probably the data is passed to constructor of mongoose schema?

Can some one please advise me how to access it?

Here is specific case I am trying to solve.

export const signup = async (req, res, next) => {
    try {

        //if user object is missing return error
        if (!req.body.user) 
            return next(boom.unauthorized('No user data received.'))        

        //get user data    
        const user                                      = req.body.user,
        { auth: { local: { password, password_2 } } }   = user        

        //check if both passwords match
        if (password !== password_2)
            return next(boom.unauthorized('Passwords do not match.'))

        //check if password is valid
        if (!Password.validate(password)) {          
            const errorData = Password.validate(password, { list: true })
            return next(boom.notAcceptable('Invalid password.', errorData))
        }    

        //creates new mongo user
        const UserData = new User(user)

        //sets user password hash   
        UserData.setPassword(password)

        //saves user to database
        await UserData.save()        

        //returns new users authorization data
        return res.json({ user: UserData.toAuthJSON() })

    } catch(err) {

        //if mongo validation error return callback with error       
        if(err.name === 'ValidationError') {
            return next(boom.unauthorized(err.message))
        }

        // all other server errors           
        return next(boom.badImplementation('Something went wrong', err))
    }

}

And part of test:

describe('Success', () => {
            it('Should create new instance of User with request data', async () => {
                const   req             = { body },
                        res             = {},
                        local           = { password: '1aaaBB', password_2: '1aaaBB'},
                        constructorStub = sandbox.stub(User.prototype, 'constructor')                

                req.body.user.auth.local    = {...local}

                await signup(req, res, next)

                expect(constructorStub.calledOnceWith({...req.body.user})).to.be.true

            })                
        })

EDIT: I can verify that is is called with expect(constructorStub.calledOnce).to.be.true

Just can't get to verify data passed.

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