Top Node.js Interview Questions: A Comprehensive Guide for Beginners and Experienced Developers (2023) - IQCode

Why JavaScript and Node.js are Great for Building Scalable Products

In 2007, Jeff Atwood said, "Any application that can be written in JavaScript, will eventually be written in JavaScript," and today we see the truth in that statement. JavaScript libraries exist for almost every technical keyword, making it a great language to learn. However, to solve practical problems, developers need more skills. One such skill is building scalable products.

After jQuery animation development moved to single-page applications for better control of UI/UX, frontend frameworks such as AngularJS and Angular were introduced. JavaScript was then made available to port into any modern machine as a standalone application, Node.js became widely accepted as a backend framework, and for 2 consecutive years (2019-2020), it topped the StackOverflow survey.

Beginner Node.js Interview Questions:

1. What is a first-class function in JavaScript?

Introduction to Node.js

Node.js is a powerful open-source server-side JavaScript runtime environment built using Chrome's V8 JavaScript engine.

It allows developers to use JavaScript for both backend and frontend development, making it a popular choice for building web applications.

Node.js operates on a single-threaded event-driven architecture which makes it highly scalable and efficient. Its event loop enables it to handle a large number of clients with a minimal amount of resources.

Overall, Node.js is a versatile and powerful tool for web/mobile application development and has a growing community of developers contributing to its ecosystem.

Managing Packages in a Node.js Project

To manage packages in a Node.js project, we can use npm (Node Package Manager) - a powerful tool that makes package installation and management easy.

Installing Packages: To install new packages, we can use the command:

npm install package_name

Listing Installed Packages: To list all the packages installed in the project, we can use the command:

npm list

Updating Packages: To update the packages to their latest versions, we can use the command:

npm update

Uninstalling Packages: To uninstall a package, we can use the command:

npm uninstall package_name

Dependency Management: To manage dependencies for our project, we can create a package.json file in the root directory of the project and add all the necessary dependencies and their versions. We can install all the dependencies listed in the package.json file using the command:

npm install

By managing packages and their dependencies effectively, we can ensure that our Node.js project runs smoothly and without errors.

Advantages of Node.js over other popular frameworks:

Node.js is better than other popular frameworks in several ways:

- Node.js has a non-blocking and event-driven architecture, making it highly scalable and efficient in handling a large number of I/O operations.
- It uses the same language (JavaScript) for both client-side and server-side programming, promoting code reusability and faster development.
- Node.js has a rich package ecosystem (npm) with over 1 million packages, making it easier for developers to add functionalities to their applications.
- It has a built-in support for real-time applications, such as chat applications and multiplayer games, through its WebSocket protocol.
- Node.js supports cross-platform development, allowing developers to build applications that can run on multiple operating systems.

Overall, Node.js is a powerful and flexible framework with a growing community of developers. Its advantages make it a popular choice for developing complex web applications.

Steps to Explain How "Control Flow" Controls Function Calls

In programming, "control flow" refers to the order in which the instructions of a program are executed. When it comes to function calls, control flow determines the order in which the functions are called and the arguments that are passed along.

Here are the steps to explain how control flow controls function calls:

1. The program begins execution in the "main" function, which contains the initial code to be executed. 2. When a function is called, control is passed to that function and the code within it is executed. 3. If the function being called contains another function call within its code, control is passed to that function and the same process repeats. 4. Once all functions have been executed, control is passed back to the "main" function and execution continues from where it left off. 5. During this process, the values of variables and other data are stored temporarily in memory until they are no longer needed.

It's important to note that the order of function calls and the passing of arguments can greatly impact the outcome of the program. Therefore, proper understanding and manipulation of control flow is key to creating effective and efficient code.

Common Timing Features in Node.js

Node.js provides several timing features that are commonly used, including:


: This function executes a specified function after a set amount of time has elapsed.


: This function executes a specified function repeatedly after a set interval of time.


: This function executes a specified function immediately after the current stack has executed.


: This function schedules a function to be executed immediately after the current operation completes, regardless of its I/O operations.

These timing functions are useful for scheduling tasks, delaying execution, and optimizing performance.

Advantages of using Promises over Callbacks

Using Promises in JavaScript has several advantages over traditional Callbacks:

  1. Promises provide a cleaner and more readable syntax for handling asynchronous operations.
  2. Promises allow for better error handling and propagation, making it easier to debug code and handle errors in a more orderly fashion.
  3. Promises allow for easy chaining of multiple asynchronous operations, leading to cleaner and more concise code.
  4. Promises can help with avoiding callback hell, a phenomenon where nested callback functions make code difficult to read and maintain.

