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Entry NodeJS Interview Questions

Curated Entry-level NodeJS interview questions for developers targeting entry positions. 30 questions available.

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NodeJS Interview Questions & Answers

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Welcome to our comprehensive collection of NodeJS interview questions and answers. This page contains expertly curated interview questions covering all aspects of NodeJS, from fundamental concepts to advanced topics. Whether you're preparing for an entry-level position or a senior role, you'll find questions tailored to your experience level.

Our NodeJS interview questions are designed to help you:

  • Understand core concepts and best practices in NodeJS
  • Prepare for technical interviews at all experience levels
  • Master both theoretical knowledge and practical application
  • Build confidence for your next NodeJS interview

Each question includes detailed answers and explanations to help you understand not just what the answer is, but why it's correct. We cover topics ranging from basic NodeJS concepts to advanced scenarios that you might encounter in senior-level interviews.

Use the filters below to find questions by difficulty level (Entry, Junior, Mid, Senior, Expert) or focus specifically on code challenges. Each question is carefully crafted to reflect real-world interview scenarios you'll encounter at top tech companies, startups, and MNCs.

Questions

30 questions
Q1:

What is Node.js?

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Node.js is a runtime environment that allows JavaScript to run outside the browser, mainly used for backend development.
Quick Summary: Node.js is a JavaScript runtime built on Chrome's V8 engine that lets you run JavaScript on the server side — outside the browser. It's built around a non-blocking, event-driven architecture, which makes it great for building fast, scalable network applications like APIs, real-time apps, and microservices.
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Q2:

Why was Node.js created?

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Node.js was created to handle a large number of concurrent requests efficiently using a non-blocking architecture.
Quick Summary: Node.js was created by Ryan Dahl in 2009 to solve the problem of handling many simultaneous connections efficiently. Traditional servers (Apache) blocked on I/O — each request tied up a thread. Node uses an event-driven, non-blocking model so one thread handles thousands of concurrent I/O operations without waiting.
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Q3:

What is the V8 engine in Node.js?

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The V8 engine is Google’s JavaScript engine that compiles JavaScript into fast machine code, helping Node achieve high speed.
Quick Summary: V8 is Google's open-source JavaScript engine — the same one that runs JavaScript in Chrome. It compiles JavaScript directly to native machine code (JIT compilation) instead of interpreting it, making it very fast. Node.js uses V8 under the hood to execute your server-side JavaScript code.
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Q4:

What is the event loop in Node.js?

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The event loop manages asynchronous operations and ensures Node remains responsive without blocking.
Quick Summary: The event loop is the core of Node.js — it's what allows JavaScript to handle multiple operations without blocking. It continuously checks if there are pending callbacks to run (timers, I/O, network). When an async operation completes, the event loop picks up the callback and runs it. This is how Node handles many connections with one thread.
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Q5:

What is single-threaded architecture?

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Node uses one main thread to execute JavaScript while delegating heavy tasks to background workers.
Quick Summary: Single-threaded means Node.js runs JavaScript on one main thread — unlike traditional servers that create a new thread per request. Instead of threading, Node uses asynchronous callbacks to handle concurrency. The single thread is never blocked because I/O operations run in the background (via libuv) and notify the main thread when done.
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Q6:

What is npm?

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npm is Node’s package manager used to install and manage external libraries.
Quick Summary: npm (Node Package Manager) is the default package manager for Node.js. It lets you install, share, and manage JavaScript libraries (packages) that you use in your project. npm install adds a package; package.json tracks which packages your project depends on. The npm registry hosts over 2 million packages.
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Q7:

What is a package.json file?

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A metadata file that stores project details, dependencies, scripts, and configuration.
Quick Summary: package.json is the configuration file for your Node.js project. It tracks your project name, version, scripts (npm start, npm test), and dependencies — the packages your app needs. Anyone who clones your repo can run npm install and get all the same packages installed automatically.
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Q8:

What are dependencies and devDependencies?

