Interview Q&A / Node.js

1. If Node.js is single-threaded, how does it handle thousands of requests at the same time?

   Your JavaScript runs on one main thread, but slow work like network and file I/O is handed to the operating system or libuv's thread pool. The main thread never sits waiting; it keeps taking new requests and runs each callback when its I/O finishes.

   BeginnerIn-depth3 min read
2. What is middleware in Express.js and how does next() work?

   Middleware is a function that runs between the request arriving and the response being sent. It receives req, res and next, and it either ends the response or calls next() to pass control to the next middleware or route. Calling next(err) jumps to error-handling middleware.

   BeginnerIn-depth2 min read
3. What is the difference between CommonJS (require) and ES modules (import) in Node.js?

   CommonJS uses require and module.exports, loads files synchronously, and was Node's original system. ES modules use import and export, are the JavaScript standard, are analysed before running, and support top-level await. Node supports both.

   BeginnerIn-depth3 min read
4. What is the difference between dependencies and devDependencies, and why do we need package-lock.json?

   dependencies are packages your app needs to run in production; devDependencies are only needed while developing, testing or building. package-lock.json records the exact version of every installed package so every machine and deployment gets the same dependency tree.

   BeginnerIn-depth3 min read
5. How do you handle errors in asynchronous Node.js code, and what is an unhandled promise rejection?

   Use try/catch around await, .catch() on promise chains, and the error-first argument in callbacks. An unhandled rejection is a promise that fails with no handler attached; since Node 15 it crashes the process by default, so every promise needs a catch somewhere.

   IntermediateIn-depth3 min read
6. What are streams in Node.js and why would you use them for large files?

   A stream processes data in small chunks as it arrives instead of loading everything into memory at once. For a large file, streams keep memory use low and let you start sending data immediately, and pipeline() handles backpressure and errors for you.

   IntermediateIn-depth3 min read
7. What is the difference between process.nextTick, setImmediate and setTimeout in Node.js?

   process.nextTick runs right after the current operation, before promise callbacks and before the event loop moves on. setImmediate runs in the check phase, right after I/O polling. setTimeout(fn, 0) runs in the timers phase after at least about 1 ms.

   IntermediateIn-depth3 min read
8. When would you use worker_threads, cluster or child_process in Node.js?

   Use worker_threads for CPU-heavy JavaScript inside one process, cluster to run several copies of a server across CPU cores sharing one port, and child_process to run a separate program or script such as ffmpeg or a Python file.

   AdvancedIn-depth3 min read