Top 50 Node.js Interview Questions and Answers

Node.js is a cross-platform server-side JavaScript runtime environment that facilitates back-end web development with JavaScript. It is lightweight and is quite efficient for developing data-intensive applications.

Moreover, Node.js is quite similar to Ruby’s EventMachine and Python’s Twisted, but Node.js is more capable. It has an event-driven architecture that supports asynchronous I/O operations, which makes it capable of handling multiple concurrent requests.

One of the main positives of Node.js which makes it a popular JS runtime environment is that it runs on various platforms, including Windows, Linux, macOS, and Unix.

Nonetheless, if you are willing to become a Node.js developer and appear for a job interview, you need to know about the most common node.js interview questions.

In this article, we will mention the commonly asked Node.js interview questions along with their appropriate answers. But before that, we will highlight some key skills to become a proficient Node.js developer.

Essentials to Become a Node JS Developer

If you’re willing to become a node.js developer, here is what you need to have:

• Knowledge of various back-end JS web frameworks and their proper usage along with an in-depth understanding of core concepts of Node.js its API .
• Although Node.js is a server-side environment and most of the work is done in the back-end, you must have knowledge of certain front-end web development technologies as well, including HTML and CSS.
• A good understanding of databases along with hands-on experience with databases to store and retrieve data.
• A thorough understanding of Node.js package management.

Top Node.js Interview Questions

We have divided the entire list of Node.js interview questions into three levels: Beginner, Intermediate, and Advanced.

Beginner-Level Node.js Interview Questions

1. Explain Node.js.

Answer: Node.js is a development platform for building fast and scalable distributed applications using JavaScript that can run on the server-side. It runs on the V8 engine, which is capable of translating the JavaScript code into low-level machine code efficiently, thereby, enhancing the application performance and reducing the complexity.

This platform is one of the most popular server-side runtime environments for developing web apps as it consists of an event-driven architecture that can perform asynchronous I/O operations.

Node.js functions are non-blocking and follow event-driven architecture, which makes it efficient to handle concurrent client requests by using callbacks. Node.js is generally coupled with a database and combined with a browser and JSON to develop fully functioning network applications. Also, Node.js supports various server-side development patterns, such as MVVM, MVP, MVC.

2. Highlight some of the key features of Node.js?

Answer: The following are the salient features of Node.js:

• Node.js uses JavaScript: This is one of the most important aspects from a developer’s point of view as the majority of the web developers are well versed with JavaScript, so they don’t have to switch to another programming language for creating back end applications.
• Asynchronous event-driven I/O: Node.js follows an event-driven mechanism that supports asynchronous I/O operations. This simply means that on receiving a request, Node.js executes that request parallelly with other requests, without interrupting their operations.
• Fast: Node.js  is fast as it works on the V8 engine that is used by Google Chrome. Moreover, Node.js is single-threaded, which means it does not block any other operations. Thus, all the operations get executed in no time.
• Highly Scalable: Node.js works on single-threaded loop events that allow it to support thousands of other existing operations, which makes it highly scalable.
• Active Community: Node.js has a large and active community that is always available to help Node JS developers with their issues.

3. Is Node.js completely based on a single thread?

Answer: Node.js is single-threaded but internally it makes use of multiple threads to process asynchronous code by using libraries that are not single-threaded. One such library is the libuv library. In the background, Node.js uses child threads that are not exposed to users. It uses the Single Threaded Event Loop mechanism to efficiently deal with many concurrent requests.

This platform is majorly based on JavaScript's Event-based model and its callback mechanism. So, it can deal with many concurrent client requests efficiently. Node.js performs non-blocking operations that enable it to delegate all callbacks to different threads for their execution. The Node.js run-time is responsible for handling this process.

4. What does event-driven programming mean?

Answer: Event-driven programming is a type of programming pattern or paradigm in which the flow of the program is controlled by some specific events, such as messages from other programs, sensor outputs, mouse clicks, etc. Event-driven programs have a main loop for event listening which then triggers the callback function after the detection of the event.

