Trouble connecting to local MongoDB server through app.js

I'm a newbie to MongoDB, and I just set up MongoDB within my Ubuntu terminal with mongod and mongo commands running in separate terminals. However, I can't get my app.js that would launch a website to run properly. Here is the beginning of the code:

var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var exphbs = require('express-handlebars');
var helpers = require('handlebars-helpers')();
var expressValidator = require('express-validator');
var flash = require('connect-flash');
var session = require('express-session');
var passport = require('passport');
var LocalStrategy = require('passport-local').Strategy;
var mongoose = require('mongoose');
var fs = require('fs');
var net = require('net');

process.on('uncaughtException', (err) => {
    console.log(err);
});

//const config = {
//    autoIndex: false,
//    useNewUrlParser: true
//};

//return mongoose.connect(uri, config);

//Log into MongoDB server
//var db = mongoose.connection;
const db = mongoose.connection
const url = 'mongodb://127.0.0.1:27017/modeltest'

mongoose.connect(process.env.MONGO_URI, {
useUnifiedTopology: true,
useNewUrlParser: true,
})
.then(() => console.log('DB Connected!'))
.catch(err => {
console.log('DB Connection Error: ' + err);
});

When I run

app.js

, I get the following error:

{ MongooseError: The uri parameter to openUri() must be a string, got "undefined". Make sure the first parameter to mongoose.connect() or mongoose.createConnection() is a string.
    at new MongooseError (/home/ubuntu/modeltest/web/node_modules/mongoose/lib/error/mongooseError.js:10:11)
    at NativeConnection.Connection.openUri (/home/ubuntu/modeltest/web/node_modules/mongoose/lib/connection.js:579:11)
    at Mongoose.connect (/home/ubuntu/modeltest/web/node_modules/mongoose/lib/index.js:333:15)
    at Object. (/home/ubuntu/modeltest/web/app.js:31:10)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Function.Module.runMain (module.js:693:10)
    at startup (bootstrap_node.js:188:16)
    at bootstrap_node.js:609:3
  message: 'The uri parameter to openUri() must be a string, got "undefined". Make sure the first parameter to mongoose.connect() or mongoose.createConnection() is a string.',
  name: 'MongooseError' }
ubuntu@ip-172-31-27-34:~/mod

I've also tried to connect to the Mongo Atlas DB I've set up through some modifications to the code:

var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var exphbs = require('express-handlebars');
var helpers = require('handlebars-helpers')();
var expressValidator = require('express-validator');
var flash = require('connect-flash');
var session = require('express-session');
var passport = require('passport');
var LocalStrategy = require('passport-local').Strategy;
var mongoose = require('mongoose');
var fs = require('fs');
var net = require('net');

process.on('uncaughtException', (err) => {
    console.log(err);
});

//const config = {
//    autoIndex: false,
//    useNewUrlParser: true
//};

//return mongoose.connect(uri, config);

//Log into MongoDB server
var db = mongoose.connection;
//const db = mongoose.connection
//console.log(process.env.MONGO_URI)
const {MongoClient} = require('mongodb');
async function main(){
        const uri = "mongodb+srv://theusernamewhichientered:thepassword@modeltest-3lpoi.mongodb.net/test?retryWrites=true&w=majority"
        const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology: true });

try {
    await client.connect();

    await listDatabases(client);

} catch (e) {
    console.error(e);

} finally {
    await client.close();

}
}

main().catch(console.error);

async function listDatabases(client){
    databasesList = await client.db().admin().listDatabases();

    console.log("Databases:");
    databasesList.databases.forEach(db => console.log( - ${db.name}));
};

But I get the following output when I run

app.js

:

DIRNAME
/home/ubuntu/modeltest/web
Server started on port 3000
{ MongoServerSelectionError: connection  to 52.203.196.37:27017 closed
    at Timeout.waitQueueMember.timer.setTimeout [as _onTimeout] (/home/ubuntu/modeltest/web/node_modules/mongodb/lib/core/sdam/topology.js:430:30)
    at ontimeout (timers.js:482:11)
    at tryOnTimeout (timers.js:317:5)
    at Timer.listOnTimeout (timers.js:277:5)
  name: 'MongoServerSelectionError',
  reason: 
   TopologyDescription {
     type: 'ReplicaSetNoPrimary',
     setName: null,
     maxSetVersion: null,
     maxElectionId: null,
     servers: 
      Map {
        'modeltest-shard-00-00-3lpoi.mongodb.net:27017' => [Object],
        'modeltest-shard-00-01-3lpoi.mongodb.net:27017' => [Object],
        'modeltest-shard-00-02-3lpoi.mongodb.net:27017' => [Object] },
     stale: false,
     compatible: true,
     compatibilityError: null,
     logicalSessionTimeoutMinutes: null,
     heartbeatFrequencyMS: 10000,
     localThresholdMS: 15,
     commonWireVersion: null },

What could I do to move past this?