Top 100+ Node.js Interview Questions (With Answers) 2025

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1. What is Node.js?

Node.js is an open-source, cross-platform runtime environment for executing JavaScript code outside of a browser. It uses the V8 JavaScript engine and is designed for building scalable network applications.

2. Why use Node.js?

Node.js is popular because of its non-blocking, event-driven architecture, allowing it to handle multiple requests simultaneously without blocking the execution. It’s fast, scalable, and uses JavaScript for both server-side and client-side programming. Node.js is ideal for I/O-bound tasks, real-time applications (like chat and gaming apps), and building APIs. It also has a large ecosystem with npm, offering reusable modules and libraries.

Real-world use case:

• Netflix: Uses Node.js for handling large numbers of simultaneous connections.

3. How does Node.js work?

Node.js uses a single-threaded event loop to handle multiple requests asynchronously. When an I/O task (e.g., reading a file) is requested, Node.js doesn’t block the execution and continues with other tasks. Once the task is complete, a callback is triggered to handle the result. This event-driven model makes Node.js highly efficient, especially for real-time applications.

4. What is the difference between Node.js and JavaScript?

JavaScript is a programming language used for client-side scripting in the browser, while Node.js is a runtime environment that allows executing JavaScript code server-side.

5. How is Node.js different from traditional web servers like Apache?

Node.js is event-driven and non-blocking, making it more suitable for I/O-heavy applications. Apache, on the other hand, is a traditional multi-threaded server that uses multiple threads for handling requests.

6. Why is Node.js single-threaded?

Node.js operates on a single-threaded event loop architecture, allowing it to handle many connections simultaneously without blocking the main thread, which enhances performance in I/O-bound operations.

7. What is a callback function in Node.js?

A callback function is a function passed as an argument to another function, which is executed after the completion of an asynchronous task. In Node.js, it is commonly used for handling operations like reading files or making HTTP requests.

Example:

const fs = require('fs');

                                            fs.readFile('file.txt', 'utf8', (err, data) => {
                                            if (err) throw err;
                                            console.log(data);  // Callback function executed when file is read
                                            });

8. What is REPL in Node.js?

REPL stands for Read-Eval-Print Loop. It is an interactive environment in Node.js that allows developers to write JavaScript code and immediately see the result. It is useful for testing small code snippets and debugging.

To start REPL in Node.js, simply type node in the terminal.

9. What is the difference between Node.js and Angular?

Node.js operates on the server-side, while Angular operates on the client-side.

10. What is the Event Loop in Node.js?

The Event Loop is responsible for executing asynchronous operations and non-blocking I/O operations in Node.js. It ensures that operations like file reading or HTTP requests don't block other tasks.

11. What does I/O (Input/Output) mean in Node.js?

I/O (Input/Output) refers to operations that involve reading or writing data from external sources, like files, databases, or networks. In Node.js, I/O operations (like reading a file or making an HTTP request) are typically asynchronous, allowing the program to handle multiple operations without blocking the execution of other tasks.

12. What is asynchronous programming in Node.js?

Asynchronous programming in Node.js allows operations to be executed without waiting for one operation to finish before starting another, which prevents the blocking of the main thread.

13. What is control flow in Node.js?

Control flow refers to the order in which statements and functions are executed in a program. In Node.js, control flow is asynchronous by default, meaning tasks like file I/O, database queries, and HTTP requests are executed without blocking the main thread. This non-blocking nature is managed through callbacks, promises, and async/await.

14. What are the main disadvantages of Node.js?

Some disadvantages of Node.js include:

• Single-threaded: While it's efficient for I/O operations, Node.js might struggle with CPU-heavy tasks due to its single-threaded nature.
• Callback Hell: Without proper handling, callbacks in asynchronous code can lead to deeply nested code, making it harder to maintain.
• Not suitable for CPU-intensive operations: Node.js is optimized for I/O-bound tasks but is less efficient for CPU-bound tasks that require heavy computation.

15. What are the core modules in Node.js?

Core modules like http, fs, path, and url are built-in libraries in Node.js that provide various functionalities. They don’t need to be installed separately.

