Node.js has become a highly popular server-side platform that is being adopted by more and more organizations. If you are planning to transition to a career in this field and have an upcoming job interview, it is advisable to prepare well and polish your interview skills. While there are some Node.js interview questions that are commonly asked across various interviews, it is also recommended that you focus on industry-specific questions to better prepare for the interview.

To assist you in your preparation, we have created a comprehensive list of frequently asked Node.js interview questions and provided tips on how to answer them effectively. This resource will not only help you to perform well in your interview but also enable you to gain a better understanding of the core concepts of Node.js.

Node.js Interview Questions and Answers For Freshers

The following section contains fundamental Node.js interview questions that are primarily geared towards individuals who are new to the field.

What is Node.js? Where can you use it?

Node.js is a JavaScript runtime built on Chrome’s V8 JavaScript engine. It allows developers to run JavaScript code on the server-side, outside of a web browser.

Node.js is commonly used for building scalable and high-performance network applications. It can be used in a variety of scenarios, such as web applications, real-time chat applications, streaming applications, and API servers, among others.

Why use Node.js?

• Node.js is built on a non-blocking, event-driven I/O model, allowing it to handle large amounts of data and requests without blocking the server’s resources.
• Node.js has a vast collection of open-source libraries and packages available through its package manager, npm, making it easy to add functionality to applications without writing code from scratch.
• Node.js enables the use of a single programming language, JavaScript, on both the client-side and server-side of an application, simplifying the development process and reducing the amount of code needed.
• Node.js has a large and active community of developers, providing a wealth of resources and support for those who use it.

How does Node.js work?

Node.js works by executing JavaScript code on the server-side, outside of a web browser. It uses the Google Chrome V8 JavaScript engine to compile and execute JavaScript code.

When a Node.js application is started, it initializes an event loop that listens for incoming requests. When a request is received, Node.js executes the relevant code and returns the response to the client. Node.js uses a non-blocking, event-driven I/O model to handle requests, which means that it can handle multiple requests simultaneously without blocking the server’s resources.

Node.js also has a built-in module system that allows developers to load and reuse code across their applications. This makes it easy to manage dependencies and keep code organized.

Overall, Node.js provides a powerful and efficient platform for building high-performance, scalable network applications.

 Why is Node.js Single-threaded?

Node.js is single-threaded because it uses a non-blocking, event-driven I/O model to handle requests. In this model, a single thread is responsible for handling all requests, and each incoming request is processed asynchronously. When a request is received, it is added to a queue, and the single thread executes the relevant code for each request in turn, without waiting for a response from each request before moving on to the next one.

This approach allows Node.js to handle a large number of requests simultaneously without blocking the server’s resources. Because Node.js does not need to create a new thread for each request, it can handle more requests with fewer resources than traditional multi-threaded servers.

While Node.js is single-threaded, it does use multiple threads internally to handle I/O operations. For example, when reading or writing data from a file or network socket, Node.js uses a pool of threads to perform the I/O operations asynchronously, without blocking the main thread. This approach further enhances Node.js’s performance and scalability.

If Node.js is single-threaded, then how does it handle concurrency?

Node.js uses an event loop to handle concurrency in a single-threaded environment. The event loop is a loop that continuously listens for events, such as incoming requests, and dispatches them to the appropriate handlers.

When a request is received, Node.js adds it to a queue and continues to listen for additional requests. Meanwhile, the request is processed asynchronously by a callback function that is registered with the event loop. This allows the main thread to continue listening for additional requests while the callback function is executed.

Because Node.js uses an event-driven model, it can handle multiple requests simultaneously without blocking the server’s resources. When a callback function is blocked by an I/O operation, such as reading data from a file or network socket, the event loop continues to process other requests until the I/O operation is complete and the callback function can resume execution.

By using this approach, Node.js can handle concurrency efficiently while remaining single-threaded. This makes it ideal for building highly scalable and performant network applications.

 Explain callback in Node.js.

