Node.js Interview Questions.

Node.js has quickly become one of the most popular and powerful tools for building scalable, high-performance web applications. Its ability to handle large volumes of data and process multiple requests simultaneously makes it an ideal choice for developing modern web applications.

The rise of Node.js as a dominant web technology is clear in the 2022 Stack Overflow Developer Survey which shows that 47.12% of all respondents use Node.js as their preferred web technology. The benefits of Node.js surpass those of its competitors, leading organizations to recognize the need to hire Node.js developers.

However, mastering Node.js can be a difficult task, and job interview questions and answers can be particularly challenging. To help you prepare, we've put together a comprehensive list of Node.js interview questions. This article will look at some of the most frequently asked questions and provide detailed answers to help you ace your next interview or identify qualifications to look for in a candidate.

So, let's start with the Node.js interview questions. In the next section, we will look at common Node js interview questions and provide some code samples where necessary. Ready to get started? Let's dive in and learn how to master Node.js interview questions!

Basic Node.js Interview Questions

This section covers basic Node.js interview questions and their answers.

1. What is the difference between installing local and global npm packages?

In Node.js, packages can be installed locally or globally. Here are the differences between the two:

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

It's important to note that Node.js is based on JavaScript and the two share similarities, but JavaScript and Node.js are different technologies. Here is a table that summarizes the main differences between the two:

3. Is Node.js free?

Node.js is free to use under the MIT license.

4. List the API functions available in Node.js.

The two types of API functions in Node.js are:

1. Asynchronous – Non-blocking functions
2. Synchronous – Blocking functions

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

Node.js uses a non-blocking, event-driven I/O approach to handle concurrency. This implies that it can manage multiple connections at the same time without launching additional threads. Node.js adds client requests to the event loop and dispatches them to the appropriate handler asynchronously. This allows Node.js to effectively manage large volumes of I/O operations with little overhead.

6. How do we use package.json in Node.js?

The package.json file is used to specify the dependencies and metadata for a Node.js project. It is typically stored in the node module root or project directory and provides information about the default node package. The details are the project name, version, author, license, and a list of dependencies.

7. When compared to Node.js, how does Angular differ?

When building online applications, developers often turn to Angular as a front-end framework and Node.js as a server-side JavaScript runtime. Client-side web applications are typically developed with Angular, while server-side applications are typically developed with Node.js. Unlike Node.js, which uses JavaScript as its main scripting language, Angular uses TypeScript.

8. Briefly explain the working of Node.js

Node.js is a server-side JavaScript environment that uses the V8 JavaScript engine. The I/O model used to manage concurrency is event-driven and non-blocking. Node.js processes requests received from clients asynchronously by adding them to the event loop, which notifies the appropriate handler to start working on them.

9. In what types of applications is Node.js most often used?

As it is a robust system, Node.js can have many different uses. Some typical Node.js applications are as follows:

• Node.js is ideal for developing real-time applications such as chat clients, multiplayer online games, and teamwork platforms. It's simple to manage a large volume of simultaneous connections and instantly update the user interface thanks to its event-driven design and non-blocking I/O.
• Node.js is also highly effective when used to create streaming applications such as media streaming services. It is ideal for such uses because of its ability to process massive sets of data without breaking the event loop.
• Node.js can also be useful for developing APIs as it can be used to create RESTful APIs and microservices. It is commonly used to build scalable and high-performance APIs due to its lightweight architecture and ability to manage a large volume of queries.
• Node.js is commonly used with popular front-end frameworks like React, Angular, and Vue to create single-page applications (SPAs). It's a great option for developing SPAs because it can serve static files and manage server-side rendering.
• Node.js can also be used to create CLIs, automation scripts, and other command-line tools. It is commonly used to develop such applications because of its ability to communicate with the file system and execute shell commands.

Intermediate Node.js Interview Questions

This section covers intermediate Node.js interview questions and their answers.