16. What is the fs module in Node.js?

The fs (File System) module in Node.js provides an API for interacting with the file system, allowing you to read, write, delete, and manipulate files.

17.What is the require() function in Node.js?

The require() function is used to load and import modules in Node.js. It allows you to include built-in, external, or custom modules in your application.

18. What are the modules in Node.js?

Modules in Node.js are reusable pieces of code that encapsulate specific functionality. There are built-in modules (like http, fs, path) provided by Node.js, third-party modules (like express, mongoose), and custom modules created by developers. Modules are imported into applications using the require() function.

19. How to import a module in Node.js?

Modules in Node.js can be imported using the require() function. For example:

const http = require('http');

This imports the http core module, and you can now use its functions in your code.

20. What is npm in Node.js?

npm (Node Package Manager) is the default package manager for Node.js. It allows developers to install and manage libraries and dependencies required for their projects.

21. What is package.json in Node.js?

package.json is a file used by npm to manage project dependencies and metadata. It contains information about the project, such as its name, version, scripts, and the libraries it depends on. This file is essential for setting up and managing the Node.js project environment.

22. How do you install, update, and delete a dependency in Node.js?

Install a dependency: To install a dependency, run the following command in the terminal:

npm install <package-name>

• Example: npm install express.

Update a dependency: To update a dependency to the latest version, run:

npm update <package-name>

• npm update express.

Delete a dependency: To remove a dependency, run:

npm uninstall <package-name>

• Example: npm uninstall express.

23. What is a callback function in Node.js?

A callback function is a function passed as an argument to another function. It is commonly used to handle asynchronous operations in Node.js, such as reading files or making HTTP requests.

24. What is a promise in Node.js?

A promise is an object that represents the eventual completion or failure of an asynchronous operation. It allows developers to handle asynchronous operations more effectively and avoid callback hell.

25. What are Promises in Node.js?

Promises are used to handle asynchronous operations. A Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

26. Which database is more popularly used with Node.js?

• MongoDB is the most popular database with Node.js due to its JSON-like format (BSON), making it a natural fit for JavaScript-based applications.
• Other popular databases include MySQL and PostgreSQL, used for applications requiring structured data and complex queries.

27. What is the difference between Synchronous and Asynchronous functions?

28. What is the difference between var, let, and const in Node.js?

• var is function-scoped and can be redeclared.
• let is block-scoped and can be reassigned but not redeclared.
• const is block-scoped and cannot be reassigned or redeclared.

29. Write a simple Node.js HTTP server.

const http = require('http');

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World
');
});

server.listen(3000, '127.0.0.1', () => {
  console.log('Server running at http://127.0.0.1:3000/');
});

This code creates an HTTP server that listens on port 3000 and responds with "Hello World".

Writing a Node.js server is just the first step; ensuring it works across browsers, devices, and environments is the real challenge. Developers often face issues with cross-browser compatibility, inconsistent rendering, and manual testing overhead.

With LambdaTest, you can easily test your Node.js apps and APIs across 3000+ real browsers and 10000+ real devices. Use the LambdaTest cloud platform to:

• Run automated tests with frameworks like Mocha, Jest, or Cypress.
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• Verify UI rendering and performance on various browser versions, all without maintaining your own infrastructure.

30. What are the most commonly used libraries in Node.js?

Some of the most commonly used libraries in Node.js include:

• Express.js: A minimal and flexible web application framework for building RESTful APIs and web applications.
• Socket.io: A library for real-time communication between clients and servers.
• Mongoose: An ODM (Object Data Modeling) library for MongoDB that provides a schema-based solution to modeling your data.
• Lodash: A utility library for JavaScript that provides helpful functions for tasks like deep cloning, debouncing, and data manipulation.
• Jest/Mocha: Testing frameworks for unit and integration testing in Node.js.

31. What are the pros and cons of Node.js?

32. What is the difference between process.nextTick() and setImmediate() in Node.js?

• process.nextTick() schedules a callback to be invoked in the same phase of the event loop before any I/O operations.
• setImmediate() schedules a callback to be invoked in the next iteration of the event loop.

