demo.js

import express from 'express';

const app = express();

// Middleware to parse req.body from application/x-www-form-urlencoded to an object
app.use(express.urlencoded({ extended: false }));

// Shared state that the server has access to.  Could be a database or some
// other external service.
let counter = 0;

app.route("/counter")
  .get((req, res) => res.send(`
    The counter is at: ${counter}.

    <form method="post">
      <label>
        Add <input type="number" name="n" value="1"> to counter
      </label>
      <input type="submit" value="do it">
    </form>
  `))
  .post((req, res) => {
    const n = Number(req.body.n);

    // Validation, authz, etc.
    if (!Number.isFinite(n))
      return res.status(400).send("Bad Request: not a number");

    counter += n;

    // Could also render again directly here and send it without
    // redirect, but best practice is redirect POST → GET.
    return res.redirect("/counter");
  });

app.listen(3000)