I have created an Node js app, I am creating session using

app.use(session({
  resave: true , 
  rolling: true,
  secret: '2853554111' , 
  genid: function(req) { return uuid(); },
  saveUninitialized: true
 }));

and this is how I am saving my data in session.

req.session.user=req.body.username;

but after sometime I am getting undefined value of user in session, and this is hapenning very frequently, Not sure what is wrong with the code.

This is how my app.js looks like

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');
var session = require('express-session');
var indexRouter = require('./routes/index');
var usersRouter = require('./routes/users');
var bodyParser = require('body-parser');
var uuid = require("uuidv4");
var app = express();
var login = require('./routes/login-admin');
var adminHome = require('./routes/adminHome');
var pmLogin = require('./routes/pmLogin');
var pmHome = require('./routes/pmHome');
var confirmEmail = require('./routes/confirmEmail');

app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
app.use(express.static('public'));
app.use(logger('dev'));
//app.use(express.json());
app.use(express.json({limit: '5120mb'}));
//app.use(bodyParser.json())
app.use(bodyParser.json({ limit: '5120mb' }))
app.use(express.urlencoded({limit: '5120mb', extended: false }));

// view engine setup
app.use(session({
  resave: true , 
  rolling: true,
  secret: '2853554111' , 
  genid: function(req) { return uuid(); },
  saveUninitialized: true
 }));

app.use('/', indexRouter);
//app.use('/', login);
app.use('/users', usersRouter);
app.use('/login-admin', login);
app.use('/adminHome', adminHome);
app.use('/login', Login);
app.use('/Home', Home);
app.use('/confirmEmail', confirmEmail);