In Node.js, a callback is a function that is passed as an argument to another function and is executed when the original function completes its task. This allows asynchronous code to be written in a non-blocking manner, as the callback function is executed once the requested operation has completed, rather than waiting for it to finish before moving on to the next task.

Here’s an example of using a callback in Node.js to read data from a file:

const fs = require('fs');

fs.readFile('/path/to/file', (err, data) => {
  if (err) throw err;
  console.log(data);
});

In this example, the fs.readFile() function is used to read data from a file. The function takes two arguments: the path to the file to be read, and a callback function that is executed when the file has been read.

The callback function is passed two arguments: err and data. If an error occurs while reading the file, the err argument will contain an error object, and the code inside the if block will be executed. Otherwise, the data argument will contain the contents of the file, and the code inside the callback function’s body will be executed.

By using a callback in this way, Node.js can continue to execute code while the file is being read, rather than waiting for the operation to complete before moving on to the next task. This allows Node.js to handle multiple requests simultaneously and is a key feature of its non-blocking, event-driven architecture.

What are the advantages of using promises instead of callbacks?

Promises offer a number of advantages over traditional callbacks in Node.js. Here are a few key advantages:

1. Promises make it easier to handle errors: With callbacks, error handling can become complicated, particularly when dealing with multiple asynchronous operations. Promises provide a more straightforward way to handle errors, using the .catch() method to catch any errors that occur during the promise chain.
2. Promises provide a cleaner code structure: Promises allow you to chain multiple asynchronous operations together in a more readable and concise way, using the .then() method to specify what should happen after the promise resolves.
3. Promises support more advanced operations: Promises allow you to perform more advanced operations, such as waiting for multiple promises to resolve using Promise.all(), or cancelling a promise chain using Promise.race(). These operations can be difficult to achieve using callbacks alone.

Overall, promises offer a more robust and reliable way to handle asynchronous operations in Node.js, with better error handling and more advanced features.

How would you define the term I/O? 

I/O stands for Input/Output, which refers to the way in which a computer system interacts with its external environment. In general, I/O refers to the process of transferring data between a computer system and external devices or networks, such as keyboards, printers, disks, and the internet.

In the context of Node.js, I/O typically refers to the way in which Node.js handles input and output operations, such as reading and writing files or sending and receiving data over the network. Node.js uses a non-blocking I/O model, which allows it to handle multiple I/O operations simultaneously without blocking the main thread of execution. This makes Node.js well-suited for building high-performance, scalable applications that rely heavily on I/O operations.

How is Node.js most frequently used?

Node.js is most frequently used for building server-side applications, particularly web applications. Some common use cases for Node.js include:

1. Building web servers and APIs: Node.js is often used to build fast, scalable web servers and APIs that can handle large volumes of traffic.
2. Real-time applications: Node.js is well-suited for building real-time applications, such as chat apps, gaming servers, and collaborative tools, that require fast, two-way communication between the client and server.
3. Data streaming: Node.js is frequently used to build data-intensive applications that involve real-time data streaming, such as IoT (Internet of Things) applications or data analytics platforms.
4. Command-line tools: Node.js is often used to build command-line tools and utilities, such as build tools or testing frameworks, that can be run directly from the command line.

Overall, Node.js is a versatile and flexible platform that can be used for a wide range of applications, but it is most frequently used for building server-side web applications.

Explain the difference between frontend and backend development?

In summary, frontend development focuses on the parts of a website or application that users see and interact with, using languages such as HTML, CSS, and JavaScript to build user interfaces and optimize website performance. Backend development, on the other hand, focuses on the server-side logic and data storage, using languages such as Java, Python, and Ruby to build databases, servers, and APIs that ensure data security, scalability, and server reliability. Both frontend and backend development are critical for building a complete and functional website or application.

What is NPM?

NPM stands for Node Package Manager, which is a command-line tool that comes bundled with Node.js. NPM is used for managing and sharing packages (libraries, frameworks, and other code modules) that are written in JavaScript and can be used with Node.js.