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Dependencies are required at runtime; devDependencies are used only during development.
Quick Summary: dependencies are packages needed in production — express, mongoose, jsonwebtoken. devDependencies are only needed during development — jest, nodemon, eslint. When you deploy to production, you can run npm install --production to skip devDependencies and keep the deployment lean. Separating them keeps production clean.
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Q9:

What is a callback function?

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A callback is a function passed as an argument to execute after an asynchronous task finishes.
Quick Summary: A callback is a function you pass to another function to be called when an operation finishes. In Node.js: fs.readFile('file.txt', callback) — Node reads the file async and calls your callback when done. Callbacks are the original way to handle async operations before Promises and async/await made it cleaner.
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Q10:

What is a module in Node.js?

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A module is a separate file containing reusable code that can be imported into other files.
Quick Summary: A module is a reusable piece of code in its own file. Node uses the module system to keep code organized and prevent variable name collisions. You export what you want to share (module.exports) and import what you need (require()). Every file in Node.js is automatically its own module.
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Q11:

What is CommonJS?

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CommonJS is Node’s module system using module.exports and require for import and export.
Quick Summary: CommonJS (CJS) is the original module system in Node.js — it uses require() to import and module.exports to export. It loads modules synchronously, which is fine for server-side code. It's still the default in most Node.js projects, though ES Modules (import/export) are now also supported.
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Q12:

What is the fs module used for?

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The fs module provides APIs to create, read, write, and modify files.
Quick Summary: The fs module provides functions for working with the file system — reading files, writing files, creating directories, checking if a file exists. Node has both sync versions (fs.readFileSync — blocks until done) and async versions (fs.readFile — non-blocking with a callback). Always prefer async in production to avoid blocking the event loop.
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Q13:

What is the path module used for?

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The path module works with file and directory paths in a cross-platform way.
Quick Summary: The path module helps you work with file and directory paths in a cross-platform way. It handles differences between operating systems (Windows uses backslashes, Unix uses forward slashes). path.join(__dirname, 'public', 'index.html') correctly builds a path regardless of the OS.
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Q14:

What is the os module?

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The os module provides system-level information such as CPU details, memory, and operating system.
Quick Summary: The os module provides information about the operating system — CPU count, total memory, free memory, hostname, platform (win32, linux, darwin). Useful for building health checks, monitoring dashboards, or scaling logic that adapts based on available system resources.
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Q15:

What is a buffer in Node.js?

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A buffer stores binary data in memory, useful for streams, files, and network operations.
Quick Summary: A Buffer is a fixed-size chunk of raw binary data — like a byte array. Node.js uses Buffers when dealing with binary data: reading files, receiving network packets, processing images. Buffers exist outside the V8 heap and are necessary because JavaScript strings are UTF-16 encoded and can't efficiently represent raw binary.
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Q16:

What is a stream in Node.js?

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A stream handles data in chunks, enabling efficient reading and writing without loading full data.
Quick Summary: A stream is a sequence of data delivered in chunks over time instead of all at once. Node.js uses streams for reading large files, piping HTTP responses, and processing data without loading everything into memory. Types: Readable (data comes in), Writable (data goes out), Duplex (both), Transform (modifies data in transit).
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Q17:

What is server-side JavaScript?

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It refers to writing backend logic in JavaScript that runs on the server instead of the browser.
Quick Summary: Server-side JavaScript means running JavaScript code on the server (Node.js) instead of the browser. You use the same language for both frontend and backend, share code between them, and build full-stack apps with one language. The server handles requests, talks to databases, and sends back responses — all in JavaScript.
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Q18:

What is the http module used for?

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The http module allows creation of HTTP servers and handling incoming requests.
Quick Summary: The http module is Node's built-in way to create HTTP servers and make HTTP requests. http.createServer() creates a server that listens on a port and receives requests. It's the foundation that frameworks like Express are built on. You rarely use it directly — Express provides a cleaner API on top of it.
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Q19:

What is middleware?

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Middleware functions run between a request and response for tasks like parsing, logging, or validation.
Quick Summary: Middleware is a function that sits between the request and response in the request pipeline. It receives the request, can modify it, perform logic (authentication, logging, parsing), and either send a response or pass control to the next middleware. In Express: app.use((req, res, next) => { ... next(); }).
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Q20:

What is Express.js?