10. In the fs module, what are the differences between synchronous and asynchronous methods?

The fs module's synchronous methods freeze all other objects and code execution until the operation is complete, while the module's asynchronous methods continue to execute and rely on callbacks to handle the result of the operation. Synchronous methods are often easier to use but can block the event loop, while asynchronous methods are more efficient and do not block the event loop.

11. What is the command to load library packages from elsewhere in Node.js?

The required function in Node.js allows importing third-party libraries. The following code shows how to import the express library:

 const express = require('express');

12. List the tasks that should be performed asynchronously using event loop?

I/O operations, network activities, and other long-running processes that can block the event loop are examples of types of tasks that are best performed asynchronously with the event loop. File I/O, database queries, and HTTP requests to external web addresses or resources are good examples.

13. What is the difference between cluster and workers_threads packages in Node.js?

Multiple Node.js processes can work together with the cluster package to manage incoming requests and share a common set of server ports. With worker thread packaging, a single Node.js process can have many threads running simultaneously, making better use of available CPU cores.

14. Explain Node.js callback function.

Node.js uses callback functions asynchronously, passing them as arguments to other functions for execution once processing is complete.

15. List the two arguments that async.queue takes as input?

To manage multiple asynchronous calls with the async.queue module, providing two parameters: a worker function that handles each item in the queue, and a concurrency value telling how many items to handle per thread.

16. What exactly does it mean when people talk about event-driven programming and Node.js?

Event-driven programming in Node.js emphasizes events that control the flow of the application at each step. An event listener is created for certain events based on your needs. Whenever this event is triggered, the application responds by invoking a callback function. It is important to note that both application and system generated events are processed by the event loop in Node.js.

17. What do you understand by first class function in Javascript?

When JavaScript functions are manipulated in the same way as any other variable, it is a first-class JavaScript function. Such functions can be assigned to variables, provided as arguments to other functions, and returned as results.

18. How does a control flow function work?

Asynchronous code can be managed with the help of Node.js control flow utilities. They allow you to perform operations in a predetermined sequence, handle errors, and call the control flow function for later execution depending on the results of previous operations. async.waterfall, async.series, and async.parallel are just some of the control flow methods in Node.js.

The async.waterfall function is an example of asynchronously executing multiple functions in sequence and passing their results as arguments to the next function in the chain.

 async.waterfall((
 function(callback) {
  callback(null, 'one', 'two');
 }, 
function(arg1, arg2, callback) {
 callback(null, 'three');
 },
 function(arg1, callback) {
 // arg1 now equals 'three'
 callback(null, 'done');
 }
 ), function (err, result) {
 // result now equals 'done'
 });

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

The URL module in Node.js provides methods for working with URLs. To use the url module, you first need to import it:

 const url = require('url');

You can then use the URL module's various methods to parse, format, and manipulate URLs. For example, to parse a URL string, you can use the url.parse method:

 const urlString = '
 const parsedUrl = url.parse(urlString);
 console.log(parsedUrl);

This will generate an object containing all the various components of the URL, such as protocol, host, path, query parameters, and return data fragment.

20. What is the preferred method for resolving unhandled exceptions in Node.js?

To handle missed errors, it is recommended that an uncaughtExeption event handler be configured at the process level. This handler is called whenever a Node.js process throws an unhandled exception, giving the opportunity to clean up and terminate the process gracefully.

To create a handler for the uncaughtException event, you can use the following code:

 process.on('uncaughtException', (err) => {
 console.error('Unhandled exception:', err);
 // Perform cleanup tasks and shut down the process
 process.exit(1);
 });

21. What is an event loop in Node.js and how does it work?

During asynchronous processing, the Node.js event loop ensures that no blocking occurs, thus allowing you to manage asynchronous I/O activities. The event loop tracks the event queue and executes all jobs waiting on it in a non-blocking approach.

