Node.js technical interview questions and answers are essential for candidates preparing for backend development, full-stack development, and server-side programming roles. Companies like TCS, Infosys, Wipro, Accenture, and Capgemini frequently test your understanding of asynchronous programming, event loops, Express.js, APIs, middleware, error handling, database integration, and performance optimization. Technical interviewers evaluate your ability to build scalable, efficient, and secure backend applications using Node.js and its ecosystem.
These questions help freshers, job seekers, and working professionals strengthen their concepts and improve their coding abilities during real interviews. Node.js interviews often include scenario-based questions, live coding challenges, and practical backend tasks to test your problem-solving skills. This guide provides the most important technical interview Q&A to help you revise key topics, practice mock questions, and become interview-ready for top IT companies and campus placements.
1. What is the event-driven architecture in Node.js
Answer: Node.js is built on event-driven architecture where actions are triggered by events. It allows for asynchronous processing, enabling non-blocking operations, which is highly efficient in I/O-bound operations.
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2. How does Node.js handle asynchronous operations
Answer: Node.js uses an event loop to handle asynchronous operations. It uses callbacks, Promises, or async/await to manage operations like reading files, making HTTP requests, and database queries without blocking the execution of other code.
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3. What is the purpose of the cluster module in Node.js
Answer: The cluster module in Node.js allows you to create child processes (workers) that share the same server port, enabling load balancing and better utilization of multi-core systems.
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4. Explain the concept of middleware in Express.js
Answer: Middleware in Express.js are functions that have access to the request object, response object, and the next middleware function in the application’s request-response cycle. They can execute code, modify the request and response objects, end the request-response cycle, or call the next middleware function.
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5. What is the difference between process.nextTick() and setImmediate()
Answer: process.nextTick() schedules a callback to be invoked in the next iteration of the event loop, before any I/O tasks. setImmediate(), on the other hand, schedules a callback to be executed on the next iteration of the event loop after I/O events.
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6. What are streams in Node.js and how are they used
Answer: Streams in Node.js are instances of EventEmitter and are used to handle reading/writing of data in a continuous manner. They are especially useful when working with large amounts of data (e.g., reading a large file). Streams can be readable, writable, duplex, or transform.
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7. What is the role of the package.json file in a Node.js project
Answer: The package.json file holds various metadata relevant to the project. It defines the project’s dependencies, scripts, version, and more. It is essential for managing the project and its dependencies.
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8. Explain how to handle errors in Node.js
Answer: Errors in Node.js can be handled using try-catch blocks for synchronous code and error-first callbacks or Promises (with catch) for asynchronous code. It is also possible to handle uncaught exceptions using process.on("uncaughtException").
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9. What is the purpose of the buffer class in Node.js
Answer: The Buffer class in Node.js is used for handling binary data directly in memory. It is particularly useful when working with TCP streams or file system operations, where the data is stored in binary format.
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10. How do you implement authentication in a Node.js application
Answer: Authentication in a Node.js application can be implemented using Passport.js, JWT (JSON Web Tokens), OAuth, or other libraries. Passport.js, for example, offers various strategies for handling authentication, while JWT is often used for token-based authentication.
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11. What is the role of the require function in Node.js
Answer: The require function is used to include modules (core, local, or third-party) in Node.js applications. It reads the JavaScript file, executes it, and returns the export object.
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12. Explain the difference between CommonJS and ES6 modules
Answer: CommonJS modules use require and module.exports, while ES6 modules use import and export. ES6 modules are static, meaning they are evaluated at compile time, whereas CommonJS modules are dynamic and can be evaluated at runtime.
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13. What is the purpose of the Express.js Router
Answer: The Express.js Router is used to define route handlers for different HTTP methods and URL paths. It allows for modularization of routes and helps in structuring large applications.
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14. How does Node.js support non-blocking I/O operations
Answer: Node.js supports non-blocking I/O operations using its event-driven architecture. Instead of waiting for I/O operations to complete, it registers a callback and moves on to execute other code. The callback is invoked once the operation is complete.
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15. What is the significance of the __dirname in Node.js
Answer: __dirname is a global variable in Node.js that holds the directory path of the current module. It is useful for constructing paths to files and directories relative to the current script.
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16. How can you improve the performance of a Node.js application
Answer: Performance of a Node.js application can be improved by using clustering, avoiding synchronous code, caching data, using streams for large data processing, and optimizing database queries.
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17. What are the main differences between Node.js and traditional web server models
Answer: Node.js uses an event-driven, non-blocking I/O model which is different from traditional web servers that create a new thread for each request. This makes Node.js more efficient in handling concurrent connections.
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18. What is a memory leak in Node.js and how can you prevent it
Answer: A memory leak in Node.js occurs when allocated memory is not properly released, leading to increased memory usage over time. It can be prevented by using tools like heap profiling, ensuring proper closure of resources, and monitoring memory usage.
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19. How does the async/await syntax work in Node.js
Answer: The async/await syntax in Node.js is syntactic sugar over Promises. It allows writing asynchronous code that looks synchronous. Functions declared with async return a Promise, and await pauses the execution until the Promise is resolved.
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20. Explain the concept of callback hell and how to avoid it
Answer: Callback hell occurs when multiple asynchronous operations are nested within each other, leading to complex and hard-to-read code. It can be avoided by using Promises, async/await, or modularizing code into smaller functions.
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21. What is the difference between process.env and dotenv in Node.js
Answer: process.env is a global object in Node.js that holds environment variables, while dotenv is a module that loads environment variables from a .env file into process.env, making it easier to manage environment-specific variables.
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22. How does Node.js handle file system operations
Answer: Node.js provides the fs module to handle file system operations. It supports both synchronous and asynchronous methods for reading, writing, and manipulating files and directories.
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23. What is the purpose of the package-lock.json file
Answer: The package-lock.json file ensures that the exact versions of dependencies are installed in a Node.js project. It locks the versions of installed packages and their dependencies, preventing any unwanted changes in subsequent installations.
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24. How do you secure a Node.js application
Answer: Securing a Node.js application involves using HTTPS, validating and sanitizing user inputs, using environment variables for sensitive data, regularly updating dependencies, and implementing security headers.
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25. Explain the purpose of the crypto module in Node.js
Answer: The crypto module in Node.js provides cryptographic functionality, including encryption, decryption, hashing, and digital signatures. It is used to secure data in various parts of an application.
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