33. What is the role of EventEmitter in Node.js?

EventEmitter is a class in Node.js used for handling events. It allows objects to emit events and register listeners (callbacks) to respond to those events. It’s commonly used to manage asynchronous operations and custom events in Node.js applications.

Example:

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('event', () => {
  console.log('Event occurred!');
});

emitter.emit('event');

In this example, an EventEmitter object listens for the "event" and triggers the callback when the event is emitted.

34. What are the two types of API functions in Node.js?

Node.js provides two types of API functions:

1. Asynchronous API:

• These functions do not block the execution of other code. They allow tasks like reading files, querying databases, etc., to be performed asynchronously.
• Example: fs.readFile(), http.request()

2. Synchronous API:

• These functions block the execution of other code until the operation is complete. They are generally used for simpler, quick tasks.
• Example: fs.readFileSync(), http.createServer()

35. How can you work with the URL module in Node.js?

The URL module in Node.js provides utilities for URL resolution and parsing. You can use it to extract parts of a URL (like protocol, hostname, and pathname) or to format a URL from its components.

Example:

const url = require('url');

const myUrl = new URL('https://www.example.com:8000/pathname/?search=test#hash');

console.log(myUrl.hostname);  // "www.example.com"
console.log(myUrl.pathname);  // "/pathname/"
console.log(myUrl.search);    // "?search=test"

In this example, the URL module helps break down the given URL into its components.

36. What are asynchronous and non-blocking APIs in Node.js?

In Node.js, asynchronous and non-blocking APIs allow tasks like I/O operations to run without blocking the main thread, improving performance.

• Asynchronous APIs: These APIs execute in the background, allowing other tasks to run while waiting for completion. Once the task is done, a callback function is triggered.
• Non-blocking APIs: These APIs don't block the execution of other code. Even if an operation takes time (like reading a file), Node.js can continue executing other code in parallel.

Example:

const fs = require('fs');

fs.readFile('file.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

In this example, readFile() is asynchronous and non-blocking, allowing Node.js to continue executing other code while reading the file.

37. Explain the role of the exports object in Node.js.

The exports object is used to expose functionality from a module. It is a shorthand for the module.exports an object, allowing other files to access the functions or variables defined within.

38. What is middleware in Node.js?

Middleware in Node.js is a function that processes HTTP requests and responses. It is often used for tasks like logging, authentication, and error handling in web applications built with Express.js.

39. What is Express.js in Node.js?

Express.js is a web application framework for Node.js. It simplifies routing, middleware integration, and the creation of web servers and APIs.

40. What are Promises, and how are they different from callbacks in Node.js?

Promises are a more modern alternative to callbacks for handling asynchronous operations. They avoid the issues of callback hell by allowing chaining of operations and handling success or failure in a more structured way.

41. What are streams in Node.js?

Streams in Node.js allow the reading and writing of data in chunks, making it more memory efficient for handling large datasets like files, HTTP responses, or database queries.

42. What is a Buffer in Node.js?

A Buffer is a temporary storage area used to handle binary data. It allows Node.js to read, manipulate, and write binary data, especially when interacting with streams or file systems.

43. What are event-driven architectures in Node.js?

In an event-driven architecture, the flow of the program is determined by events. Node.js uses this model to handle I/O operations asynchronously, where events are triggered and handled by listeners.

44. What is the cluster module in Node.js?

The cluster module allows you to create child processes (workers) to run on multiple CPU cores, enabling Node.js to better handle multi-core systems and improve performance.

45. How does Node.js handle concurrency?

Node.js handles concurrency using its event loop and non-blocking I/O. Although it is single-threaded, it can handle many requests simultaneously by delegating blocking tasks to external services or systems.

46. What are the differences between the fork() and spawn() methods in Node.js?

In Node.js, both fork() and spawn() methods are used to create child processes, but they differ in their use cases and behavior. Here's a comparison:

Real-world Example:

• fork(): When you need to run a parallel Node.js script.
• spawn(): When executing shell commands, like listing files or running a Python script.

