Node.js is one of the most popular Javascript frameworks used for back-end development. Node.js is a server-side platform that is open-source and can be used for cross-platform development. It is a runtime environment that helps us to develop server-side web apps and networking applications. It is a library-rich framework that can significantly reduce the development complexity of web apps.

In this article, we've compiled a comprehensive list of the most common Node.js interview questions that are asked most frequently in interviews, as well as the best ways to respond to them. Furthermore, it will provide you with an introduction to the fundamental concepts of Node.js.

Node.js Interview Questions Based On Basic And Technical Concepts

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

Node.js is an open-source, cross-platform JavaScript runtime environment and library that allows web applications to run outside of the client's browser. It is used for creating server-side web applications.

Due to its asynchronous, event-driven model, Node.js is perfect for data-intensive applications. You can use I/O-intensive web applications such as video streaming websites. Additionally, it can be used to create real-time web applications, network applications, general-purpose applications, and distributed systems.

2) Why are some advantages of Node.js?

Node.js simplifies the development of scalable network programs. The following are some of its advantages:

• Generally, it is fast
• Rarely blocks
• Offers a unified programming language and data type
• Everything is asynchronous
• Provides great concurrency

3) Explain how Node.js works?

A web server using Node.js typically has a workflow, which can be explained with the following points:

• To interact with the web application, clients send requests to the webserver. Requests can be non-blocking or blocking:
• Asking for data
• Deleting data
• Updating data
• Node.js retrieves the incoming requests and adds them to the Event Queue.
• The requests are then passed through the Event Loop one by one. It determines if the requests are simple enough not to require any external resources.
• The Event Loop processes simple requests (non-blocking operations), such as I/O Polling, and delivers the responses to the corresponding clients.

4) What is the first-class function in Javascript?

When functions can be treated like any other variable then those functions are called first-class functions. There are many other programming languages, such as Scala, Haskell, etc, which follow this approach, including JS. As a result, this function can be passed as a parameter to another function (callback) or a function can return another function (higher-order function). Map() and filter() are popular higher-order functions.

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

• setTimeout/clearTimeout – This is used to delay code execution.
• setInterval/clearInterval – Used to run a code block multiple times.
• setImmediate/clearImmediate – You can use this to set the code to be executed after the event loop completes.
• process.nextTick – You can use this to set the code to be executed at the start of the next event loop.

6) How do you manage packages in your node.js project?

Several package installers can manage it, as well as their configuration files. Out of them, most use yarn or npm. They both provide almost all javascript libraries with extended features for controlling environment-specific configurations. We use package.json and package-lock.json to maintain lib versions in a project to ensure there are no issues in porting it to a different environment.

7) Explain the steps of how “Control Flow” controls the function calls?

Following are the steps of how Control Flow controls the function calls:

• Control the order of execution
• Collect data
• Limit concurrency
• Call the following step in the program.

8) How is Node.js better than other frameworks most popularly used?

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

• Node.js offers simplicity in development due to its non-blocking I/O and even-based model, which results in short response times and concurrent processing, unlike other frameworks that require thread management.
• It uses the chrome v8 engine, written in C++, and is extremely performant with continuously improving performance.
• In addition, since Javascript will be used both on the front-end and on the back-end, the development will be faster.
• Last but not least, there are ample libraries, so we don't have to reinvent the wheel.

In Node.js, there is always a great need for professionals. Therefore, it is highly recommended to learn Node.js from the best Node.js tutorials.

9) What are the advantages of using promises instead of callbacks?

When you use promise, you get an object to decide what action needs to be taken after completing the async task. As a result, the code is more manageable and callback hell is avoided.

10) Why is Node.js single-threaded?

Node.js was created specifically as an experiment in async processing. This was done to analyze a new theory of doing async processing on a single thread over the existing thread-based implementation of scaling through different frameworks.

11) What is the fork in node JS?

In general, a fork is used to spawn child processes. Node uses it to create a new instance of the v8 engine to run several workers to execute the code.

12) How many types of API functions are there in Node.js?

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

1. Asynchronous, non-blocking functions- Mainly I/O operations that can be fork out of the main loop.

2. Synchronous, blocking functions- Primarily operations that affect the process running in the main loop.

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

var http = require("http");
http.createServer(function (request, response) {
  response.writeHead(200, {'Content-Type': 'text/plain'});
  response.end('Hello World\n');
}).listen(3000);

14) What is REPL?

REPL in Node.js stands for Read-Eval-Print-Loop, which further means evaluating code on the go.

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

These are the two arguments that async.queue takes as input:

• Concurrency Value
• Task Function

16) What is the purpose of module.exports?

The aim of this is to expose functions of a particular module or file so they can be used elsewhere in the project. In this way, all similar functions can be compiled into one file, improving the structure of your project.

