Question 1

What is Node.js and how does it differ from traditional web servers?

Accepted Answer

Node.js is a JavaScript runtime built on Chrome's V8 engine that executes JavaScript server-side. Unlike traditional servers (Apache, Nginx) that create new threads per request, Node.js uses a single-threaded event loop with non-blocking I/O, making it highly efficient for concurrent connections and I/O-heavy applications like real-time apps and APIs.

Question 2

What is the event loop in Node.js?

Accepted Answer

The event loop is Node.js's mechanism for handling asynchronous operations on a single thread. It continuously checks the call stack and callback queue, executing callbacks when the stack is empty. Phases include timers, I/O callbacks, idle/prepare, poll, check (setImmediate), and close callbacks. This enables non-blocking I/O without multi-threading complexity.

Question 3

What is the difference between CommonJS and ES Modules?

Accepted Answer

CommonJS uses require()/module.exports, loads synchronously, and is Node.js's original module system. ES Modules use import/export, load asynchronously, support static analysis and tree-shaking, and are the JavaScript standard. Use .mjs extension or "type": "module" in package.json for ES Modules. ES Modules are increasingly preferred for new projects.

Question 4

What are Promises and how do they improve on callbacks?

Accepted Answer

Promises represent eventual completion/failure of async operations, returning an object with .then(), .catch(), and .finally() methods. They solve callback hell by enabling chaining, provide better error handling with catch propagation, and allow Promise.all()/Promise.race() for concurrent operations. Promises are the foundation for async/await syntax.

Question 5

How does async/await simplify asynchronous code?

Accepted Answer

async/await is syntactic sugar over Promises that makes async code look synchronous. 'async' marks a function as returning a Promise; 'await' pauses execution until a Promise resolves. Benefits: cleaner code, easier debugging, try/catch for error handling, and better readability than .then() chains. Always handle errors with try/catch or .catch().

Question 6

What is npm and what is package.json?

Accepted Answer

npm (Node Package Manager) is the default package manager for Node.js, providing access to thousands of reusable packages. package.json is the project manifest containing metadata, dependencies, devDependencies, scripts, and configuration. It enables reproducible builds, defines project entry points, and manages version constraints for dependencies.

Question 7

What is the difference between dependencies and devDependencies?

Accepted Answer

dependencies are packages required for production runtime (Express, database drivers). devDependencies are only needed during development (testing frameworks, linters, build tools). Install with npm install (dependencies) or npm install --save-dev (devDependencies). Production installs (npm install --production) skip devDependencies to reduce deployment size.

Question 8

What are middleware functions in Express?

Accepted Answer

Middleware are functions with access to request, response, and next() that execute sequentially in the request-response cycle. They can modify req/res objects, end the cycle, or call next() to pass control. Types: application-level, router-level, error-handling (4 params), built-in (express.json()), and third-party (cors, helmet). Order matters.

Question 9

How does error handling middleware work in Express?

Accepted Answer

Error-handling middleware has four parameters: (err, req, res, next). Define it after all routes. When next(err) is called or an error is thrown in async code, Express skips to error middleware. Use try/catch in async handlers or express-async-errors package. Always send appropriate status codes and avoid exposing sensitive error details in production.

Question 10

What is clustering in Node.js?

Accepted Answer

Clustering creates multiple Node.js processes (workers) sharing the same server port, utilizing multiple CPU cores. The master process manages workers via the cluster module. Each worker runs independently with its own event loop and memory. This improves performance and provides fault tolerance—if a worker crashes, others continue. PM2 simplifies cluster management.

Question 11

What are streams in Node.js?

Accepted Answer

Streams are objects for handling data piece-by-piece without loading everything into memory. Four types: Readable (fs.createReadStream), Writable (fs.createWriteStream), Duplex (TCP sockets), and Transform (zlib compression). Use pipe() to connect streams. Ideal for large files, network data, and real-time processing. They're memory-efficient and enable backpressure handling.

Question 12

How do you manage authentication in Node.js applications?

Accepted Answer

Common approaches: JWT (stateless tokens stored client-side), session-based (server-stored sessions with cookies), and OAuth/OpenID for third-party auth. Use Passport.js for strategy abstraction. Best practices: hash passwords with bcrypt, use HTTPS, implement refresh tokens, set appropriate cookie flags (httpOnly, secure, sameSite), and validate/sanitize all inputs.