// Example of using Promises in JavaScript

// Creating a Promise
const myPromise = new Promise((resolve, reject) => {
  // Asynchronous operation 
  setTimeout(() => {
    // If success, resolve the Promise with a result
    resolve('Promise resolved successfully!');
    // Else, reject the Promise with an error
    // reject('Promise rejected due to an error.');
  }, 2000);

// Consuming the Promise using 'then' and 'catch' methods
myPromise.then((result) => {
}).catch((error) => {

Understanding Forking in Node.js

In Node.js, forking is the process of spawning child processes from the main or parent process. These child processes are often used to perform computationally intensive tasks or to perform long-running tasks in parallel with the main process.

Forking in Node.js is done using the `child_process` module, which provides a way to create and interact with child processes.

When a fork is created, a new instance of the V8 JavaScript engine is launched, which can execute the same Node.js code as the parent process. However, the child process is completely independent of the parent process, with its own memory space, event loop, and other resources.

To create a child process using forking in Node.js, you can use the `fork()` method of the `child_process` module. This method takes a path to a JavaScript file that will be executed in the child process, along with any command-line arguments that should be passed to the child process.

Once the child process is created, you can communicate with it using the `send()` and `on('message')` methods. These methods allow you to pass messages between the parent and child processes, which can be used to coordinate their activities.

Overall, forking is a powerful feature of Node.js that allows you to leverage the full power of modern hardware by executing intensive tasks in parallel. By using child processes, you can improve the performance and scalability of your Node.js applications.

Why is Node.js single-threaded?

Node.js is single-threaded because it is based on an event-driven architecture that uses an event loop to handle I/O operations asynchronously. This means that it can handle a large number of connections with a single thread, without the need for multithreading to handle blocking operations. While this can limit the performance of CPU-bound tasks, it makes Node.js a great choice for building scalable, high-performance web applications that rely heavily on I/O.

Creating a Simple Server in Node.js that Returns "Hello World"

To create a basic server in Node.js that returns "Hello World", follow these steps:

1. Open a text editor and create a new file. 2. Add the following code:

const http = require('http');

const hostname = '';
const port = 3000;

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

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);

3. Save the file with a .js extension (e.g. server.js). 4. Open your terminal and navigate to the directory where the file is saved. 5. Run `node server.js` to execute the file. 6. In your web browser, go to `http://localhost:3000/`. You should see the message "Hello World" displayed on the page.

This is a very basic example and can be expanded upon to create more complex servers.

How many types of API functions does Node.js have?

In Node.js, there are several types of API functions including:

- Asynchronous APIs

- Buffers

- File System APIs

- Global Object APIs


- Module APIs

- Timers

Understanding these types of API functions is crucial for developing efficient and effective Node.js applications.


Arguments for async.queue:

The async.queue() function takes two arguments:

  • worker: A function that performs the actual task for each item in the queue.
  • concurrency: An optional integer that determines how many tasks get executed at the same time.

H3 tag: Purpose of module.exports in Node.js

In Node.js, `module.exports` is used to export functionalities from a module to be used in another module or file. It allows us to organize our code into reusable modules and keep the global namespace clean. By setting `module.exports` equal to a function or object, we can make that function or object available to be imported by other modules using the `require` function. Without `module.exports`, it would be difficult to share code between different files in Node.js.

Tools for Ensuring Consistent Code Style

Consistency in code style is crucial for the readability and maintainability of the codebase. Here are some tools that can be used to ensure consistent code style:

  1. ESLint:

    A powerful and widely used linter for JavaScript that helps detect and fix errors and enforce consistent code style rules.

  2. Prettier:

    A code formatter that automatically formats code to a consistent style, reducing the need for manual formatting.

  3. EditorConfig:

    A simple syntax configuration file that can be used to define and maintain consistent coding styles across different editors and IDEs.

  4. Stylelint:

    A linter for CSS that helps detect errors and enforce consistent coding styles.

By using these tools, developers can ensure that their code is consistent, maintainable, and adheres to best practices.

Intermediate Node.js Interview Questions

Callback Hell

refers to the situation when there are many nested callbacks within each other. It becomes difficult to read and maintain code when callbacks are used in this way. This situation arises when multiple asynchronous operations are required to take place in a specific order, and each operation is dependent on the completion of the previous operation. This leads to code that is difficult to debug and results in errors that are hard to trace. To avoid callback hell, various techniques can be used, such as promises or async/await functions.