For example, you can put a file that contains all utils functions with util in different programming languages to get solutions to the problem statement.

const getSolutionInJavaScript = async ({
 problem_id
}) => {
...
};
const getSolutionInPython = async ({
 problem_id
}) => {
...
};

Therefore, by using module.exports we can use these functions in some other file: const { getSolutionInJavaScript, getSolutionInPython} = require("./utils")

17) What tools can be used to assure consistent code style?

ESLint can be used with any IDE to ensure a consistent coding style, which in turn facilitates code maintenance.

18) What are node.js buffers?

In general, buffers are temporary memory used by streams to hold some data until it is consumed. Buffers are introduced with additional use cases than JavaScript's Unit8Array and are primarily used to represent a fixed-length sequence of bytes. It also supports legacy encodings such as ASCII, UTF-8, etc. It is a fixed(non-resizable) allocated memory outside the v8.

19) What do you understand by callback hell?

async_A(function(){
   async_B(function(){
       async_C(function(){
           async_D(function(){
           ....
           });
       });
   });
});

In the above example, we are passing callback functions, which makes our code unreadable and unmaintainable, therefore we should modify our async logic to avoid this situation.

20) How does Node.js overcome the problem of blocking of I/O operations?

It is possible to handle all I/O operations asynchronously via the event loop in the node without blocking the main function.

Therefore, for example, if a network call is needed, it would be scheduled in the event loop and not in the main thread (single thread). Similarly, if there are multiple I/O requests, each one will be queued independently to be executed separately (other than the main thread).

Even though JS is a single-threaded language, I/O operations are handled in a non-blocking way.

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

Listener functions can use both process.nextTick() and setImmediate() to transition into an asynchronous mode.

process.nextTick() sets the callback to execute but setImmediate accelerates the callback in the queue to be executed. As a result, the event loop runs as follows:

timers–>pending callbacks–>idle,prepare–>connections(poll,data,etc)–>check–>close callbacks

The process.nextTick() method adds the callback function to the start of the next event queue and the setImmediate() method places the function in the check phase of the next event queue.

22) What are node.js streams?

Streams in Node.js are instances of EventEmitter that can work with streaming data. They are often used for handling and modifying streaming large files (videos, mp3s, etc.) over the network. To temporarily store data, they use buffers.

There are mainly four types of the stream:

• Writable: streams in which data can be written (for example, fs.createWriteStream()).
• Readable: streams from which data can be read (for example, fs.createReadStream()).
• Duplex: Read-write streams (for example, net.Socket).
• Transform: Duplex streams that can modify or transform the data as it is read and written (for example, zlib.createDeflate()).

23) What is middleware?

Middleware sits between your request and business logic. It mainly captures logs and enables rate limits, routing, authentication, and anything else that does not belong to business logic. Furthermore, third-party middleware is available, such as body-parser, and you can write your middleware for a specific use case.

24) Why Google uses the V8 engine in Node.js?

What other options are there? While we do have Spiderman from Firefox and Chakra from Edge, Google’s v8 is the most evolved and fastest(since it’s written in c++) we got till now as a JavaScript and WebAssembly engine. Almost any machine can run it.

25) Explain what a Reactor Pattern in Node.js?

The Reactor pattern is another pattern for nonblocking I/O operations. Generally, this is used in any event-driven architecture.

There are two components in this:

1. Reactor: Handles I/O events by dispatching them to the appropriate handler.
2. Handler: It takes care of the events that occur.

26) Why should you separate the Express app and server?

The server initiates the routes, middleware, and other application logic, while the app has the business logic, which will be served by the routes the server initiates. This ensures that the business logic is encapsulated and decoupled from the application logic, making the project more readable and maintainable.

27) Describe the exit codes of Node.js?

Using exit codes, we can find out how a process terminated/the reason behind it. Here are a few of them:

• Uncaught fatal exception - (code - 1) - An exception has occurred that has not been handled.
• Unused - (code - 2) - This is reserved by bash
• Fatal Error - (code - 5) - An error occurred in V8 with stderr output of description.
• Internal Exception handler Run-time failure - (code - 7) - An exception has occurred when bootstrapping function was called.
• Internal JavaScript Evaluation Failure - (code - 4) - An exception occurred when bootstrapping process was not able to return function values.

28) Explain the concept of stub in Node.js?

Stubs are used in writing tests, which are an integral part of the development process. It replaces the entire function that is being tested.

It helps us test scenarios such as:

• External calls that make tests slow and difficult to write (e.g., HTTP/DB calls)
• Different outcomes can be triggered by a piece of code (e.g. what happens if an error is thrown/ if it passes).

29) What is an Event Emitter in Node.js?

Node.js's EventEmitter class wraps all objects that are capable of emitting events. EventEmitter.on() can be used to attach named events emitted by the object. Thus, whenever this object throws an event, its attached functions are invoked synchronously.

30) How can clustering improve Node.js performance?

