15 Node.js Interview Questions You Should Be Able to Answer (2026)

Preparing for a Node.js interview? Here are 15 frequently asked questions with concise answers. Mastering these basics will put you in good stead.

1. What exactly is Node.js? It's a runtime environment to execute JavaScript code outside of web browsers, built upon the V8 engine. It's event-driven and non-blocking, making it perfect for I/O-intensive applications such as APIs and real-time servers.

2. Is Node.js single-threaded? Yes, your JavaScript runs on one main thread, but Node delegates heavy I/O operations to libuv's background thread pool. While it appears single-threaded, it leverages multiple threads behind the scenes.

3. What's the difference between CommonJS and ES Modules? CommonJS uses the 'require' and 'module.exports' syntax (synchronous and classic). ES Modules employ 'import' and 'export' syntax (modern standard, asynchronous, and tree-shakeable). Enable ES Modules by using 'type: module' or '.mjs' file extension.

4. Explain the event loop. The event loop enables Node's single-threaded nature to manage multiple operations concurrently. It processes callbacks for completed asynchronous tasks (timers, I/O) once the call stack is clear.

5. How do 'setImmediate' and 'setTimeout' compare? Both run within asynchronous phases, but 'setImmediate' executes in the 'check' phase, while 'setTimeout(fn, 0)' operates in the timers phase. Within an I/O callback, 'setImmediate' will be executed before 'setTimeout(fn, 0)'.

6. What is async/await? This is syntactic sugar built on top of Promises, allowing asynchronous code to be written in a more synchronous manner. An async function yields a Promise, and the await keyword pauses execution until the Promise settles. Errors should be handled using try/catch.

7. What are the differences among Promise.all, allSettled, race, and any? all stops execution on the first rejection. allSettled waits for all Promises to settle, reporting the outcome of each. race returns as soon as any Promise settles. any mirrors the behavior of race, but only rejects if every Promise fails.

8. How can you manage CPU-intensive tasks? Offload them to worker_threads for parallel processing, child processes, or use the cluster module to utilize all available CPU cores. Avoid blocking the single JavaScript thread at all costs.

9. What are streams? Streams are used to process data in chunks instead of loading it all into memory. This is particularly useful for handling large files or network data. Types include Readable, Writable, Duplex, and Transform streams.

10. How does the 'require()' function handle caching? Upon the first require(), the module is executed and its exports are cached. Subsequent calls simply return the cached exports, ensuring top-level module code executes only once.

11. What role do middleware functions play? Middleware functions follow the (req, res, next) signature and operate within the request-response cycle. They can read or modify the request/responses, terminate the response, or invoke next() to pass control to the subsequent middleware.

12. Outline the key REST principles. Resources are identified by URLs (nouns), standard HTTP methods (verbs) represent actions, requests should be stateless, and appropriate status codes must be returned.

13. How does JWT authentication function? After login, the server generates a token (header.payload.signature). The client includes this token in every request's header; the server validates the signature. Note that JWTs are Base64-encoded only and should not contain sensitive data.

14. How should passwords be handled? Use a secure hashing algorithm like bcrypt with a salt, and compare hashes using bcrypt.compare during login. Never store plain text passwords.

15. How can you scale a Node.js application? Utilize the cluster module or PM2 to harness multiple CPU cores, employ Redis for caching, enhance database indexing, avoid blocking the event loop, and paginate large response sets.

For a more comprehensive list, check out the full 64-question guide available here: https://asbackendinstitute.com/blog/nodejs-interview-questions/top-60-nodejs-interview-questions. Which Node.js question do you feel is the most commonly asked in interviews? Share your thoughts below.