Understanding the Event Loop in Node.js

The event loop is a critical part of the Node.js architecture, responsible for handling asynchronous operations and ensuring smooth and efficient execution of code. It works by continuously monitoring a queue of pending events and executing them in a non-blocking way. This allows Node.js to handle high volumes of I/O operations without getting bogged down or becoming unresponsive. In essence, the event loop acts as the backbone of the Node.js runtime environment, enabling it to deliver fast, scalable, and highly efficient performance for a wide range of web applications and services.

How does Node.js Handle Concurrency Despite Being Single-Threaded?

Node.js is built on the V8 engine, which is a JavaScript runtime that processes JS code into machine code. Node.js leverages a non-blocking I/O model, which means that it can handle multiple requests simultaneously without getting blocked or waiting for any I/O requests to complete. This I/O model is implemented using an event loop that listens for events and triggers callbacks once an event is detected. Therefore, Node.js can handle concurrency by delegating tasks to its event loop, which allows it to process multiple requests concurrently on a single thread without getting blocked or slowing down.

Differentiating between process.nextTick() and setImmediate()




are both functions in Node.js that allow the developer to execute a callback function asynchronously, but the differences between the two are important to note.


adds the given callback function to the top of the Event Queue. This means that the function will be executed before any other I/O events, timers, or queued events. This can lead to some edge cases where a process may become blocked, as the function will continue to execute until the queue is empty.


, on the other hand, adds the given callback function to the check phase of the Event Queue. This means that the function will be executed after any I/O events, but before any timers.

In general,


is recommended for most asynchronous tasks, but


can be useful in certain edge cases where a function needs to be executed asynchronously before any other events in the Event Queue.

// Example usage:
process.nextTick(() => {
  console.log('This will be executed before any other queued events');
setImmediate(() => {
  console.log('This will be executed after I/O events, but before any timers.');

How Node.js Addresses Blocking I/O Operations

Node.js utilizes an event-driven, non-blocking I/O model, which helps address the problem of blocking I/O operations. Instead of waiting for operations to complete before moving on to the next one, Node.js uses callbacks to allow the program to continue running while the I/O operation is being processed.

This approach allows Node.js to handle large numbers of simultaneous requests with high efficiency and scalability. Additionally, Node.js incorporates features such as streams, which enable data to be processed in small chunks as it's being transferred, rather than waiting for the entire block of data to be received before processing begins.

Overall, Node.js's approach to handling I/O operations helps improve the performance and responsiveness of web applications, making it a popular choice for building real-time, high-traffic applications.

Using Async Await in Node.js

Async Await is a feature in Node.js that allows developers to write asynchronous code using synchronous syntax. It makes code easier to read and write by eliminating the need for callbacks.

Here is an example of how to use Async Await in Node.js:

async function getData() {
  try {
    const response = await fetch('');
    const data = await response.json();
  } catch (error) {

In this example, we define an async function called getData which uses the await keyword to wait for the response from the Github API. If the request is successful, the function will log the data to the console. If there is an error, it will log the error.

Overall, using Async Await can make Node.js code easier to read and write, especially for asynchronous tasks.

Understanding Node.js Streams

Node.js Streams allow the processing of large amounts of data in smaller, more manageable chunks. They are a powerful feature that allow data to be read from or written to a source synchronously. Streams are used for reading and writing file system operations, as well as HTTP requests and responses.

Streams can be of four types: Readable, Writable, Transform, and Duplex. Each type of stream serves a unique purpose. A Readable stream provides data to a program, a Writable stream receives data, a Transform stream can modify the data, and a Duplex stream may both accept data and transform it.

Streams in Node.js can be used with either a push or pull approach. In a push approach, data is sent to a consumer automatically, while in a pull approach, the consumer requests data. Node.js uses the push approach, which means that data is pushed to a receiver without that receiver explicitly requesting it.

Overall, Node.js Streams provide high performance, efficiency, and flexibility when handling large amounts of data. They are an essential tool for developers building scalable applications.

Understanding Node.js Buffers

Buffers in Node.js are used to represent raw binary data. It is used to store and manipulate binary data before being sent to a network or file. Buffers are useful when normal strings cannot handle binary data. They are instances of the Buffer class in Node.js, which is a global object.

Creating a buffer in Node.js can be done in several ways. You can create a buffer with a specific length, create a buffer from an array, or create a buffer from a string. Once a buffer is created, you can read from or write to it using various methods provided by the Buffer class.

Buffers are commonly used in scenarios where data needs to be manipulated or stored in a binary format. For example, when working with image files, audio files, or network protocols, buffers are used to store and process binary data efficiently.