47. What is the V8 engine, and how does Node.js use it?

The V8 engine is an open-source JavaScript engine developed by Google, which is used in Chrome and other web browsers for executing JavaScript code. It compiles JavaScript directly into machine code using Just-In-Time (JIT) compilation, which improves performance significantly.

In Node.js, the V8 engine is used to execute JavaScript code on the server-side. Here's how Node.js utilizes V8:

• JavaScript Execution: Node.js uses the V8 engine to interpret and run JavaScript code efficiently. This enables Node.js to handle asynchronous I/O and event-driven programming effectively, making it suitable for building scalable network applications.
• JIT Compilation: The V8 engine compiles JavaScript into native machine code at runtime, optimizing performance for each specific operation. This helps Node.js achieve faster execution speeds compared to traditional interpreted JavaScript engines.
• Garbage Collection: V8 also manages memory and garbage collection automatically, which helps in managing memory usage efficiently during runtime.

Real-world Use Case: Node.js benefits from V8’s fast execution engine, which allows it to process multiple I/O requests concurrently, making it ideal for applications like real-time chat servers, live-streaming apps, and APIs.

48. What is the purpose of the Buffer class in Node.js?

The Buffer class in Node.js is used to handle binary data directly. It allows the manipulation of raw binary data without converting it into strings. Buffers are particularly useful for dealing with I/O operations (like reading files or receiving data from network protocols) in Node.js, where data is not always in string format.

Key Points:

• No conversion: Buffers store binary data exactly as it is, without needing encoding.
• Efficient memory usage: Buffers provide an efficient way to manage binary data in memory.
• Fixed size: Once created, a Buffer has a fixed size.

Example:

const buf = Buffer.from('Hello, Node.js!');
console.log(buf); // <Buffer 48 65 6c 6c 6f 2c 20 4e 6f 64 65 2e 6a 73 21>

In this example, Buffer.from() creates a buffer from a string, representing the string data in binary format.

49. What is piping in Node.js?

Piping in Node.js is a method used to pass data from one stream to another. It allows you to read data from a readable stream and write it to a writable stream efficiently.

Example:

const fs = require('fs');
fs.createReadStream('input.txt').pipe(fs.createWriteStream('output.txt'));

This example pipes data from input.txt to output.txt without blocking the event loop.

50. What are the commonly used flags for reading and writing files in Node.js?

In Node.js, file read/write operations can be customized using various flags. Below is a table summarizing some of the commonly used flags for file operations.

51. What is the Reactor Pattern in Node.js?

The Reactor Pattern in Node.js is an event-driven design pattern used to handle I/O-bound operations asynchronously. It allows Node.js to handle multiple events or requests without blocking the execution, by delegating tasks to the appropriate event handlers when I/O operations are complete.

• How it works: The Reactor pattern listens for events (like incoming HTTP requests or file read events) and reacts by invoking the corresponding callback or handler function. This non-blocking approach ensures that the event loop is not blocked, making Node.js highly efficient for I/O-heavy tasks.

52. What is the Test Pyramid in Node.js?

The Test Pyramid in Node.js is a concept for organizing different types of tests in an application, aiming to maintain a balanced test suite. It suggests that there should be:

• Unit Tests (Base): The foundation of the pyramid. These tests are fast, numerous, and focus on testing individual components or functions of your application in isolation.
• Integration Tests (Middle Layer): These tests check how multiple components work together. They are fewer in number than unit tests but more complex.
• End-to-End (E2E) Tests (Top): The smallest number of tests, these tests simulate real user scenarios and ensure the entire system works as expected.

The pyramid suggests having more unit tests than integration and end-to-end tests, to ensure fast feedback and maintainable code.

53. What are the Node.js exit codes and how do they work?

In Node.js, exit codes indicate the status of a process when it finishes execution. These codes help determine whether the program ran successfully or encountered an error.