The event loop validates that there are no tasks remaining in the stack before adding a new job to the event queue. When the stack is empty, the event loop will execute the first task in the queue. When the stack is empty, the task is dequeued and executed in a single-threaded event loop.

The asynchronous job function runs, and then the event loop checks the event queue for additional tasks. Work can continue running while asynchronous event loop I/O activities, including file I/O, or network requests are processed in the background, thanks to the delegation of these event loop actions to a thread pool separate.

The thread pool communicates with the event loop, which is an event mechanism for returning the raw data of an asynchronous I/O action to the event queue upon its completion. As the I/O operation completes, the event loop continues its work including any code that was waiting.

Tough Node.js Interview Questions

This section covers intermediate Node.js interview questions and their answers.

22. What is callback hell and what is the main cause of it?

When there are too many levels of callbacks in an asynchronous application, the code becomes “hell” or difficult to understand and maintain. This happens when there is a complicated hierarchy of callbacks, with callbacks within callbacks within callbacks.

JavaScript is a single-threaded language, so it uses many asynchronous programming methods such as callbacks, promises, and async/await to avoid blocking the main thread while waiting for I/O operations to complete, which is the main source from callback hell. This leaves developers having to write multiple callbacks, which can become a maintenance nightmare if not organized properly.

It can also be challenging for developers to keep up with the program flow when dealing with countless layers of callbacks. This can reduce code readability, maintainability, and scalability and introduce glitches and errors.

Let's take a look at the code to see how promises, async/await, and event emitters help developers write more manageable asynchronous code and avoid the risks of callback hell.

 asyncFunc1(function (error, result1) { 
if (error) {
 // handle error
 } else {
 asyncFunc2(function (error, result2) {
 if (error) {
 // handle error
 } else {
 asyncFunc3(function (error, result3) {
 if (error) {
 // handle error
 } else {
 // do something with result1, result2, and result3
 }
 });
 }
 });
 }
 });

The code snippet above is made up of asynchronous functions. These functions must be performed one after the other. Since they are asynchronous, callbacks must be applied to handle their results. The code then becomes nested and difficult to understand. This makes it difficult to follow the code flow.

Here's an example of how the same code can be rewritten using Promises to avoid callback hell:

 asyncFunc1
  .then(result1 => {
    return asyncFunc2 ;
  })
  .then(result2 => {
    return asyncFunc3 ;
  })
  .then(result3 => {
    // do something with result1, result2, and result3
  })
  .catch(error => {
    // handle error
  });

Instead of using callbacks in the code, promises are used in this example. Each function results in a promise and the then method is used to connect all the promises.

23. Differentiate between Fork and Spawn methods in Node.js.

There are several differences between the Node.js fork and spawn functions for spawning child processes.

With the fork method, a new instance of the Node.js process with identical functioning is created. The child process has access to the parent's shared resources and can communicate with it through interprocess communication (IPC) because they both run in the same context. Node.js code can be split into its own processes using fork as worker processes in a cluster.

The spawn function essentially starts a new operating system process. Communication between child processes is achieved through streams or pipes, as the child process does not have access to the same resources as the parent. The spawn method is often used to start new processes that can execute other code.

24. What is middleware in Node.js

A middleware is a function that runs before or after the main request handler. Middleware functions can perform tasks such as logging, authentication, or error handling, and can modify request or response objects before passing them to the next middleware function or main request handler.

Middleware functions in Node.js are often organized into a pipeline or chain-attached functions, with each function in the chain responsible for a specific task. Middleware functions can be added to the pipeline using the use method of a middleware framework like Express.