Overall, understanding how to work with buffers in Node.js is essential for anyone who wants to work with binary data in their applications.

Middleware in Web Development

Middleware is software that acts as a bridge or intermediary between an application and its server infrastructure. It can be used to handle tasks such as authentication, logging, routing, and error handling, among others. In web development, middleware sits between the front-end application and the backend server. It helps in abstracting the complexity of building scalable, reliable, and high-performance web applications.

Explaining the Reactor Pattern in Node.js

The Reactor pattern in Node.js is a design pattern for handling I/O operations in a non-blocking and scalable way. It relies on a central event loop that listens for I/O events and dispatches them to the appropriate handlers.

When an application is using the Reactor pattern, it registers its I/O operations with the event loop. The event loop then listens for I/O events, such as data being received on a socket, and dispatches them to the appropriate registered handlers. This allows the application to perform I/O operations in a non-blocking way, allowing for more concurrency and scalability.

One of the key advantages of using the Reactor pattern in Node.js is its performance. Since it relies on non-blocking I/O, it can handle a large number of concurrent connections without becoming overwhelmed. Additionally, it is well-suited for applications that require real-time communication, such as chat applications or online gaming.

Overall, the Reactor pattern is an important pattern for Node.js developers to understand, as it provides a powerful tool for building scalable and performant applications.

Benefits of Separating Express App and Server

Separating Express app and server can provide numerous benefits such as modularizing your code, improving scalability and reducing code coupling. By separating your app and server, you can easily change or replace one without affecting the other. Additionally, it allows better organization of the codebase and enables developers to work on specific areas without interfering with other parts. It also simplifies your code and enhances its reusability, thus making it more maintainable. Overall, separating your Express app and server can lead to more efficient and robust applications.

Why does Google use the V8 engine for Node.js?

Google chose to use the V8 engine in Node.js because of its speed and efficiency in compiling JavaScript code. V8 is an open-source JavaScript engine created by Google that is known for its fast performance. Since Node.js is a JavaScript runtime built on top of the V8 engine, it benefits from V8's speed and performance while executing code. This allows Node.js to handle high levels of traffic and process data quickly and efficiently. Additionally, V8's garbage collector helps to manage memory in Node.js, making it a popular choice for server-side applications.

Exit Codes in Node.js

In Node.js, exit codes are used to signify the result of a process or operation. The following are the exit codes and their meanings in Node.js:

- 0: Success. The program executed successfully without any errors. - 1: General error. This could mean that there was an unhandled exception, a process was terminated abnormally, or some other error occurred. - 2: Misuse of shell built-ins. This indicates that a shell command was used incorrectly. - 3: Invalid argument to a command. This means that the arguments passed to a command were invalid. - 4: Something tried to use the operating system's fork system call but it failed, either because the system lacked the necessary resources or because the system did not permit it. - 5: Internal state of Node.js was found to be invalid. - 6: An unknown error occurred.

These exit codes can be used by developers to determine the outcome of their Node.js processes and take appropriate actions based on the result.

Concept of Stub in Node.js

In Node.js, a stub is a small piece of code that mimics the behavior of a more complex component in the application. It is used in unit testing to isolate a specific piece of code and fake the results of function calls or API requests.

A stub can be created using various libraries such as Sinon or Jest. It allows developers to test the code logic without having to actually make a request to an external service or API. This makes testing faster and more efficient.

For example, let's say we have a function that sends an email using a third-party email service. We can create a stub for that email service that always returns a success result, regardless of whether the email was actually sent or not. This way, we can test our email sending function without actually having to rely on the third-party service.

To create a stub, we can use the following syntax in Sinon:

const sinon = require('sinon');

// Create a stub function to mimic an API request
const apiStub = sinon.stub();
apiStub.withArgs('user123').returns({ name: 'John Doe', email: '[email protected]' });

In the above example, we create a stub function called `apiStub` that returns a specific object when called with the argument `'user123'`. We can then use this stub in our unit tests to simulate the behavior of an external API.

Overall, using stubs in Node.js testing is a powerful technique for isolating specific pieces of code and making testing faster and more efficient.

Node.js Interview Question: Event Emitter

In Node.js, an `EventEmitter` is a class that is used to create event-driven architectures. It allows objects to emit named events and bind callbacks to them. When an event is emitted, all of the bound callbacks are called synchronously, in the order that they were added.

Here is an example of how to use an `EventEmitter` in Node.js:

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();

myEmitter.on('myEvent', () => {
  console.log('Event has occurred');


In this example, we first require the `events` module, which is where the `EventEmitter` class is defined. We then create a new class `MyEmitter` that extends `EventEmitter`, and instantiate a new `MyEmitter` object.