• Exit Code 0: Indicates the program executed successfully without any errors.
• Exit Code 1: Typically indicates a general error, such as an uncaught exception or a failure in the script.
• Exit Code 2: Used when there is an incorrect command-line argument or a syntax error.
• Exit Code 128: Indicates the process was terminated by a signal (e.g., SIGKILL).
• Other Exit Codes: Custom exit codes can be set within your application using process.exit(code) to reflect specific errors or conditions.

Example:

54. What are the different types of HTTP requests in Node.js?

55. How would you connect a MongoDB database to Node.js?

To connect a MongoDB database to a Node.js application, you can use the Mongoose library, which provides an elegant way to interact with MongoDB. Here's how to do it:

1. Install the MongoDB driver and Mongoose: First, install the required dependencies using npm:

2. Set up the connection in Node.js: Use Mongoose to connect to the MongoDB server. Here’s how you can set it up:

const mongoose = require('mongoose');

// MongoDB URI (replace with your own connection string)
const uri = 'mongodb://localhost:27017/mydatabase';

mongoose.connect(uri, { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => {
    console.log('MongoDB connected successfully');
  })
  .catch(err => {
    console.error('Error connecting to MongoDB:', err);
  });

3. Create a Schema and Model: After establishing the connection, you can define a schema and create a model to interact with the database.

const Schema = mongoose.Schema;

const userSchema = new Schema({
  name: String,
  email: String
});

const User = mongoose.model('User', userSchema);

4. Perform database operations: You can now use the User model to perform database operations like adding or retrieving users.

// Add a new user
const newUser = new User({ name: 'John Doe', email: 'john@example.com' });
newUser.save()
  .then(user => console.log('User saved:', user))
  .catch(err => console.error('Error saving user:', err));

56. What is the purpose of process.env in Node.js?

process.env is an object in Node.js that provides access to environment variables. It is used to manage configuration settings and sensitive data, like API keys and database credentials, outside the codebase.

Example:

console.log(process.env.NODE_ENV);  // Outputs environment variable value

57. What does WASI stand for, and what is its purpose?

WASI (WebAssembly System Interface) is a system-level interface that allows WebAssembly (Wasm) modules to interact with the underlying system, providing access to system resources like files, networking, and other I/O. It is being introduced to enable WebAssembly to run outside the browser in a secure and portable way, bringing its performance benefits to server-side applications and other environments.

58. What does it mean for a function to be first-class in JavaScript?

In JavaScript, a first-class function means that functions are treated as first-class citizens. This means functions can:

1. Be assigned to variables

2. Be passed as arguments to other functions

3. Be returned from other functions

4. Be stored in data structures like arrays or objects

This flexibility allows functions to be used dynamically and makes JavaScript a powerful language for functional programming.

Example:

// Assigning a function to a variable
const greet = function() { return "Hello!"; };

// Passing a function as an argument
function callFunction(fn) {
  return fn();
}

console.log(callFunction(greet));  // Outputs "Hello!"

59. How does Node.js differ from Ajax?

Node.js and Ajax are both widely used in web development, but they serve different purposes and operate in different contexts. Here's a comparison:

60. Does Node.js run on Windows?

Yes, Node.js runs on Windows. It is cross-platform and can be installed and used on various operating systems, including Windows, macOS, and Linux.

• Installation: Node.js provides an installer for Windows, making it easy to set up.
• Compatibility: Node.js supports Windows environments and works well with tools like PowerShell, Command Prompt, or Windows Subsystem for Linux (WSL) for running Node.js applications.

61. Explain memory management in Node.js.

Node.js uses garbage collection (GC) to manage memory. The V8 engine automatically frees up memory by reclaiming unused memory objects, though developers must be cautious of memory leaks.

62. What are Worker Threads in Node.js?

Worker Threads allow Node.js to handle CPU-bound tasks in parallel by spawning separate threads. This helps prevent blocking the event loop and improves the performance of multi-core systems.

63. Explain the concept of async/await in Node.js.

async/await is a syntactic sugar built on top of Promises. It allows developers to write asynchronous code in a synchronous style, improving readability and reducing callback hell.