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Express.js is a lightweight web framework for Node that simplifies routing, middleware, and server creation.
Quick Summary: Express.js is a minimal web framework for Node.js that makes it easy to build APIs and web apps. It provides routing (match URLs to handler functions), middleware support (plug in authentication, logging, parsing), template rendering, and error handling — with far less boilerplate than raw Node.js http module.
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Q21:

What is routing in Node and Express?

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Routing determines what happens when a specific URL and HTTP method are accessed.
Quick Summary: Routing maps incoming HTTP requests (URL + method) to handler functions. In Express: app.get('/users', handler) handles GET requests to /users; app.post('/users', handler) handles POST. Routes let you organize your API into clean, meaningful endpoints. Express Router lets you group related routes into separate files.
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Q22:

What is JSON in Node.js?

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JSON is a lightweight data format used for configuration, APIs, and storing structured data.
Quick Summary: JSON (JavaScript Object Notation) is the standard data format for APIs. Node.js has JSON.parse() to convert a JSON string to a JavaScript object, and JSON.stringify() to convert an object to a JSON string. Express's res.json() automatically serializes your response object and sets the Content-Type header to application/json.
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Q23:

What is an API endpoint?

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An API endpoint is a URL where clients send requests and receive responses.
Quick Summary: An API endpoint is a specific URL that your server exposes for clients to interact with. GET /api/users returns the user list; POST /api/users creates a new user; DELETE /api/users/123 deletes user 123. Each endpoint maps to a handler function in your server code that performs the appropriate operation.
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Q24:

What is an environment variable?

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Environment variables store external configuration such as API keys or database URLs.
Quick Summary: Environment variables are key-value pairs set outside your code that configure your application — database URL, API keys, port numbers, environment name. They let the same code run differently in dev vs prod without changing the code. You access them in Node.js via process.env.MY_VARIABLE.
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Q25:

What is process.env?

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process.env is a built-in object providing access to environment variables.
Quick Summary: process.env is an object in Node.js that contains all environment variables set in the system or .env file. process.env.PORT gives you the port to listen on; process.env.DB_URL gives the database connection string. Never hardcode secrets in code — read them from process.env instead.
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Q26:

What is nodemon?

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Nodemon automatically restarts a Node application when file changes are detected.
Quick Summary: nodemon is a development tool that automatically restarts your Node.js server when you change a file. Instead of manually stopping and restarting after every code change, nodemon watches your files and does it for you. Install as devDependency and run: nodemon server.js. It's a standard part of the Node.js dev workflow.
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Q27:

What is asynchronous programming in Node?

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Asynchronous programming allows Node to run tasks without waiting, improving scalability.
Quick Summary: Asynchronous programming means starting an operation (file read, database query, API call) and continuing to do other work while waiting for it to finish. When the operation completes, the callback, Promise, or async/await resumes. This is how Node.js serves many requests simultaneously with one thread — nobody waits for nobody.
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Q28:

What is synchronous programming?

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Execution happens step-by-step, blocking the thread until each operation completes.
Quick Summary: Synchronous programming runs operations one at a time — each line waits for the previous to finish. Simple to reason about but inefficient for I/O: fs.readFileSync blocks the entire Node.js thread while reading, preventing it from handling other requests. Fine for scripts; avoid in web servers handling concurrent requests.
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Q29:

What is an error-first callback?

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A callback pattern where the first argument is the error and the second is the result.
Quick Summary: An error-first callback is Node.js's convention: the first parameter of the callback is always an error (null if no error, Error object if something went wrong). The second parameter is the result. Always check the error first: if (err) return handleError(err). This pattern predates Promises but is still common in many Node APIs.
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Q30:

Difference between console.log and return?

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console.log prints output to the console; return sends a value back from a function to its caller.
Quick Summary: console.log outputs something to stdout — it's a side effect for visibility/debugging. return ends the function and passes a value back to the caller. They serve completely different purposes: console.log is for printing; return is for passing data. A function can console.log and return at the same time.
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People Also Ask - Related NodeJS Questions

What is Node.js?
Why was Node.js created?
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