Did you put the two parts of code in the same file(ie. main.js) or two different files.

put them in the same file, and run node main.js do throw exceptions.

// main.js
var mongoose = require('mongoose');
var uri = 'mongodb://localhost/mydb';
mongoose.Promise = global.Promise;
var options = { server: {socketOptions: { keepAlive: 300000, 
connectTimeoutMS: 10000 } } } ;
mongoose.connect(uri,options);
var db = mongoose.connection;
db.on('error',console.log.bind(console,'connection refused !!!!!'));
db.once('open', console.log.bind(console,'connection success !!!!!'));

var express = require('express');
var router = express.Router();
var mongoose = require('mongoose');
var rootRes = require('../model/rootresources.js');

router.get('/', function(req, res, next) {
  rootRes.find({},function (err, rootResource) {
  if (err){
    console.log('Error occurd !!!') }
  res.json(rootResource);
  });
});

exceptions are:

connection refused !!!!! { MongoError: failed to connect to server [localhost:27017] on first connect [MongoError: connect ECONNREFUSED 127.0.0.1:27017]
etc...

So, I think maybe you put codes about express in a file like index.js and codes about mongoose connection in another file. And just run node index.js in command line. While running codes in index.js will not include codes in other files, codes in main.js will not be executed. As the result, there is no error info.

Updates Two ways of I know two ways of doing this:

1.In main.js create function which creates connection to database and returns a instance of db so that you can call it function in you main code.

// main.js like this
var mongoose = require('mongoose');
function createConnection(url) {
  mongoose.connect(url,options);
  var db = mongoose.connection;
  db.on('error',console.log.bind(console,'refused !!!!!'));
  db.once('open', console.log.bind(console,'success !!!!!'));
  return db;
}
// export function
module.exports = createConnection;

// in your index.js 
var createConnection = require('./main.js');
var db = createConnection(url);
// other codes here

2.Using require or vm to compile and run javascipt code. You can find vm api detail here

//main.js
var mongoose = require('mongoose');
var uri = 'mongodb://localhost/mydb';
mongoose.Promise = global.Promise;
var options = { server: {socketOptions: { keepAlive: 300000, 
connectTimeoutMS: 10000 } } } ;
mongoose.connect(uri,options);
var db = mongoose.connection;
db.on('error',console.log.bind(console,'connection refused !!!!!'));
db.once('open', console.log.bind(console,'connection success !!!!!'));

// index.js
// require will load file and execute automaticly
var scriptSrc = require('./main');
// other codes here

You can think of the second way as using eval('var mongoose = require('mongoose'); var uri = 'mongodb://localhost/mydb'; etc...)

mongoose connection do not happen unless you hit a request. so its best you handle it in your first request middleware. see code insight bellow.

module.exports = function () {

return function (req, res, next) {

            mongoose.connect(URL, MONGO_OPTIONS);
            mongoose.connection
                .once('open', () => { })
                .on('error', (error) => {
                     res.status(401).json({});
                });
...

Then pass the middleware above to your router: let me know if you need more explanation

router.get('/', myMiddleware(){})