Aniketh - The session needs to be stored in order to persist, which you already seem to have part of. For MongoDB, I use:

npm i connect-mongodb-session --save

Example:

const mongoose = require('mongoose');
const session = require('express-session');
const MongoDBStore = require('connect-mongodb-session')(session);

const MONGODB_URI = 'mongodb://localhost:27017/[database_name]';

const app = express();
const store = new MongoDBStore({
  uri: MONGODB_URI,
  collection: 'sessions'
})

// define database via mongoose
mongoose.connect(MONGODB_URI, {
  useNewUrlParser: true
});
mongoose.set('useCreateIndex', true);

// connect to mongodb
const conn = mongoose.connection;
conn.on('error', console.error.bind(console, '[!] Connection Error '));

// session
app.use(
  session({
    secret: 'my secret',
    resave: false,
    saveUninitialized: false,
    store: store
  })
);

Now, when you check the database, you'll notice db.collection.session.

I use Browsersync in my Gulp pipeline, and needed to learn how to retain the session along with that and nodemon. In my case, this was the solution.