64. What is JWT, and how do you implement it in Node.js?

JSON Web Tokens (JWTs) are used for securely transmitting information between parties. In Node.js, JWT can be implemented using libraries like jsonwebtoken for authentication and authorization in web applications.

65. What is the difference between require and import in Node.js?

• require() is a synchronous function used in CommonJS modules.
• import is part of ES6 modules, and it allows asynchronous loading and better modularity, making it more future-proof.

66. What does "non-blocking" mean in Node.js?

In Node.js, non-blocking refers to the ability of the system to handle I/O operations asynchronously without halting the execution of other code. This means that while Node.js is waiting for tasks like reading files, making network requests, or querying databases, it doesn’t block the program from running other code. As a result, Node.js can handle many requests concurrently, making it highly efficient, especially for I/O-heavy applications.

67. How can we use async/await in Node.js?

In Node.js, async/await makes it easier to work with asynchronous code.

• async: Marks a function as asynchronous, meaning it will always return a promise.
• await: Pauses the execution of the function until the promise is resolved.

Example:

const fs = require('fs').promises;

async function readFile() {
  try {
    const data = await fs.readFile('file.txt', 'utf8');
    console.log(data);
  } catch (err) {
    console.log(err);
  }
}

readFile();

Explanation: In this example, readFile() waits for fs.readFile() to finish before logging the file content.

68. What are some of the cluster methods in Node.js?

In Node.js, the cluster module helps utilize multiple CPU cores by creating child processes. Some key methods are:

1. cluster.fork(): Creates a new child process (worker).

2. cluster.on(): Used to listen for events like exit when a worker process terminates.

3. cluster.isMaster: Checks if the current process is the master process.

4. cluster.isWorker: Checks if the current process is a worker process.

5. cluster.worker: Provides access to the worker instance.

These methods help manage multiple worker processes in a Node.js application to improve performance.

69. How do we implement authentication and authorization in Node.js?

In Node.js, authentication and authorization can be implemented using various strategies, typically involving JWT (JSON Web Tokens) or OAuth.

1. Authentication: Verifies the identity of the user, often using methods like:

• JWT: The user logs in with credentials, and the server issues a token. The client sends this token with each request to authenticate.
• Passport.js: A popular middleware for handling various authentication strategies like local authentication or third-party logins (Google, Facebook, etc.).

2. Authorization: Determines what resources the authenticated user is allowed to access.

• Based on roles (e.g., admin, user), access control lists (ACLs), or permissions.
• Implemented by checking the user’s token (or session) against the roles defined in the database.

Example:

• Authentication: Using JWT to send a token after login.
• Authorization: Checking the token’s roles/permissions before allowing access to protected routes.

70. How does Node.js differ from server-side scripting languages like Python?

71. What is the Node.js Redis module?

The Node.js Redis module is a client library that allows Node.js applications to interact with a Redis database. It provides a simple API to perform operations such as setting/getting keys, caching data, and using pub/sub for real-time messaging. Redis is often used to improve application performance by storing frequently accessed data in memory. You can install it using npm install redis and use it to efficiently handle caching and messaging within your Node.js applications.

72. How to handle environment variables in Node.js?

In Node.js, environment variables are accessed via process.env. You can use the dotenv package to load variables from a .env file into process.env. This helps keep sensitive information like API keys or database credentials separate from your code.

Steps:

1. Install dotenv:

2. Create a .env file with key-value pairs.

3. Use require('dotenv').config() to load the variables.

This approach keeps your configuration secure and flexible.

73. Explain the DNS module in Node.js

The DNS module in Node.js allows you to interact with the Domain Name System (DNS). It provides methods for resolving domain names to IP addresses, looking up DNS records (like A, MX), and performing reverse lookups. Common methods include dns.lookup() for resolving hostnames and dns.resolve() for querying various DNS records.

74. Explain the use of the timers module in Node.js

The timers module in Node.js provides methods like setTimeout(), setInterval(), and setImmediate() to schedule code execution after a delay or at regular intervals.

