How to reference a variable inside a module.exports in Node.js

Just declare it above the module.exports as an ordinary variable:

var port = '1337';
module.exports = {
    port: port,
    facebook: {
        clientID: '123456789',
        clientSecret: 'a1b2c3d4e5f6g7h8i9j0k',
        callbackURL: 'http://localhost:'+ port + '/oauth/facebook/callback'
    }
};

You can put module.exports wherever you want in your file, you can even perform some logic (like retrieving settings from a file or another resource).