Question 13

What are common security vulnerabilities in Node.js applications?

Accepted Answer

Key vulnerabilities: injection attacks (SQL, NoSQL, command), XSS (cross-site scripting), CSRF (cross-site request forgery), insecure dependencies, sensitive data exposure, and broken authentication. Mitigations: use parameterized queries, sanitize inputs, implement CSP headers, use helmet middleware, audit dependencies (npm audit), and follow OWASP guidelines.

Question 14

How do you connect Node.js to a database?

Accepted Answer

Use database-specific drivers: mongoose for MongoDB, pg for PostgreSQL, mysql2 for MySQL. ORMs like Sequelize, TypeORM, or Prisma provide abstraction. Best practices: use connection pooling, handle connection errors gracefully, use environment variables for credentials, implement retry logic, and close connections properly. Async/await simplifies database operations.

Question 15

What tools are used for testing Node.js applications?

Accepted Answer

Test runners: Jest (most popular, built-in mocking), Mocha (flexible, needs assertion library). Assertion libraries: Chai, Node's assert. Mocking: Sinon, Jest mocks. HTTP testing: Supertest. Coverage: Istanbul/nyc. E2E: Playwright, Cypress. Best practices: test pyramid (many unit, fewer integration, few E2E), aim for high coverage, use CI/CD integration.

Node.js Fundamentals(5 questions)

What is Node.js and how does it differ from traditional web servers like Apache or Nginx?

Can you explain how the Node.js runtime handles I/O operations?

What are the key differences between the CommonJS and ES Modules module systems in Node.js?

Describe the role and significance of the single-threaded event loop in Node.js.

How do callbacks work in Node.js, and what limitations or pitfalls might they bring?

Asynchronous Programming and Event Loop(5 questions)

Can you explain the concept of asynchronous programming in Node.js?

What are Promises, and how do they improve the developer experience compared to callbacks?

How does async/await simplify asynchronous code? Provide an example scenario.

What is the event loop and how does it manage asynchronous operations?

Describe what the terms "callback hell" or "pyramid of doom" refer to and how to avoid them.

NPM and Package Management(5 questions)

What is npm and why is it vital for Node.js development?

How do you manage dependencies in a Node.js project?

What is the purpose of the package.json file?

How would you handle version updates to dependencies in a production application?

Can you explain the difference between dependencies and devDependencies?

Express.js and Web Frameworks(5 questions)

How do you set up a basic Express.js application?

What are middleware functions in Express and how are they used?

How would you structure an Express app for scalability and maintainability?

What is the role of error handling middleware in an Express application?

How can you implement routing in an Express project and what considerations should be made for nested routes?

Databases and Data Management(5 questions)

How do you connect a Node.js application to a database (e.g., MongoDB, PostgreSQL)?

What are the best practices for handling asynchronous database operations in Node.js?

Can you discuss the benefits and challenges of using an ORM/ODM in Node.js projects?

Describe how you would secure database queries to prevent injection attacks.

How do you manage database migrations in a Node.js environment?

Testing and Debugging(5 questions)

What tools and libraries are available for testing Node.js applications?

How do you perform unit testing in a Node.js application?

What are integration tests, and why are they important in the context of fullstack development?

How can you debug a Node.js application effectively?

What strategies can be employed to monitor and log errors in Node.js applications?

Performance Optimization and Scaling(5 questions)

How does Node.js handle high concurrency and what strategies exist to scale Node applications?

What is clustering in Node.js and how does it improve performance?

Describe some common performance pitfalls in Node.js applications.

How can you leverage caching strategies to optimize performance in a Node.js app?

What profiling tools might you use to identify performance bottlenecks in Node.js?

Security Best Practices(5 questions)

What are some common security vulnerabilities in Node.js applications?

How do you secure sensitive data within a Node.js application?

What measures can be taken to prevent cross-site scripting (XSS) and cross-site request forgery (CSRF) in Node.js apps?

How do you manage authentication and authorization in a Node.js application?

What are some considerations for securing RESTful APIs built with Node.js?

Advanced Topics and Specialized Knowledge Areas(5 questions)

How does Node.js integrate with microservices architecture?

What are streams in Node.js and how can they be utilized for efficient data processing?

Can you explain how WebSockets work in Node.js and provide a use case for their application?

Describe the process of building a real-time application with Node.js.

How do you approach error handling and resilience in a large-scale Node.js service?