We then bind a callback function to the `myEvent` event using the `on()` method of the `myEmitter` object. This callback function simply logs a message to the console. Finally, we emit the `myEvent` event using the `emit()` method of the `myEmitter` object, which triggers the bound callback function.

This is a basic example of how to use the `EventEmitter` class in Node.js, but it can be used for much more complex event-driven architectures.

Enhancing Node.js Performance through Clustering

Node.js is a popular choice for building scalable network applications, but its single-threaded nature can limit performance in some cases. Clustering is a technique that allows Node.js to fully utilize multi-core systems, thus improving performance.

//Simple example of clustering in Node.js:

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${} died`);
} else {
  // Worker processes have a http server.
  http.createServer((req, res) => {
    res.end('hello world\n');

  console.log(`Worker ${} started`);

In the above example, the master process forks worker processes and distributes incoming connections among them. Each worker has its own event loop and listens on the same port. This allows Node.js to handle more concurrent requests and improve the overall performance of the application.

Thread Pool in Node.js and its Library

In Node.js, a thread pool is a collection of worker threads that execute tasks in the background to improve performance. It manages a queue of pending tasks and assigns them to idle worker threads available in the pool for processing. By default, the size of the thread pool is determined by the number of available CPU cores on the machine.

The `worker_threads` module in the Node.js standard library handles the thread pool in Node.js. This module provides a simple yet powerful way to create and manage worker threads for running intensive CPU-bound operations in parallel. It allows you to spawn new threads, communicate with them, and share data between them.

When you need to perform computationally expensive operations in Node.js, you can employ worker threads to run them asynchronously and improve the overall performance of your application.

What is WASI and why is it being introduced?

WASI stands for WebAssembly System Interface. It is a new standard that provides a modular approach to running code outside of a web browser. WASI allows developers to create applications that can run on any platform, regardless of the underlying hardware or operating system. This is important because it eliminates the need for developers to write platform-specific code, which can be time-consuming and error-prone.

WASI is being introduced because it makes it easier to write and run applications across multiple platforms. It also provides a more secure environment for applications to run in, as it isolates them from the underlying operating system. Additionally, WASI allows applications to run in a sandboxed environment, which provides an extra layer of security. Overall, WASI is an important standard that will make it easier for developers to write cross-platform applications while making them more secure and reliable.

Differences Between Worker Threads and Clusters

Worker threads and clusters are two different ways to scale Node.js applications. Worker threads provide a way to execute JavaScript code in parallel, while clusters allow multiple instances of a Node.js process to run on multiple CPU cores.

The main differences between worker threads and clusters are:

  • Worker threads are lightweight and designed for in-process concurrency, while clusters are heavyweight and designed for inter-process concurrency.
  • Worker threads can share memory and state with the main thread, while clusters cannot.
  • Worker threads are more efficient for CPU-bound tasks, while clusters are more efficient for I/O-bound tasks.

In short, worker threads are suitable for scenarios where a single Node.js process needs to perform multiple tasks simultaneously, while clusters are suited for scenarios where multiple Node.js processes need to share the load of handling incoming requests.

Measuring the Duration of Asynchronous Operations:

When working with asynchronous operations in JavaScript, it can be useful to measure the duration of these operations in order to optimize performance. Here is an example of how to measure the duration of an asynchronous operation:

async function fetchData() {
  const startTime =; // getTime() method also can be used
  const data = await fetch('');
  const duration = - startTime;
  console.log('The operation took ' + duration + ' milliseconds.');
  return data;

This function uses the


API to make an asynchronous HTTP request and then measures the duration of the request using the

method before and after the request. This duration is then logged to the console.

By measuring the duration of asynchronous operations, you can identify bottlenecks in your code and optimize performance.

Measuring Performance of Asynchronous Operations

Asynchronous operations can be measured for performance using various metrics such as completion time, throughput, latency, and error rate.

To measure completion time, start a timer before the operation and stop it after the operation completes. The difference between these two values provides the completion time.

Throughput refers to the number of operations completed per unit of time. This can be calculated by dividing the total number of operations completed by the total time taken to complete them.

Latency is the time it takes for a single operation to complete. This can be calculated by measuring the time between the start of an operation and its completion.

Error rate can be calculated by dividing the number of failed operations by the total number of operations attempted.

To ensure accurate measurements, it is important to perform multiple tests and average the results. Tools such as JMeter, LoadRunner, and Gatling can be used to automate and accurately measure performance of asynchronous operations.

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