For example, the following code defines a simple middleware function that registers the request method and URL:

 function logger(req, res, next) {
 console.log(`${req.method} ${req.url}`);
 next ;
 }

This middleware function can be added to an Express application using the use method:

const express = require('express');
 const app = express ;
 app.use(logger);
 app.get(" (req, res) => {
 res.send('Hello, world!');
 });
 app.listen(3000, => {
 console.log('Server started on port 3000');
 });

25. Explain the tasks of terms used in Node REPL

You can communicate interactively with Node.js using the Read-Eval-Print Loop (REPL) command-line interface. The following terms are widely used in Node REPL:

• Read: Input is accepted from the user in single-line or multi-line blocks during the read phase of the REPL.
• Eval: The REPL evaluation phase evaluates the data read in the previous phase and executes any JavaScript code added at this time.
• Print: The output from the REPL evaluation phase is printed to the console during the print phase.
• Loop: The loop phase of the REPL continually returns to the read phase for further user input.

26. Would you read files sequentially in Node.js? Provide a code example.

By using callbacks or promises, you can ensure that each file is read in order in Node.js. Here is an example of leveraging callbacks:

 const fs = require('fs');
 function readFile(path, callback) {
 fs.readFile(path, 'utf8', (err, data) => {
  if (err) {
   return callback(err);
  }
  callback(null, data);
 });
 }
 readFile('file1.txt', (err, data1) => {
 if (err) {
  return console.error(err);
 }
 console.log(data1);
 readFile('file2.txt', (err, data2) => {
  if (err) {
   return console.error(err);
  }
  console.log(data2);
  readFile('file3.txt', (err, data3) => {
   if (err) {
    return console.error(err);
   }
   console.log(data3);
  });
 });
 });

This code reads each file in sequence and records its contents in the console.

27. Explain some error handling approaches in Node.js that you know. Which will you use?

With Node.js, you can handle errors in several ways, some of which are:

• Callback Error Handling: Errors can be handled using a callback function, sending an error object as its first parameter. If the error value is true, then an error occurred, so the error object would specify what went wrong.
• Promise Error Handling: To handle unfulfilled or failed promises, developers can use the .then and .catch methods in a chain. If an error occurs in the promise chain, it can be resolved using the .catch function.
• Error events: When an error occurs, an error event is sent and an event listener is registered to process the error. Stream-based applications often use this method.
• Try-catch blocks: code is encapsulated in a try block and any problems are handled in a subsequent catch block, in the try-catch pattern. Synchronous applications benefit most from this method.

How errors are handled is a personal preference and depends on the type of web application framework being built. It is advisable that a combination of these strategies be used to provide reliable error handling.

28. How many different stream options does Node.js have?

Node.js supports four different types of streams:

• Readable streams: Data can be read into memory from a file or a network connection using a readable stream.

• Writable Streams: Writable streams are those that can be used to write information to a file or network.

• Duplex Streams: A stream where data can be read and written is called a duplex stream.

• Transformation streams: Transformation streams are bidirectional streams that can undergo transformations during reading or writing.

29. Explain the use of a buffer class in Node.js.

A buffer is a temporary storage location for raw binary data. In Node.js, this functionality is leveraged using the Buffer class. The Buffer class provides an efficient way to manipulate binary data in Node.js whenever used with streams, network protocols, and system file operations. This makes the Buffer class a critical component of the Node.js I/O system.

There are a variety of useful functions provided by the Buffer class. They can then be used to create and manipulate the buffers. The most popular are Buffer.from , Buffer.alloc and Buffer.concat .

Let's now see how to create a buffer for a string.

 const str="hello world";
 const buf = Buffer.from(str, 'utf8');
 console.log(buf); //

In this example, a buffer is created from the string 'hello world' using the Buffer.from method.

