Node.js — Files, HTTP, Express

9. Practice Problems

1. Write a runnable in-memory CRUD store for "books".
   ✨ Show Answer

   a1.js

   const books = [];
   const add  = (t) => books.push({ id: books.length + 1, t });
   add("পথের পাঁচালী"); add("আগুনের পরশমণি");
   console.log(books);

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2. Sketch a GET /books endpoint in Express.
   ✨ Show Answer

   app.get("/books", (req, res) => res.json(books));

3. Why use express.json() middleware?
   ✨ Show Answer

   Answer: It parses incoming JSON bodies and attaches them to req.body. Without it, req.body is undefined for JSON requests.

4. Add a 404 handler.
   ✨ Show Answer

   app.use((req, res) => res.status(404).json({ error: "not found" }));

5. Read process arguments.
   ✨ Show Answer

   // node app.js hello world
   console.log(process.argv.slice(2));   // ["hello", "world"]

6. Use fs/promises to read a JSON file (sketch).
   ✨ Show Answer

   import { readFile } from "node:fs/promises";
   const cfg = JSON.parse(await readFile("config.json", "utf8"));

7. Why is Node single-threaded yet good for I/O?
   ✨ Show Answer

   Answer: Node uses libuv to push I/O onto a thread pool while JS itself runs single-threaded. Your code never blocks waiting for disk or network — callbacks/Promises fire when the result arrives. Result: thousands of concurrent connections handled by one process.

8. Build an Express middleware that logs request time.
   ✨ Show Answer

   app.use((req, res, next) => {
       const t0 = Date.now();
       res.on("finish", () => console.log(req.method, req.url, Date.now() - t0, "ms"));
       next();
   });

9. Add CORS headers manually (sketch).
   ✨ Show Answer

   app.use((req, res, next) => {
       res.setHeader("Access-Control-Allow-Origin", "*");
       res.setHeader("Access-Control-Allow-Methods", "GET,POST,PATCH,DELETE");
       res.setHeader("Access-Control-Allow-Headers", "Content-Type,Authorization");
       if (req.method === "OPTIONS") return res.sendStatus(204);
       next();
   });

10. Build a runnable validate(body) helper for {text required}.
    ✨ Show Answer

    a10.js

    const validate = b => {
        if (!b || typeof b.text !== "string" || !b.text.trim())
            return { ok: false, error: "text required" };
        return { ok: true };
    };
    console.log(validate({}));
    console.log(validate({ text: "hi" }));

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11. Why do production apps put Node behind a reverse proxy (nginx)?
    ✨ Show Answer

    Answer: nginx terminates TLS, serves static files faster, gzip-compresses, rate-limits, and load-balances multiple Node processes. It's also far better at gracefully handling slow clients than Node's built-in HTTP server.

12. Add a graceful shutdown on SIGINT (sketch).
    ✨ Show Answer

    const server = app.listen(3000);
    process.on("SIGINT", () => {
        console.log("shutting down");
        server.close(() => process.exit(0));
    });

13. Demonstrate an in-memory rate-limiter (logic only).
    ✨ Show Answer

    a12.js

    function limiter(maxPerMin) {
        const hits = new Map();
        return ip => {
            const now = Date.now();
            const arr = (hits.get(ip) || []).filter(t => now - t < 60_000);
            arr.push(now);
            hits.set(ip, arr);
            return arr.length <= maxPerMin;
        };
    }
    const ok = limiter(2);
    console.log(ok("ip1"), ok("ip1"), ok("ip1"));

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14. In one paragraph, why is Node so popular for backends in 2026?
    ✨ Show Answer

    Answer: One language end-to-end, the largest package ecosystem (npm), brilliant async I/O performance, and a tiny memory footprint per connection. Hire one engineer, they ship both ends. The big trade-off — single-threaded CPU per process — is worked around with worker threads or cluster mode when needed.