Embedded systems can also achieve the equivalent with the help of interruptions in hardware instead of running the main loop constantly. Any programming language can be used to write an event-driven program, but if a language with high-level abstractions is chosen, then it can make the task much easier and simpler. Today, several GUI development tools and architectures depend on event-driven programming.

5. Define “I/O” operations?

Answer: Input/Output (I/O) is defined as a program , operation, or device that can convey and receive information to and from another device. I/O devices are hardware components that allow users to interact with a system, and the operations that facilitate this interaction are I/O operations.

6. How Node.js works?

Answer: Node.js is a cross-platform JavaScript runtime environment. Node.js is single-threaded which has an asynchronous I/O mechanism i.e., it does not block other functions while running. Although Node.js is single-threaded, all the I/O runs on single threads as the I/O APIs in Node.js are asynchronous/non-blocking by design, making it more efficient.

Whenever Node.js receives a request, it first completes its processing before moving on to subsequent requests. The loop is responsible for the conversion of all the external events into a mere callback function. This event loop calls all the event handlers at the correct time.

Hence, multiple functions get executed at a single time at the server-side during the processing of one request. This results in no delay and the other functions do not have to wait for their turn to get processed.

7. Name the various API functions supported by Node.js?

Answer: API stands for Application Programming Interface. Node.js supports the following types of API functions:

• Asynchronous, Non-blocking functions
• Synchronous, Blocking functions

Asynchronous Functions, Non-blocking functions: These functions, as the name suggests, operate asynchronously. It simply means that when a request is made by Node.js, it doesn’t get blocked until it is received. Instead, it will be sent to a subsequent API, and the server will be responded to by a notification mechanism. In layman’s terms, these functions allow working further while the request is being handled. Example: Emails, online forums, etc.

Synchronous, Blocking functions: These functions, unlike asynchronous functions, act as blocking functions. It simply means that these functions will make the calling system await a response. Thus, when a system uses synchronous APIs, it expects to urge immediate data when requests are made. These sorts of APIs are used where there are ample connectivity and availability with low latency. Example: Instant messaging, video meetings.

8. What do you understand by the control flow functions?

Answer: Control flow in programming is a way in which a method or statement is computed. There is a linear flow in the majority of the programming languages, which means that statements get executed at a time. These functions are chunks of code, which when the request is made by the user, gets executed. Control Flow functions help in the execution of the functions according to the order in which the call is made i.e, they follow the first come first serve (FIFO) order.

Node.js is asynchronous and single-threaded, so it does not let other functions await while it executes a function. Instead, it registers the callback and returns it so that there is no delay and other functions get executed simultaneously.

9. Explain NPM.

Answer: NPM or Node Package Manager is the default package manager for Node.js. The command-line client of NPM enables communication to a remote registry. It enables users to preoccupy and distributes JavaScript modules that are available within the registry.

There is metadata in JSON format contained in the packages within the registry that is in CommonJS format. The registry doesn't have any vetting process for submission, which suggests that packages found there are often inferior, insecure, or malicious. Also, user reports play a crucial role in taking down packages that violate policies.

Projects that are dependent on certain packages can also be managed by npm. In a local project, if npm is used as the package manager, it provides the facility to install all dependencies of the project with a single command.

10. What are the two arguments that async.queue takes as input?

Answer: Async::Queue is an async function for processing a queued task with the required concurrency. The tasks that are passed as arguments to the async.queue are processed simultaneously with its worker routine up to the given concurrency level. Asyn::Queue takes two arguments as input:

• Task function: It's a set of functions that are needed to run. It is often an array, an object, or any iterable structure. The task function also takes up two parameters as input: 1. The task that needs to be performed 2. The callback function.

These tasks will be processed in FIFO order.

• Concurrency value: It is the number of queued functions (concurrency level) that run in parallel. The tasks passed to the async.queue is processed simultaneously with its worker routine up to the required concurrency level.

11. State the difference between Node.js and Ajax?

Answer: The common thing about Node.js and AJAX is that both of them are advanced JavaScript implementations. However, the purposes they serve vary in many ways.

• Node.Js: It's a server-side JavaScript runtime environment for developing server-side applications.

• AJAX (Asynchronous Javascript And XML): Ajax is a group of technologies that works hand in hand for developing asynchronous client-side applications.

