Node.js Interview Questions PDF Downloads

Key takeaways

• Node runs JavaScript on a single main thread and offloads I/O so it can serve many requests.
• Blocking the event loop with heavy computation slows every request.
• Express middleware runs in order and shapes every request and response.
• Handle errors in async code explicitly and never leak internal details to clients.
• Validate input, keep secrets in environment variables and update dependencies.

What is Node.js interview questions?

What is Node.js and when is it a good fit?

Node.js runs JavaScript outside the browser on the V8 engine. It is a good fit for I/O-heavy services such as APIs, real-time apps and tools that wait on networks and databases. It is less suitable for long CPU-heavy work on the main thread, which should go to worker threads or separate services.

How does the event loop work?

Node runs your JavaScript on one main thread. When it starts an operation such as reading a file or a network call, it hands the work to the system or a thread pool and continues. When the work finishes, the callback is queued and the event loop runs it when the call stack is empty. The loop has phases (timers, pending callbacks, poll, check and close). Promise callbacks (microtasks) and process.nextTick run between phases, before the next timer.

console.log("A");
setTimeout(() => console.log("B"), 0);
Promise.resolve().then(() => console.log("C"));
console.log("D");
// A, D, C, B

What does non-blocking mean?

A non-blocking call returns immediately and signals completion later, so the thread can serve other requests. A blocking call such as fs.readFileSync or a long loop stops everything until it finishes. Use async versions in servers.

How do modules work?

CommonJS uses require and module.exports. ES modules use import and export and are enabled with "type": "module" in package.json or the .mjs extension. Modules are cached after the first load, so a module's top-level code runs once.

Callbacks, promises and async await?

Callbacks are functions passed to be called later; nesting them gets hard to read. Promises represent a future value and chain with .then. async and await give a sequential style on top of promises, with try/catch for errors. Run independent tasks together with Promise.all.

What are streams?

Streams process data in chunks instead of loading it all in memory, for example reading a large file or piping a file to an HTTP response. Types are readable, writable, duplex and transform. Streams reduce memory use and start sending data sooner.

What is Express middleware?

Middleware functions receive the request, the response and a next function. They run in the order registered and can modify the request, end the response or call next. Common uses are logging, parsing JSON, authentication and error handling.

app.use(express.json());
app.get("/users/:id", async (req, res, next) => {
  try {
    const user = await getUser(req.params.id);
    if (!user) return res.status(404).json({ error: "Not found" });
    res.json(user);
  } catch (err) { next(err); }
});
app.use((err, req, res, next) => res.status(500).json({ error: "Server error" }));

How should I handle errors?

Use try/catch with async code, pass errors to a central error handler, return correct status codes and log details on the server only. Handle unhandledRejection and uncaughtException by logging and shutting down cleanly, because the process may be in an unsafe state.

How do I configure an app?

Read settings from environment variables, keep secrets out of source control, provide an example file without real values and validate required settings at startup. Use different settings for development and production.

What security basics should I know?

Validate and sanitise input, use parameterised database queries to prevent injection, hash passwords with a strong algorithm, set secure HTTP headers, limit request rates, keep dependencies updated and avoid exposing stack traces. The OWASP Top Ten is a good checklist.

How do I scale a Node app?

Run several processes with a process manager or the cluster module to use all CPU cores, put a load balancer in front, keep the app stateless by moving sessions to a shared store and use caching for repeated reads.

How do I test a Node service?

Write unit tests for functions and integration tests that call the API with a test client against a test database. Mock external services, and run the tests in CI.

What small coding tasks are common?

Typical tasks are writing a REST endpoint that returns JSON, reading a file with streams, adding authentication middleware, implementing a rate limiter or reading from a database and handling failure. Explain the routes, the status codes you return, how you validate input and where you handle errors. Mention how you would test the endpoint with a client tool.

How do I discuss performance?

Measure before changing: use logging and profiling to find slow queries or blocking code. Typical fixes are adding database indexes, caching repeated reads, using streams for large payloads, compressing responses, moving heavy computation to workers and using connection pools. Always state the trade-offs, such as cache staleness.

Download Node.js Interview Questions PDF

Sources and further reading

• Node.js documentation
• Express documentation
• OWASP Top Ten