With NPM, developers can easily install and use packages in their own projects, as well as share their own packages with the wider development community. NPM also includes a registry, which is a public repository of open-source packages that can be easily installed and used in projects.

NPM simplifies the process of managing dependencies and makes it easy for developers to reuse code, which can save time and improve the quality of code. NPM also includes features for updating, removing, and publishing packages, as well as for managing package versions and dependencies.

What are the modules in Node.js?

In Node.js, modules are reusable blocks of code that can be easily imported and used in a project. Node.js includes a number of built-in modules that provide functionality for working with the file system, creating web servers, handling network connections, and more. Some of the built-in modules in Node.js are:

These are just a few examples of the many modules available in Node.js. Modules in Node.js are reusable blocks of code that can be easily imported and used in a project to save time and improve code quality. The Node.js community has created a large ecosystem of modules that can be easily installed and used via NPM, the Node Package Manager.

What is the purpose of the module .Exports?

In Node.js, the module.exports object is used to define the public interface of a module. It allows developers to expose certain functions, objects, or variables to other modules, while keeping other internal implementation details hidden. By assigning properties to module.exports, developers can control what is visible and accessible outside of the module.

Why is Node.js preferred over other backend technologies like Java and PHP?

There are several reasons why Node.js is preferred over other backend technologies like Java and PHP:

• Node.js is built on JavaScript, which is a popular and widely-used programming language, making it easier for developers to learn and use.
• Node.js is lightweight and scalable, making it suitable for building high-performance applications that can handle large amounts of traffic.
• Node.js has a non-blocking I/O model, which allows it to handle many requests at once without blocking other requests.
• Node.js has a large and active community of developers and a wide range of open-source libraries and tools available.

What is the difference between Angular and Node.js ?

Which database is more popularly used with Node.js?

MongoDB is one of the most popular databases used with Node.js due to its flexibility, scalability, and support for JSON documents.

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

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

• Express: A minimalist web framework for building web applications and APIs.
• Async: A library for handling asynchronous operations in JavaScript.
• Lodash: A utility library for working with arrays, objects, and functions.
• Request: A library for making HTTP requests from Node.js.
• Socket.IO: A library for real-time, bidirectional communication between clients and servers.

What are the pros and cons of Node.js?

What is the command used to import external libraries?

The command used to import external libraries in Node.js is require(). For example, to import the express library:

const express = require('express');

Node.js Interview Questions and Answers For Intermediate Level

What does event-driven programming mean?

Event-driven programming is a programming paradigm where the flow of the program is determined by events such as user input, messages from other programs, or system events. Instead of executing code in a linear fashion, the program waits for events to occur and responds to them.

What is an Event Loop in Node.js?

An Event Loop in Node.js is a mechanism that allows the runtime environment to execute non-blocking I/O operations. It works by continuously monitoring the event queue and executing callback functions when events are detected. This allows Node.js to handle a large number of requests efficiently.

Differentiate between process.nextTick() and setImmediate() ?

What is an EventEmitter in Node.js?

An EventEmitter in Node.js is a class that allows developers to create and emit custom events. This is useful for building applications where different components need to communicate with each other. For example, an EventEmitter could be used to create a custom event when a file is saved or when a user logs in.

Example code:

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

// Event listener
myEmitter.on('greet', (name) => {
  console.log(`Hello, ${name}!`);
});

// Emit event
myEmitter.emit('greet', 'John');

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

The two types of API functions in Node.js are blocking and non-blocking. Blocking functions execute synchronously and can cause the program to halt until the operation is complete. Non-blocking functions execute asynchronously and allow the program to continue executing while the operation is being performed.

What is the package.json file?

The package.json file is a configuration file that contains metadata about a Node.js project, including its name, version, dependencies, scripts, and more. It is used by npm to install and manage packages and can be used to define custom scripts for running tasks.

How would you use a URL module in Node.js using example?

