NodeJS module.exports does not work as expected

Question

I am writing a web service in NodeJS using Express. My problem occurs when I want to use the app (Express instance) in other modules. The following configuration works just fine, but if I replace

var app = module.exports = express();

with

var app = express();

and then use

module.exports = app;

at the bottom of app.js, everything will break down. There will be an error when calling app.get() in the auth module (TypeError: app.get is not a function). Can somebody explain to me why it matters where I export the app object?

app.js

var mongoose = require('mongoose');
var express = require('express');
var bodyParser = require('body-parser');
var debug = require('debug')('app');
var morgan = require('morgan');
var config = require('./config');
var app = module.exports = express();

// --- globals ---
app.set('jwtTokenSecret', config.jwtTokenSecret);

// --- middleware ---
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(morgan('dev'));

// --- routes ---
var courses = require('./routes/courses');
var users = require('./routes/users');
var auth = require('./routes/auth');

app.use('/auth', auth);
app.use('/courses', courses);
app.use('/users', users);

// --- database connection ---
mongoose.set('debug', true);

mongoose.connect(config.database, function(err) {
    if (err) {
        debug('Could not establish connection to mongodb');
    } else {
        debug('Successfully connected to mongodb');
    }
});

routes/auth.js

var express = require('express');
var router = express.Router();
var moment = require('moment');
var jwt = require('jwt-simple');
var User = require("../models/User");
var debug = require('debug')('app');
var app = require("../app");

// POST /auth
router.post('/', function(req, res, next) {
    User.findOne({
        'username' : req.body.username
    }, function(err, user) {
        if (err || !user) {
            res.status(401).json({ error: "No user found"});
            return;
        }
        if (user.password != req.body.password) {
            res.send(401);
        }

        debug(app.get('database'));

        var expires = moment().add(7, 'days').valueOf();
        var token = jwt.encode({
            user: user.username,
            exp: expires
        }, app.get('jwtTokenSecret'));

        res.json({
            success: true,
            token: token
        });
    });
});

module.exports = router;

Answer 1

It's because you have a circular dependency between app.js and auth.js.

In your app.js file, you are requiring auth.js, and inside auth.js, you are requiring app.js. By moving module.exports = app to the bottom of app.js, it is being called AFTER var auth = require('./routes/auth');. This means that when var app = require("../app"); is called inside auth.js, module.exports will not be yet defined inside app.js, hence leading to the error.