UserSchema.pre('save', function (next) {
  //contents of function
});

module.exports = mongoose.model('User', UserSchema);

var newUser = new User({
            firstName: req.body.firstName,
            lasttName: req.body.lastName,
            email: req.body.email,
            password: req.body.password
            });
            // save the user
            newUser.save(function(err) {
   ...