REST API Nodejs server files

//******* users.js: ********//
var express = require('express');
var router = express.Router();
var mongoose = require('mongoose');
var User = mongoose.model('User');

 /* When accessing http://localhost:3000/users/ */
router.route('/')
 // create a user (accessed at POST http://localhost:3000/users/)
    .post(function (req, res) { // create a new instance of the User model
        new User({ // set the user's attributes (comes from the request)
        firstname: req.body.firstname,
        lastname: req.body.lastname,
        age: req.body.age,
        update_at: Date.now()
    })
    .save(function (err) {
        if (err)
            res.send(err);
            res.json({message: 'User created!'});
        });
    })
    // get all the users (accessed at GET http://localhost:8080/api/users)
    .get(function (req, res) {
        User.find(function (err, users) {
            if (err)
                res.send(err);
                res.json(users);
            });
    });

 /* When accessing http://localhost:3000/users/:userId */
 router.route('/:userId')
 // get the user with that id (accessed at GET http://localhost:3000/users/:userId)
    .get(function (req, res) {
        User.findById(req.params.userId, function (err, user) {
            if (err)
                res.send(err);
            res.json(user);
        });
    })

    // update the user with this id (accessed at PUT http://localhost:3000/users/:userId)
    .put(function (req, res) {
        //use our user model to find the user we want
        User.findById(req.params.userId, function (err, user) {
            if (err) res.send(err);
            if (!!req.body.firstname)
                user.firstname = req.body.firstname;
            if (!!req.body.lastname)
                user.lastname = req.body.lastname;
            if (!!req.body.age)
                user.age = req.body.age;

            // save the user
            user.save(function (err) {
                if (err)
                    res.send(err);
                res.json({message: 'User successfully updated!'});
            });
        });
     })

     // delete the user with this id (accessed at DELETE http://localhost:3000/users/:userId)
     .delete(function (req, res) {
        User.remove({ _id: req.params.userId }, function (err, user) {
            if (err)
                res.send(err);
            res.json({message: 'Successfully deleted'});
     });
});
module.exports = router;

//******* userDB.js: ********//
 var mongoose = require('mongoose');
 var Schema = mongoose.Schema ;

 var User = new Schema({
    firstname: String,
    lastname: String,
    age: Number,
    update_at: Date
 });

 mongoose.model('User', User);
 mongoose.connect('localhost', 'MyNodeDB');