//index.js

const app = require('express')();
const http = require('http').Server(app);
...
var databaseInstance = new databaseManager({
    host: process.env.MONGO_HOST,
    port: process.env.MONGO_PORT,
    user: process.env.MONGO_USER,
    pass: process.env.MONGO_PASS,
    database: process.env.MONGO_DATABASE
});
var authInstance = new authManager();
var chatInstance = new chatManager();

// auth.js

class AuthManager extends EventEmitter {

    constructor() {

        super();

        ...

        initalizeRoutes();

    }

}

function initalizeRoutes() {

    console.log(main.expressApp)

    // I would like to access app here!
    app.post('/auth', function(req, res) {

    ...

    });

}

module.exports = AuthManager;