how to add custom function to express module - node.js

Question

I am trying to add a method
loadSiteSettings to express module
In app.js
var express = require('express');
var path = require('path');
var mongoose = require('mongoose');
//Set up default monggose connection for mongo db
var mongoDB = 'mongodb+srv://***:*****#cluste******j.mongodb.net/cms?retryWrites=true&w=majority';
mongoose.connect(mongoDB,{useNewUrlParser: true});
//Get the default connection
var db = mongoose.connection;
//Bind connection to error event (to get notification of connection errors)
db.on('error',console.error.bind(console, 'MongoDB connection error:'));///????????
var app = express();
///////////////////////////////////////////////////////////
var indexRouter = require('./routes/index');
app.loadSiteSettings = async function()
{
let setting = await db.collection('settings').findOne();
app.locals.siteSettings = setting;
}
app.loadSiteSettings();
//////////////////////////////////////////////////////
module.exports = app;
Index.Js for router
var express = require('express');
var router = express.Router();
var app = require('../app');
var util = require('util');
/* GET home page. */
router.get('/', function(req, res, next) {
res.render('index');
});
///////////////////////////////////////////
router.get('/reloadSettings', function(req,res,next){
app.loadSiteSettings();
})
///////////////////////////////////////
module.exports = router;
so problem lies here, when server start it calls app.loadSiteSettings() in app.js
but when i use route '/reloadSettings' it seems app is undefined in index.js

Answer 1

This is an issue with circular dependencies and module.exports. This answer shows the same problem.
What's happening is app.js is required first and starts processing. The important thing to understand is that a file pauses execution while requiring a new file.
So when app.js requires ./routes/index, it has not finished processing, and has not reached module.exports = app. This means that when your routes file requires app.js, it's requiring it in its current state, which is the default export {}.
Thankfully there's a very simple fix:
// can be imported and tested separately from the app
const loadSiteSettings = function() {
return db.collection('settings').findOne();
}
router.get('/reloadSettings', async function(req,res,next){
let settings = await loadSiteSettings();
req.app.locals.siteSettings = settings
res.send(200); // signal the response is successful
})
This explains the issue in a lot more depth in case you're interested