30. What are some commonly used timing features of Node.js?

There are a variety of pre-installed modules in Node.js that support timing and event scheduling. The following are examples of frequently used modules:

• setTimeout is a function that allows you to set another function to run after a predetermined period of time. The first parameter of the setTimeout method is the name of a function that will be executed, and the second parameter is the amount of time in milliseconds.
• setInterval is a function that allows the user to schedule another function to run repeatedly after a predetermined period of time. The first parameter of the setInterval method is the name of a function that will be executed and the second parameter is the amount of time in milliseconds.
• setImmediate is a function that schedules another function to run immediately after the current event loop iteration completes.
• process.nextTick is a function that will schedule the execution of another function at the beginning of the subsequent event loop iteration.
• The process.hrtime function is responsible for returning the most recent real-time high-resolution tuple in the format of (seconds, nanoseconds).
• Date.now function gives the current time in milliseconds.
• The process.uptime function returns the number of seconds that have passed since the start of the current Node.js process.

31. What is the use of Connect module in Node.js?

The connection module in Node.js provides a middleware system. It provides a collection of middleware operations that can be reused to perform routine tasks in the application. Tasks such as request body processing, serving static files, and managing authentication fall into this category. Connect is based on the Node.js http module and simplifies creating a pipeline of request handlers by chaining middleware functions. Connect was once a popular web application framework, but Express replaced it and is no longer being updated.

32. Differentiate between readFile and createReadStream in Node.js.

It's important to note that when working with large files, createReadStream is preferable to readFile as it reads the file in chunks rather than reading the whole thing at once, saving memory.

readFile may not be the ideal option for large files or applications that need to read multiple files simultaneously, despite its simplicity. createReadStream is the superior choice in such situations.

Here is an example of how to use readFile:

 const fs = require('fs');
 fs.readFile('file.txt', 'utf8', (err, data) => {
 if (err) throw err;
 console.log(data);
 });
 And here is an example of how to use createReadStream:
 const fs = require('fs');
 const readStream = fs.createReadStream('file.txt', 'utf8');
 readStream.on('data', (chunk) => {
 console.log(chunk);
 });
 readStream.on('end', => {
 console.log('Finished reading file');
 });

In this example, createReadStream creates a stream of binary data from the file, which can be read in chunks using the write data event. The final event is fired when the entire file is read.

33. Explain the concept of Punycode in Node.js?

Unicode characters can be represented in ASCII using a character encoding system called Punycode. Its main application is in the Domain Name System (DNS), where it is used to convert domain names, including non-ASCII characters, to ASCII-only representations.

The punycode module in Node.js offers tools for working with Punycode strings, including encoding and decoding. Unicode strings can be sent to the punycode.encode method to be converted to equivalent Punycode-encoded ASCII strings. When called with an ASCII string encoded using Punycode, the punycode.decode method will return the equivalent Unicode text.

Punycode “ xn—wgv71a119e.com ” represents the Unicode string “日本語.com ” for example. Therefore, even if a computer environment or certain system does not support Unicode, at least it will be able to read the domain name because it will be rendered in ASCII.

You must use the following command to access it:

 punycode = require('punycode');

Because it allows programmers to work with non-ASCII characters in URLs, email addresses, and other binary data formats, Punycode is an essential tool for enabling internationalization in many browsers and web applications.

Conclusion

This article will help you prepare for your next Node.js job interview by covering a wide range of Node.js interview questions and answers, from the most basic to the most advanced questions and answers.

Questions are not meant to be exhaustive and details may change based on the company and the NodeJS job description you are interviewing for. Having a firm grasp of these concepts along with the ability to explain them properly, however, can significantly increase your chances of getting the job. Non-technical questions, like those about your experience with Node.js, problem-solving skills, and teamwork, are just as likely to come up in an interview as technical questions.

Lastly, it's a good idea to keep up with the latest news and trends in the Node.js ecosystem by subscribing to the Node.js blog, visiting conferences and meetings, and interacting with online communities. If you dedicate yourself to learning and growing, you can become an invaluable addition to any company using Node.js.

In short, Node.js is a robust platform that helps programmers create responsive and scalable applications. If you learn the fundamentals and best practices of Node.js, you will open doors and advance your career. The guidance and knowledge provided in this article should be helpful as you prepare for your next job interview.