1. #1

   Juniorfundamentalsruntime

   What is Node.js and what is it good for?

   Node.js is a JavaScript runtime built on V8 with a non-blocking, event-driven I/O model, well-suited to scalable network and I/O-bound applications.

2. #2

   Mid-levelevent-loopasync

   How does the Node.js event loop work?

   libuv runs phases (timers, pending callbacks, poll, check, close) each tick, with microtasks and process.nextTick draining between phases.

3. #3

   Mid-levelmodules

   How do CommonJS and ES modules differ in Node.js?

   CommonJS uses synchronous require/module.exports; ESM uses static import/export, is asynchronous, supports top-level await, and is enabled via .mjs or "type":"module".

4. #4

   Mid-levelstreamsio

   What are streams in Node.js and why use them?

   Streams process data in chunks as it arrives instead of buffering it all in memory, enabling efficient handling of large files and network data.

5. #5

   Mid-levelexpresshttp

   What is middleware in Express/Node web frameworks?

   Middleware are functions with (req, res, next) that run in sequence to process a request — parsing bodies, authenticating, logging — before the route handler.

6. #6

   Mid-levelerrorsasync

   How do you handle errors properly in Node.js?

   Use try/catch with async/await, handle callback error-first arguments, propagate via next() in Express, and add global handlers as a last resort.

7. #7

   Juniornpmtooling

   What is package.json and what are dependencies vs devDependencies?

   package.json describes the project, its scripts, and dependencies. dependencies are needed at runtime; devDependencies only for development and building.

8. #8

   Seniorscalingcluster

   How do you scale a single-threaded Node.js app across CPU cores?

   Run multiple processes — via the cluster module, a process manager like PM2, or container replicas — since one Node process uses a single main thread.

9. #9

   Seniorsecuritybest-practices

   What are key security practices for Node.js backends?

   Validate all input, use parameterised queries, set security headers, keep secrets out of code, manage dependencies, and never trust client data.

10. #10

    Seniorconcurrencyperformance

    When should you use worker threads in Node.js?

    Use worker threads for CPU-intensive tasks that would block the event loop — parsing, encryption, image processing — keeping the main thread responsive.