12. Describe the advantages and disadvantages of Node.js?

Answer: The following are the advantages and disadvantages of Node.js:

Advantages

• Asynchronous event-driven paradigm makes it possible to handle multiple concurrent requests.
• It follows the JavaScript model, which is quite easy to learn.
• Node.js helps to speed up the overall software development process.
• It has a large and active community.

Disadvantages

• It does not provide complete scalability. CPU-heavy applications can get complex to work with.
• Using a relational database with Node.js can be challenging.
• Node.js demands a lot of changes in the code due to the instability of its API.
• It requires in-depth knowledge of JavaScript concepts to work with Node.js.

13. What are the types of streams present in Node.js?

Answer: Streams in Node.js is a way to handle end-to-end information exchange, such as network communication and reading and writing files. They are memory efficient as we don’t have to load a huge amount of data in the memory. Mainly, Node.js has four types of Streams:

1. Writable: This stream supports the write operation. Example: fs.createWriteStream()
2. Readable: This stream supports the read operation. Example: fs.createReadStream()
3. Duplex: This stream supports both read and write operations. Example: net.Socket
4. Transform: This stream enables the modification of a duplex stream.

14. Name some of the popularly used libraries in Node.js?

Answer: Node.js offers a lot of libraries that developers can use to create server-side apps. However, there are certain libraries that are highly popular, and they are listed as follows:

• Express: Express is the most popular Node.js web framework known for its minimalism and flexibility.

• Gulp: Gulp is a command-line toolkit for Node. js that helps automate processes and enhance workflow by running repetitive tasks with ease.

• Async: This utility module enables us to work with asynchronous JavaScript.

• Moment: Moment is a popular module used for formatting date and time in JavaScript. It is used to manipulate date and time in JS by operations such as parse and validate.

• Lodash: Lodash is a handy reference library that provides helper functions for common programming tasks.

• Cheerio: Cheerio is an open-source library that helps developers interpret and analyze web pages by parsing the HTML and XML pages using a jQuery-like syntax.

• Nodemailer: Nodemailer is a module for Node. js applications and is used for sending emails from Node JS.

15. What is Libuv?

Answer: Libuv is a widely used multi-platform library that can enhance the asynchronous I/O functionality of Node.js. Following are some key features of the libuv library:

• File system event handling
• Child forking and handling
• Asynchronous UDP and TCP sockets
• Asynchronous file handling and operations
• High-resolution clock

16. Mention the types of modules offered by Node.js.

Answer: Node.Js offers majorly three 3 types of modules:

1. Third-party modules: These modules are external, and come with advanced functionalities. One can download and install them on their system using NPM.
2. Core Modules: Core modules consist of basic functionalities only to keep the Node.js lightweight. These modules load automatically when the Node.js process initiates. Example: URL, HTTP, path, etc.
3. Local Modules: These modules are created by the user itself. These modules have much more functionalities than core modules.

17. What do you know about callback hell?

Answer: Callback hell is a severe issue that is caused due to multiple nested callbacks. Due to this, callback hell is also referred to as the pyramid of doom.

18. What is package.json?

Answer: Package.json is the core of Node.js, and JSON stands for JavaScript Object Notation. It consists of all information about a particular project. In order to use this file, it must be present in the outermost or main folder of every Node.js Package or Module. The file is named “package” because it contains multiple directive elements which govern the handling of other modules. Package.json is used by Node Package Manager (NPM).

19. Does Node.js support multi-core platforms?

Answer: Yes, Node.js can support multi-core systems. However, since it is single-threaded, you need to fork multiple processes in order to facilitate multicore CPUs in it. This process is known as “clustering”, and is supported by the Node.js Cluster module.

The cluster module provides a way for starting multiple Node.js worker processes that run simultaneously and share the same port and helps you to spawn new child processes on the operating system using the child_process.fork() method. Each process communicates independently with the main process by IPC, so a shared state between the child processes can not be used.

20. Explain the role of the request process in Node.js.

Answer: In Node.js, the request module is used to make the HTTP calls and handle the requests from the server. The request module, by default, follows redirects to handle HTTP calls.

