Special thanks to @hoangdv's. Thanks to him, I realized that I was using bcrypt.compareSync() wrongly: because the first two parameters of the method are the non-encrypted password and the encrypted password. Instead, I was inserting as parameters the password I was encrypting in the client side and then the DB password.

So, as @hoangdv said, I should delete formData.userPwd = this.encryptPassword(formData.userPwd); in the client side. And it worked!