Node.js Interview Questions
MCQ Quiz · All Difficulty Levels
Prepare for Node.js interview questions with MCQs covering server-side JavaScript fundamentals and production patterns. Topics include the Node.js event loop and its phases, non-blocking I/O, the module system (CommonJS vs ESM), streams and buffers, the cluster module and worker threads, child processes, the built-in http and https modules, error handling in async code, EventEmitter, path and fs module usage, environment configuration, and common Node.js design patterns. Node.js powers most JavaScript backends, and interviewers at product companies, consulting firms, and startups expect candidates to understand how it differs from browser JavaScript and how to design systems around its single-threaded event-driven architecture. Difficulty levels cover Trainee (core module usage, basic async/callback patterns), Junior Dev (streams, event emitter, Express middleware patterns), Mid Level (event loop phases, memory management, clustering, performance), and Senior Dev (production architecture, scalability patterns, profiling, microservice integration). Every explanation targets real interview gaps.
Frequently Asked Questions: Node.js Interviews
Common questions developers ask when preparing for Node.js technical interviews.
What Node.js topics are most commonly tested in backend interviews?
The event loop and non-blocking I/O model come up in almost every Node.js interview, interviewers want to verify you understand why Node.js is single-threaded yet handles concurrency. Other frequent topics include streams and buffers for handling large data, the module system (CommonJS vs ESM), error handling in async code, EventEmitter patterns, clustering for multi-core utilization, and common Express.js middleware patterns.
What is the Node.js event loop and why is it important?
The Node.js event loop is the mechanism that allows Node to perform non-blocking I/O despite being single-threaded. It processes callbacks in phases: timers (setTimeout/setInterval), pending callbacks, idle/prepare, poll (incoming I/O), check (setImmediate), and close callbacks. Understanding the phases explains why setImmediate runs before setTimeout in I/O callbacks, and why process.nextTick runs before both. This knowledge is critical for writing correctly ordered async code.
When should I use Node.js vs other backend technologies?
Node.js excels at I/O-bound workloads: real-time applications (chat, live notifications), API gateways, streaming pipelines, and microservices that spend most of their time waiting for databases or external APIs. It is less suited for CPU-intensive tasks (image processing, heavy computation) without worker threads. Its main advantages are the shared JavaScript ecosystem with frontend teams, npm, and the large community of Node-specific libraries.
How does Node.js handle concurrent requests if it is single-threaded?
Node.js delegates I/O operations (file reads, network requests, database queries) to the operating system via libuv, freeing the event loop thread to continue processing other requests. When the I/O completes, the callback is queued and runs on the next available event loop iteration. This means Node can serve thousands of concurrent connections without spawning a thread per connection, unlike traditional thread-per-request servers.
What is the difference between require() and import in Node.js?
require() is CommonJS (CJS), it loads modules synchronously, supports dynamic requires (require inside conditions), and is the traditional Node.js module system. import/export is ESM (ECMAScript Modules), it is statically analyzed, supports tree-shaking, and is the standard across browsers and modern Node.js. Node.js supports both: .cjs files use CommonJS, .mjs files use ESM, and .js files follow the type field in package.json. Most new Node.js projects default to ESM.