• setTimeout(): Executes a function after a specified delay.
• setInterval(): Executes a function repeatedly at specified intervals.
• setImmediate(): Executes a function immediately after the current event loop cycle.

75. What is the difference between setImmediate() and setTimeout()?

76. What does an EventEmitter do in Node.js?

An EventEmitter in Node.js is a class that allows objects to emit events and listen for event handlers (callbacks). It enables event-driven programming by allowing different parts of an application to react to specific events, making it ideal for handling asynchronous operations.

• Emit Events: The emit() method triggers an event.
• Listen to Events: The on() method listens for an event and executes a callback when the event occurs.

Example:

• Common use: Handling HTTP requests, file operations, or custom events in applications.

77. What is the difference between global and local installation of npm packages?

78. What is the difference between readFile and createReadStream in Node.js?

79. What is the HTTP2 module in Node.js?

The HTTP2 module in Node.js enables support for the HTTP/2 protocol, which improves performance by allowing multiplexing, header compression, and stream prioritization. It helps create more efficient HTTP2 servers and clients, reducing latency and speeding up web applications.

80. What is a Zombie Process, and can it occur in Node.js?

A Zombie Process is a terminated process that still has an entry in the process table because its parent hasn't read its exit status. In Node.js, zombie processes can occur if child processes are not properly handled, but they are usually cleaned up when the parent process handles the termination correctly.

81. What is TLS/SSL in Node.js?

TLS (Transport Layer Security) and SSL (Secure Sockets Layer) are cryptographic protocols used to secure communication over the internet. In Node.js, you can implement TLS/SSL using the tls module to encrypt data between the server and client, ensuring privacy and data integrity during transmission. It is commonly used to create secure HTTPS servers.

82. How does Node.js differ from other backend technologies?

Unlike traditional multi-threaded backend systems (like Java or .NET), Node.js is single-threaded and uses an event-driven, non-blocking I/O model. This makes it more lightweight and efficient for I/O-bound applications.

83. How would you scale a Node.js application?

Scaling a Node.js application can be achieved by using load balancers, clustering, or a microservices architecture. Node.js’ ability to work asynchronously allows it to efficiently manage numerous simultaneous requests.

84. What are the best practices for error handling in Node.js?

Use try/catch with async/await, implement centralized error handling middleware in Express.js, and log errors effectively using libraries like Winston or Bunyan to monitor the application’s health.

85. Why does Google use the V8 engine in Node.js?

• High Performance: V8 compiles JavaScript directly to native machine code, improving execution speed.
• Just-In-Time (JIT) Compilation: V8 optimizes code during runtime, boosting performance.
• Efficiency: Ideal for I/O-bound tasks, making Node.js fast and scalable for handling multiple connections.
• Real-time Applications: V8’s speed makes it perfect for real-time applications like chat servers and live-streaming services.

86. Why are Java developers increasingly adopting Node.js?

Java developers are quickly adopting Node.js for several reasons:

1. Faster Execution: Node.js, with its non-blocking, event-driven architecture, offers better performance for I/O-heavy applications, making it faster than traditional Java frameworks for handling concurrent requests.

2. JavaScript Everywhere: Node.js allows developers to use JavaScript on both the client-side and server-side, streamlining the development process and reducing context switching.

3. Scalability: Node.js is designed for highly scalable applications, making it a good choice for real-time apps like chat applications and gaming platforms.

4. Large Ecosystem: The npm (Node Package Manager) offers a vast collection of libraries and modules, enabling rapid development and easier integration with other technologies.

5. Non-blocking I/O: Java developers accustomed to blocking I/O operations benefit from Node.js's asynchronous, non-blocking model, which ensures that tasks don’t block the main thread.

87. What are the common use cases of Node.js in real-world applications?

• Real-time applications: Ideal for chat applications, live data streaming, and gaming platforms due to its non-blocking nature.
• APIs & Microservices: Frequently used for building fast, scalable APIs and microservices.
• Example 1: Netflix uses Node.js for handling high concurrency and fast data processing.
• Example 2: LinkedIn switched its mobile backend to Node.js to improve performance and scalability.

