I worked on a MERN project and everything was going well in development(local) mode. When i hosted my code on vercel i noticed that session are not working properly. my mentor suggested to create a small app to debug the issue so i created the following one. It is meant to store a value and returns it in the next ‘/get’ call?

const express = require('express');
const app = express();
const cors = require('cors');
const session = require('express-session');
const bodyParser = require('body-parser');
const MongoDBStore = require('connect-mongodb-session')(session);
app.use(cors({
    origin: 'my_domain',
    credentials: true
}));
var store = new MongoDBStore({
    uri: "my_atlas_uri?retryWrites=true&w=majority",
    collection: 'mySessions',
});

app.use(
    session({
        secret: "Session_Key",
        resave: true,
        store: store,
        saveUninitialized: false,
        cookie: {
            httpOnly: true,
            sameSite: true,
            secure: true,
            maxAge: 6 * 60 * 60 * 1000, //6 hours
            rolling: true,
        },
    })
);
app.use(bodyParser.json());
app.get('/', (req, res) => {
    res.json('hello world');
});
app.get('/store', (req, res) => {
    req.session.store = 'hi0';
    req.session.save(err => {
        if(err) {
            console.log(err);
        }
        console.log(req.session.store,"hlo");
        res.json('stored');
    });
});
app.get('/get', (req, res) => {
    console.log(req.session.store);
    res.json({data:req.session.store,id:req.sessionID,id1:req.session.id,cookie:req.session.cookie});
});

so it is returnig empty on req.session.store. the data(cookie) is being stored in my mongodb database. the staus code for “/store” is 304 and for “/get” is 200. i am deploying on vercel

this is my vercel.json

{
  "version": 2,
  "builds": [
    { "src": "*.js", "use": "@vercel/node" }
  ],
  "routes": [
    { "src": "/(.*)", "dest": "/"}
  ]
}