An HTTP request is basically a message that is a request made by the client over the web. HTTP is the fundamental format used to structure such requests for effective communication between the client and the server.

The request module is used to fulfill several requests from the server and if there is a large file that takes a long time for the server to process, a request can be made to ensure that other requests don’t get affected by that file and remain uninterrupted.

Intermediate-Level Node.js Interview Questions

21. What is the chaining process in Node.js?

Answer: In Node.js, chaining can be performed using the async npm module. The async module provides the following methods for chaining functions:

• parallel(tasks, callback): In the parallel chaining process, all the functions get executed parallelly through I/O switching. The callback function is triggered if any of the functions throws an error.

• series(tasks, callback): As the name suggests, functions are executed in series i.e, a function is being executed only after the completion of the previous function. Functions will not get executed if any of the previous functions gives an error.

22. What is a child_process module in Node.js?

Answer: The child_process module in Node.js helps to access the OS functionalities, it runs any system command inside a child process. This module is used to spin a child's process to establish communication between the processes within a messaging system.

As Node.js follows an event-driven model, it also supports the creation of child processes in parallel with the event-driven architecture. In Node.js, there are 4 different methods to create a child process:

• spawn(): This method is used to generate a new process using the command line arguments.

• fork(): This method is used to create a separation of tasks from the main event loop that needs evaluation.

• exec(): This method is used to create a child process by first creating a shell and then executing a command.

• execFile(): This method is used to create the specified executable file folder as a new process instead of generating a shell by default.

23. Explain the concept of middleware in Node.js?

Answer: A middleware function is a function that has access to all the requests made by the user. This simply means that in an application’s request cycle, these functions have access to all the response objects and the next subsequent function present in the memory stack. This function of middleware is represented by a keyword, usually named next .

Below are some of the most important tasks performed by middleware in Node.js:

• Execution of all types of code
• Modification of all the request objects
• Completion of the request-response cycle
• Call the next middleware in the stack

24. Differentiate between spawn and fork methods in Node.js?

Answer: The spawn() function generates a new process using the command line arguments. This function creates a process and gets called by Node.js when the child processes return data. The fork() method, on the other hand, is an object of the already existing process created by the spawn() method. Spawning makes sure that the tasks get handled at any given point, so it keeps more than one worker node active all the time.

25. What are stubs in Node.js?

Answer: In Node.js, stubs are functions that assess and analyze the behavior of individual components. Stubs provide the details of the executed functions while running the test cases. Following are some of the key highlights of the stubs:

• They can be anonymous.
• Stubs can be wrapped in already existing functions.
• They affect the behavior of components or modules.
• Stubs are dummy objects that are used for testing.

26. Why do module exports exist in Node.Js?

Answer: Module exports are the set of instructions, responsible for commanding Node.js to choose chunks of the program (arrays, class, etc) to “export” from the existing file to provide its access to the other files. Modules in Node.Js are responsible for combining multiple pieces of code to form a single code. This process is done by transferring all the related functions into one common file. If you’re using Node.js, making use of the modules is vital to create fully-functional applications.

27. What is the use of the connect module in Node. js?

Answer: Connect module is a module in Node.js that helps to create middleware for handling requests efficiently. It provides a safe channel for HTTP and Node.js to communicate. Also, the connect module provides a utility function that helps in registering it with HTTP so that the user does not have to invoke the HTTP.createServer(app) Connect module also has a method called createServer, which returns an instance.

This instance inherits an extended version of HTTP.Server. The extensions of Connect module usually make it easy to plug in the middleware. That is one of the main reasons why the Connect module gets symbolized as a "middleware framework".

28. What is tracing in Node.js?

Answer: Tracing is a method in Node.js that helps to trace and identify the node application pattern. Tracing makes monitoring of the application data quite easy whether it's on the function or system level. It also checks the validation of these functions by ditching them into a log file.

29. How can you scale a Node.js application?

Answer: We can scale a Node.js app using three ways that are as follows:

1. Cloning: To scale a big application, the easiest way would be to clone it multiple times so that each cloned instance can handle some part of the workload.
2. Decomposing: An application can also be scaled by decomposing it on the basis of functionalities. This way, we will have multiple applications with their corresponding code bases. Some of these can also have their own dedicated interfaces and databases.
3. Splitting: Another way to scale an application would be to split it into multiple instances, where each instance will be having the responsibility to handle a part of the overall data. This technique also goes by the name horizontal partitioning, in databases.