The URL module in Node.js provides utilities for working with URLs. Here’s an example of how to use it:

const url = require('url');

const myUrl = new URL('https://example.com/path/?query=string');

console.log(myUrl.href); // "https://example.com/path/?query=string"
console.log(myUrl.host); // "example.com"
console.log(myUrl.pathname); // "/path/"
console.log(myUrl.searchParams.get('query')); // "string"

What is the Express.js package?

Express.js is a popular web application framework for Node.js. It provides a set of features for building web applications, including routing, middleware, and template engines.

How do you create a simple Express.js application?

To create a simple Express.js application, you can follow these steps:

1. Install Node.js and npm on your system.
2. Create a new directory for your application and navigate to it in your terminal.
3. Initialize a new npm package in your directory by running npm init and following the prompts.
4. Install the Express.js package by running npm install express.
5. Create a new file called index.js in your directory.
6. In index.js, require the Express.js module by adding the following code at the top of the file:

const express = require('express');
const app = express();

7. Define a route for your application by adding the following code to index.js:

app.get('/', (req, res) => {
  res.send('Hello, world!');
});

This code creates a route for the root URL of your application and sends a response with the text “Hello, world!”.

8. Start your application by adding the following code to index.js:

const PORT = process.env.PORT || 3000;

app.listen(PORT, () => {
  console.log(`Server listening on port ${PORT}.`);
});

This code starts the server and listens for requests on port 3000 (or the port specified by the PORT environment variable).

9. Run your application by running node index.js in your terminal.

Your Express.js application should now be running and accessible at http://localhost:3000/. When you visit that URL in your web browser, you should see the text “Hello, world!” displayed on the page.

What are streams in Node.js?

Streams in Node.js are objects that allow you to read or write data continuously, in chunks, rather than reading or writing the entire data at once. Streams can be used to process large amounts of data or to work with data that is being transmitted in real-time, such as network data or audio/video streams.

There are four main types of streams in Node.js:

1. Readable stream: These streams allow you to read data from a source, such as a file or network socket.
2. Writable stream: These streams allow you to write data to a destination, such as a file or network socket.
3. Duplex stream: These streams allow you to both read and write data.
4. Transform stream: These streams allow you to modify data as it is being read or written.

How do you install, update, and delete a dependency?

To install a dependency in a Node.js project, you can use the npm install command followed by the name of the package you want to install. For example, to install the express package, you would run the following command in your terminal:

npm install express

To update a dependency to the latest version, you can use the npm update command followed by the name of the package you want to update. For example, to update the express package to the latest version, you would run the following command:

npm update express

To delete a dependency from your project, you can use the npm uninstall command followed by the name of the package you want to uninstall. For example, to uninstall the express package, you would run the following command:

npm uninstall express

These commands will update or delete the package from your package.json file as well as the node_modules directory in your project.

How do you create a simple server in Node.js that returns Hello World?

To create a simple server in Node.js, you can use the built-in http module. Here’s an example:

const http = require('http');

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

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

In this example, we create an HTTP server using http.createServer() and pass a callback function that gets executed each time a request is made to the server. The callback function takes two arguments, req and res, which represent the incoming request and the outgoing response, respectively. We set the status code and content type of the response using res.writeHead(), and then send the response body using res.end(). Finally, we tell the server to listen on port 3000 using server.listen().

Explain asynchronous and non-blocking APIs in Node.js.

Node.js is designed to be non-blocking and asynchronous, which means that it can handle a large number of simultaneous connections without using threads. This is achieved through the use of event-driven programming and callbacks. When a request is made to a Node.js server, it is handled asynchronously, which means that the server does not block while waiting for a response. Instead, it continues processing other requests while waiting for the response to come back.

Asynchronous programming in Node.js is typically achieved through the use of callback functions. When a function needs to perform an asynchronous operation, it takes a callback function as an argument. The function then performs the operation and calls the callback function when it is finished. This allows other code to continue running while the operation is in progress.

How do we implement async in Node.js?