88. Your Node.js app becomes unresponsive under high load. How would you diagnose and fix it?

Use profiling tools like clinic.js or node --inspect to detect blocking operations. Check for synchronous code, optimize database queries, and use clustering or worker threads for CPU-bound tasks. Introduce caching with Redis or similar to reduce load.

89. You accidentally exposed your .env file to GitHub. What are the next steps?

Immediately revoke all exposed credentials (API keys, DB passwords). Remove the .env file from git history using tools like bfg or git filter-branch, and add .env to .gitignore. Consider rotating keys and auditing access logs.

90. You need to build an API rate limiter in Express. How would you implement it?

Use the express-rate-limit middleware:

This limits each IP to 100 requests per 15 minutes, preventing abuse.

91. Users report slow responses when uploading files. How would you optimize file uploads?

Use multer for handling uploads efficiently. Stream files directly to cloud storage (e.g., AWS S3) to avoid local storage overhead. Validate file types and sizes early, and process large uploads in background workers.

92. Your application frequently crashes on unhandled promise rejections. How do you address this?

Handle all promises with .catch() or try/catch in async functions. Also, listen to process.on('unhandledRejection') for logging and graceful shutdown:

process.on('unhandledRejection', (reason, promise) => {  console.error('Unhandled Rejection:', reason);  process.exit(1);});

93. You need to run a background job every midnight. How do you schedule it?

Use node-cron or Agenda.js. Example with node-cron:

process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection:', reason);
  process.exit(1);
});

94. A user’s session is hijacked. How can you prevent this in a Node.js app?

Use secure cookies (httpOnly, secure, SameSite=Strict), implement token rotation, use CSRF protection, and enable session timeouts. For JWTs, avoid long expiration times and store them securely (never in localStorage).

95. You receive an out-of-memory error in production. What steps do you take?

Enable memory monitoring (--max-old-space-size), use heapdump to inspect memory leaks, and check for unclosed DB connections, retained objects in closures, or large in-memory arrays. Use tools like clinic heapprofiler.

96. Your API is slow due to repeated DB queries. How can you speed it up?

Introduce caching with Redis or memory-cache for frequently accessed data. Optimize DB indexes and use lazy loading or pagination. You can also use batching or debounce patterns for repeated user requests.

97. How would you structure a large Node.js project for scalability and maintainability?

Use a modular architecture:

• routes/ – route definitions
• controllers/ – logic per route
• services/ – business logic
• models/ – DB schema
• middlewares/ – custom logic

Use dependency injection and maintain .env-based config separation (dev/staging/prod).

98. A user uploads a malicious file. How do you protect the system?

Use multer with file type checks, limit file sizes, and scan uploaded files using tools like ClamAV. Store uploads in isolated directories and don’t allow direct execution or download without validation.

99. You're integrating a third-party API that often fails. How do you handle reliability?

Use retry logic with libraries like axios-retry, implement circuit breakers (e.g., opossum), and fallbacks or cached responses when the API is down. Log all failures and monitor response times.

100. How do you protect your API from brute-force attacks?

Implement rate limiting (express-rate-limit), CAPTCHA on login, account lockout on repeated failed attempts, IP blocking for suspicious activity, and log authentication failures for analysis.

101. You’re working with millions of records in MongoDB. How do you optimize performance in Node.js?

Use pagination (limit/skip or cursor-based), index frequently queried fields, avoid large $in queries, and use aggregation pipelines. For high-performance reads, use Redis cache where appropriate.

102. Your microservice needs to communicate with others. How do you ensure reliable messaging?

Use message brokers like RabbitMQ, Kafka, or Redis Pub/Sub. Wrap communication in retries and use durable queues for guaranteed delivery. Use message IDs to ensure idempotency.

Mastering Node.js is more than just understanding APIs or writing JavaScript on the backend, it's about architecting efficient, non-blocking systems, managing asynchronous flows, and solving real-world performance bottlenecks.

We hope this detailed guide helps you prepare thoroughly for your next Node.js interview. Keep practicing, stay curious, and don't just learn Node.js, build with it.