Serve front-end web pages and backend APIs under the same URL

index.js

const express = require("express");
const path = require("path");

const PORT = process.env.PORT || 5000;
const app = express();
const router = express.Router();

// this middleware function must come before
// express.static middleware
router.use("/", (req, res, next) => {
  // Performs access permission check and
  // then retrieves JSON data or return 
  // error message base on user input.
  
  // Following statement simply echo the 
  // query string back to user.
  if (/^\/\?.+/.test(req.url)) {
    return res.send(req.query);
  }
  
  next();
});

router.use("/", express.static(path.join(__dirname, "public")));

app.use("/", router);

app.listen(PORT, () => {
  console.log(`Some magic is happening on port ${PORT}`);
});