In Node.js, we can implement asynchronous operations using callback functions, promises, or async/await syntax. Here’s an example of each approach:

Callback function:

function doAsyncTask(callback) {
  setTimeout(() => {
    callback('Done!');
  }, 1000);
}

doAsyncTask((result) => {
  console.log(result); // Prints 'Done!' after 1 second
});

Promise:

function doAsyncTask() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('Done!');
    }, 1000);
  });
}

doAsyncTask().then((result) => {
  console.log(result); // Prints 'Done!' after 1 second
});

Async/await:

async function doAsyncTask() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('Done!');
    }, 1000);
  });
}

(async () => {
  const result = await doAsyncTask();
  console.log(result); // Prints 'Done!' after 1 second
})();

In each example, we define a function that performs an asynchronous operation (in this case, a simple timeout), and then call the function using a callback, a promise, or async/await syntax to handle the result when it is ready.

What is a callback function in Node.js?

A callback is a function that is executed after a specific task is completed, allowing other code to continue running in the meantime without being blocked.

What is the purpose of using callback functions in Node.js?

Callback functions are used in Node.js for handling asynchronous operations such as reading and writing files, making network requests, and accessing databases. These operations can take time to complete, and it is important to ensure that the program does not block while waiting for them to finish. Callback functions allow the program to continue executing while waiting for the asynchronous operation to complete.

How do callback functions work in Node.js?

In Node.js, a callback function is a function that is passed as an argument to another function and is called by that function when it has completed its task. The callback function is executed asynchronously, which means that it is executed after the completion of the function that it was passed to.

What is the syntax for using a callback function in Node.js?

The syntax for using a callback function in Node.js involves passing the callback function as an argument to another function. Here is an example using the fs module to read a file:

const fs = require('fs');

fs.readFile('/path/to/file', (err, data) => {
  if (err) throw err;
  console.log(data);
});

Node.js Interview Questions and Answers For Experienced Professionals

I’m ready to help you with any Advanced Node.js interview questions you may have. Just let me know how I can assist you.

What is REPL in Node.js?

REPL stands for Read-Eval-Print-Loop, and it is a built-in interactive shell for Node.js. It allows developers to execute JavaScript code snippets and view their output in real-time.

What is the control flow function?

In Node.js, control flow functions are used to manage the flow of asynchronous operations. They provide a way to execute multiple asynchronous tasks in a specific order, ensuring that each task is completed before moving on to the next one.

How does control flow manage the function calls?

Control flow manages function calls by using callbacks or promises. When a function is called, it is passed a callback or promise as an argument. Once the function completes its task, it calls the callback or resolves the promise, signaling to the control flow function that it is ready to move on to the next task.

What is the difference between fork() and spawn() methods in Node.js ?

1. Both fork() and spawn() methods are used to create child processes in Node.js, but there are some differences between them. Here’s a comparison table:

What is the buffer class in Node.js?

The buffer class in Node.js is used to handle binary data. It provides a way to store and manipulate binary data in memory, which can be useful for tasks such as file I/O, network communication, and cryptography.

What is piping in Node.js?

Piping in Node.js refers to the process of connecting the output of one stream to the input of another stream. It allows developers to create a pipeline of streams that can process data in a sequential manner, without the need to buffer all the data in memory at once. This can be useful for tasks such as file I/O and network communication.

What are some of the flags used in the read/write operations in files?

There are several flags used in file read/write operations in Node.js, including ‘r’ for reading, ‘w’ for writing, ‘a’ for appending, and ‘x’ for creating a new file. Additionally, there are ‘r+’ and ‘w+’ flags that allow for reading and writing simultaneously.

How do you open a file in Node.js?

To open a file in Node.js, you can use the ‘fs’ (file system) module and its ‘createReadStream’ or ‘createWriteStream’ functions. Alternatively, you can use the ‘readFile’ or ‘writeFile’ functions to read and write files, respectively.

What is callback hell?

