app.js

const express = require('express');
const app = express();
const session = require('express-session');

app.use(session({
  secret: 'keyboard cat',
  resave: false,
  saveUninitialized: true,
  cookie: { secure: true }
}))

app.post('/login', (req, res) => {
  //check against whatever database you want and return a value (ie. userid, username)
  let username = req.body.username;
  req.session.user = username;
})

app.get('/private', (req, res) => {
  //check req.session.user and do whatever else you want
})
app.listen(3000)