What does 'require('./app/routes.js')(app);' mean in this node server example? [duplicate]

Question

Server.js

    // set up ======================================================================
    var express = require('express');
    var app = express();                        // create our app w/ express
    var mongoose = require('mongoose');                 // mongoose for mongodb
    var port = process.env.PORT || 8080;                // set the port
    var database = require('./config/database');            // load the database config
    var morgan = require('morgan');
    var bodyParser = require('body-parser');
    var methodOverride = require('method-override');

    // configuration ===============================================================
    mongoose.connect(database.localUrl);    // Connect to local MongoDB instance. A remoteUrl is also available (modulus.io)

    app.use(express.static('./public'));        // set the static files location /public/img will be /img for users
    app.use(morgan('dev')); // log every request to the console
    app.use(bodyParser.urlencoded({'extended': 'true'})); // parse application/x-www-form-urlencoded
    app.use(bodyParser.json()); // parse application/json
    app.use(bodyParser.json({type: 'application/vnd.api+json'})); // parse application/vnd.api+json as json
    app.use(methodOverride('X-HTTP-Method-Override')); // override with the X-HTTP-Method-Override header in the request


    // routes ======================================================================
    require('./app/routes.js')(app);

    // listen (start app with node server.js) ======================================
    app.listen(port);
    console.log("App listening on port " + port);

I understand a majority of this code. But I have never seen this:

require('./app/routes.js')(app);

I understand we are loading our routes but why are we passing (app) as if it is a function parameter? Why is this necessary and what would happen if I remove it?

Answer 1

It just means that require('./app/routes.js') returns a function. You can then call this function with another set of parantheses.

It's basically the same as:

var func = require('./app/routes.js');
func(app);