30. Differentiate between readFile and createReadStream in Node.js?

Answer: Following are some of the major differences between readFile and createReadStream:

31. What do you understand by global objects in Node.js?

Answer: Global objects in Node.js are globally accessible objects, and they are accessible in every module. Global objects don’t need to be added to the application, they can be accessed directly. Such objects can be functions, modules, objects, and strings. Global objects are not necessarily in the global scope, they can also be in the module scope. A global object in Node.js can be accessed using the global keyword. Also, they are built-in objects and are a part of JavaScript, and that’s the reason that they can be used directly in the application without actually importing any module.

32. What is a passport in Node.js?

Answer: Passport is an extremely flexible and popular middleware in Node.js which is used for authentication. It can be easily integrated with applications based on express.js.

Unique authentication mechanisms are required for every application, so single modules are provided using the passport middleware. This makes it an easy task to assign strategies to the applications, and it also avoids any kind of dependencies.

Such strategies are an extensible set of plugins that the passport uses to authenticate requests. Passport delegates all other functionalities to the application so that encapsulation in the modules remains intact.

33. Explain the usage of a buffer class in Node.js.

Answer: The buffer class in Node.js provides a way to deal with TCP streams (file system) and handle the octet streams. Following are the key highlights of the buffer class:

• This class provides instances to store and manipulate streams of raw binary data, which is similar to storing an array of integers. However, the difference is that there is a raw memory allocation outside the V8 heap.
• Buffer class in Node.js is a global object so it can be accessed directly in an application without actually importing any modules.
• After creating a buffer, there are multiple interactions that can be done with the buffer.

34. Explain REPL in Node.js.

Answer: REPL (Read Eval Print Loop) is an interactive shell used to process Node expressions. It is a computer environment such as a UNIX/LINUX shell that interacts by evaluating output when a command is executed in the shell.

Below are some of the tasks performed by REPL:

• Read: It reads the input passed by the user and then sends the input into JavaScript data structure, which is then passed onto the memory stack.
• Eval ? This task evaluates the input from the JavaScript data structure.
• Print ? This is the task of printing the result.
• Loop ? This task loops a command until ctrl+c is pressed twice.

Advanced-Level Node.js Interview Questions

35. W hat is the major drawback of Node.js?

Answer: One major challenge that the developers face with Node.js is to process a single thread to measure multiple core servers.

36. How does the DNS lookup function work in Node.js?

Answer: The DNS lookup function/method in Node.js is an inbuilt API of the DNS module that uses a web address as its parameter and resolves IP addresses of the required hostname for given parameters into the primary found A (IPv4) or AAAA (IPv6) record accordingly.

This method has three parameters namely hostname, options, and callback. The return value of this method is an error, IP addresses, and family of IP addresses through a callback function depending upon the values passed to the method as parameters.

37. Mention the sections of event-driven programming?

Answer: The two sections of event-driven programming are:

• Event selection: Event selection occurs on click events. It responds when we make a click or use a shortcut key to execute a selection in a pull-down menu.

• Event handling: Event handling is a callback routine that controls the events and operates asynchronously when an event occurs. Event-driven programming is based on events and event handling.

38. What do you understand about a test pyramid?

Answer: A test pyramid is a tool used to help developers by giving an idea of the required number of test cases executed in three stages, unit testing, integration testing, and combined testing. This indicates how testing is done using multiple layers.

Test pyramid reduces the time required for developers to analyze the impact or behavior of any change made to the code by maintaining an ample number of test cases to develop an end-to-end project. The test pyramid is a fundamental and crucial topic that helps developers and QA testers to deal with the entire stack of the end product.

39. Explain debugging in Node.js.

Answer: Node.Js has an in-built debugger. Debugging takes place when we start the Node.js with the debug statement along with the path of the script that needs to be debugged. After that, a breakpoint will be generated into the source code, once the statement debugger is inserted in it. This is the process of debugging in Node.js.