Callback hell is a term used to describe the situation when multiple nested callbacks are used, leading to difficult-to-read and maintain code. This can occur when dealing with asynchronous operations that depend on the result of other asynchronous operations.

What is a reactor pattern in Node.js?

The reactor pattern is a design pattern used in Node.js to handle I/O operations in a non-blocking way. It involves a single event loop that manages I/O operations and callback functions, allowing for efficient handling of a large number of requests without blocking the event loop.

What is a test pyramid in Node.js?

The test pyramid is a testing strategy used in Node.js that involves a balanced distribution of unit tests, integration tests, and end-to-end tests. Unit tests are at the base of the pyramid and are the most numerous, while end-to-end tests are at the top and are the least numerous.

For Node.js, why does Google use the V8 engine?

Google uses the V8 engine in Node.js because it is a fast and efficient JavaScript engine that is open source and compatible with a variety of platforms.

Describe Node.js exit codes.

Node.js exit codes are integer values that indicate the reason for a process exiting. A code of 0 indicates success, while other codes indicate various types of errors or issues.

Explain the concept of middleware in Node.js.

Middleware is a function that sits between a request and a response in Node.js and can modify or handle the request/response as needed. Middleware can be used for a variety of purposes, such as authentication, logging, or error handling.

What are the different types of HTTP requests?

There are several types of HTTP requests, including GET, POST, PUT, DELETE, HEAD, OPTIONS, CONNECT, and TRACE. These requests are used to retrieve, add, modify, or delete resources on a web server.

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

To connect a MongoDB database to Node.js, we can make use of the MongoDB Node.js driver. Here are the steps to connect a MongoDB database to Node.js:

1. Install the MongoDB Node.js driver using the following command:

npm install mongodb --save

Create a MongoDB client object by calling the MongoClient constructor function and passing in the connection URL as a parameter. The connection URL should contain the hostname, port number, and the name of the database we want to connect to.

const MongoClient = require('mongodb').MongoClient;
const url = 'mongodb://localhost:27017/mydb';

MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  console.log("Database connected successfully");
  // Perform database operations here
  db.close();
});

3. Once the client object is created, we can use it to perform database operations such as inserting, updating, and querying documents.

Note that in the above code snippet, we are using the connect() method of the MongoClient object to establish a connection to the MongoDB database. We also have a callback function that gets called once the connection is established. This callback function takes two arguments – an error object and the db object, which we can use to perform database operations.

Also, note that we are calling the close() method on the db object to close the connection once we are done with the database operations.

What is the purpose of NODE_ENV?

NODE_ENV is an environment variable that is used to specify the current environment in Node.js. It is commonly used to differentiate between development, staging, and production environments, and can be used to modify the behavior of the application accordingly.

List the various Node.js timing features.

Node.js provides several timing features, including setTimeout, setInterval, setImmediate, and process.nextTick. These features allow for asynchronous execution of code and can be used for scheduling tasks or delaying execution.

What is WASI, and why is it being introduced?

WASI stands for WebAssembly System Interface. It is a new system interface that allows WebAssembly modules to run in a wide variety of environments beyond the web, including standalone desktop and server environments.The WASI class provides an implementation of the WASI system API, along with additional helper methods to facilitate interaction with applications built on top of the WASI system. Each instance of the WASI class represents a distinct sandboxed environment. To ensure security, each instance of the WASI class must be configured with its own set of command-line parameters, environment variables, and sandbox directory structure.

Conclusion

These Node.js interview questions can help you prepare for your upcoming interview by giving you an idea of the types of questions you may be asked. By reviewing these questions, you can better equip yourself to confidently answer them and increase your chances of success in your interview.

For a more comprehensive understanding of this popular web application development framework, consider enrolling in Simplilearn’s Post Graduate Program in Full Stack Web Development course. This program can provide you with even more advanced training to prepare you for any upcoming Node.js interviews.

Good luck with your job interview! If you have any questions or concerns about these interview questions, please feel free to leave a comment below, and our team will respond as soon as possible.