40. Explain CLI in Node.js.

Answer: Command Line Interface or CLI is a program, built-in on the computer where users type commands to perform the desired action. Different operating systems have different command lines:

• Bash in Linux.
• Terminal in macOS.
• Command Prompt or Powershell in Windows
• Shell/ Command line/terminal in Unix and Ubuntu

41. State the difference between Node.js and JavaScript?

Answer:

1. NodeJS : Node.js is a server-side (back-end) runtime environment that allows JavaScript code to execute outside the browser, i.e., on the server-side. In simple words, Node.js makes it possible to create server-side web applications.

2. JavaScript : Javascript is a client-side (front-end) scripting language. It is limited to the browser, which means that it can not execute outside the browser by default. JavaScript is the upgraded version of ECMAScript.

42. What are the common challenges with Node.js?

Answer: Following are the most common challenges with Node.js:

• It does not provide complete scalability. CPU intensive applications can get complex to work with.
• Integrating relational databases with Node.js can be complex.
• Node.js demands a lot of changes in the code due to the instability of its API.
• It requires in-depth knowledge of JavaScript concepts to work with Node.js.

43. Mention the framework most commonly used in Node.js?

Answer: Currently, Express is the most popular framework that is used in Node.js. Apart from that, some other well-known Node.js frameworks are:

• Koa.js
• Meteor.js
• Socket.io
• Nest.js
• Sails.js

44. What does “non-blocking” means in Node.js?

Answer: Node.js supports the non-blocking of input and output operations by using the libuv library to handle the I/O operations. It uses completion ports for Windows and poll or queue for UNIX. It sends a non-blocking request and that request is then queued within the event loop. The event loop calls the JavaScript on the main thread of JavaScript, which is also called callback.

45. What is the purpose of require() method in Node.js?

Answer: The most important use of require() method is that it provides the easiest way to include a module from external files in Node.js. It takes up a function having a string parameter that contains the location of a file that needs to be included. The entire JavaScript file is first read, executed, and then proceeds to return the export object.

46. How are child threads handled in Node.js?

Answer: Since Node.js is single-threaded, it does not exhibit the methods to manage threads. However, the use of the child threads can still be made using the spawn() method. To embed threading in a Node.js application, the ChildProcess module has to be included explicitly in it.

Handling of the child threads is done by spawning some asynchronous I/O tasks executing in the background. Using libraries that are not single-threaded, multiple threads can be handled efficiently in Node.js.

47. Why does Node.js use the V8 engine?

Answer: The V8 engine is an open-source engine by Google that is used by Chrome browser and Node.js. Node.js uses the V8 engine because it is capable of translating the JavaScript code into low-level machine code efficiently. It does not use an interpreter to convert the code.

48. What is the use of EventEmitter in Node.js?

Answer: The instances of the EventEmitter class emit an error event whenever the instances face any error. In such a situation, all of the specific functions of that particular event are synchronously called, this is known as calling the synchronous attachments. The objects of the EventEmitter class have a function that allows an attachment between the instances and the events. EventEmitter is present in the events module and enables the handling of custom events along with their creation.

49. Do you have any previous experience of working in a similar environment?

Answer: Interviewers ask such questions to assess your previous work or internship experience where you addressed similar working environments and similar technologies were involved. Answers to such questions should be honest and concise. Provide details wherever necessary, and discuss your relevant work experience.

50. Can you provide any certification to give a boost to your application for this Node.js role?

Answer: Possessing a certificate of the tech stack and skills that are required in the job description is always beneficial. This creates a positive impression on the interviewer that you are familiar with the required technology and have an in-depth understanding of the concepts as well as the practical applications. This also boosts up your resume and helps it to stand out, adding immense value to it as well as to your knowledge.

Wrapping Up

In this article, we discussed the top Node.js interview questions that are frequently asked by interviewers. These questions cover all the essential concepts of Node.js, starting from Node.js basics and moving forward to advanced topics. By going through these Node.js interview questions, it will become easier for you to give your best in your upcoming interviews.

If you want to share some questions that you have faced during your previous interviews and are not listed above, you can do that in the comments section below.