By default, Node.js applications run on a single processor, which means that they can't take advantage of a multi-core system. Using cluster mode, multiple instances of the event loop are spun up for multiple instances of the node.js process. Whenever we start using cluster in a node.js app behind the scenes multiple node.js processes are created, but there is also a parent process called the cluster manager which monitors the health of individual instances of our application.

31) What is WASI and why is it being introduced?

Web assembly provides an implementation of WebAssembly System Interface specification via WASI API in node.js executed using WASI class. When WASI was introduced, several POSIX-like functions were incorporated as a means to help the application access the underlying operating system more efficiently and use features that require system-level access.

32) What is a thread pool and which library handles it in Node.js?

Thread pools are managed by the libuv library. libuv is a multi-platform C library that provides support for asynchronous I/O-based operations like file systems, networking, and concurrency.

33) How to measure the duration of async operations?

The Performance API provides us with tools to determine the necessary performance metrics. We can use async_hooks and performance_hooks as simple examples. It will show us the exact time the callback took to complete.

34) How are worker threads different from clusters?

Cluster:

• Each CPU has one process with an IPC to communicate.
• When we want several servers to accept HTTP requests over the same port simultaneously, clusters can be useful.
• Since the processes are spawned in each CPU, they will have their memory and node instances, which will further cause memory problems.

Worker threads:

• In total, there is only one process with multiple threads.
• With most APIs accessible, each thread has one Node instance (one event loop and one JS engine).
• Shares memory with other threads (e.g. SharedArrayBuffer)
• It can be used for CPU-intensive tasks like processing data or accessing the file system since NodeJS is single-threaded, the worker threads can make synchronous tasks more efficient.

35) How to measure the performance of async operations?

The Performance API provides us with tools to determine the necessary performance metrics. Here is a simple example-

const { PerformanceObserver, performance } = require('perf_hooks');
const obs = new PerformanceObserver((items) => {
 console.log(items.getEntries()[0].duration);
 performance.clearMarks();
});
obs.observe({ entryTypes: ['measure'] });
performance.measure('Start to Now');
performance.mark('A');
doSomeLongRunningProcess(() => {
 performance.measure('A to Now', 'A');
 performance.mark('B');
 performance.measure('A to B', 'A', 'B');
});

36) How would you define the term I/O?

• Generally, I/O is defined as any program, operation, or device that transfers data to or from a medium and to or from another medium.
• Whenever a transfer occurs, an output from one medium is converted into an input into another. The medium can be a physical device, a network, or files within a system.

37) Explain callback in Node.js.

After a given task is completed, a callback function is called. It prevents any blocking by allowing other codes to run in the meantime. As an asynchronous platform, Node.js heavily relies on callbacks. Callbacks are supported by all the APIs of Node.

38) How is Node.js most frequently used?

Several applications use Node.js:

• Internet of Things
• Streaming applications
• Complex SPAs (Single-Page Applications)
• Microservices architecture
• Real-time collaboration tools
• Real-time chats

39) What is NPM?

NPM stands for Node Package Manager, which is responsible for managing all the packages and modules for Node.js. There are two main functions of Node Package Manager:

• Provides online repositories for node.js packages/modules, searchable on search.nodejs.org.
• Manages Node.js versions and dependencies and also provides command-line utility to install Node.js packages.

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

Node.js is preferred for the following reasons:

• Incredibly Fast
• The Node Package Manager provides developers with access to over 50,000 bundles.
• Node.js is ideal for data-intensive, real-time web applications because it doesn't wait for an API to return data.
• Better synchronization of code between server and client because of the same code base.
• It is easy for web developers to use Node.js because it is a JavaScript library.

41) Which database is more popularly used with Node.js?

Node.js is most commonly used with MongoDB. It is a cross-platform, NoSQL, document-oriented database that is easy to use, scalable, and provides high performance.

Beginners can learn MongoDB from the best MongoDB tutorials online.

42) What is the difference between Angular and Node.js?

Angular:

• Front-end development framework.
• It is written in TypeScript.
• Used for building single-page, client-side web applications.
• Splits a web application into MVC components.

Node.js:

• Server-side environment.
• It is written in C and C++ languages.
• Used for building fast and scalable server-side networking applications.
• Generates database queries.

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

In Node.js, there are two commonly used libraries:

• ExpressJS - Express is a flexible Node.js web application framework that offers a wide variety of features for developing mobile and web applications.

• Mongoose - Mongoose is also a Node.js web application framework that allows you to connect an application to a database.

44) What is the command used to import external libraries?

Using the "require" command, you can import libraries from external sources. For example - “var http=require (“HTTP”).” Through the HTTP variable, the HTTP library and the single exported object will be loaded.

45) What does event-driven programming mean?

Various functions are triggered by events in an event-driven programming approach. An event can be anything, such as a keystroke or a mouse click. Whenever an event occurs, the element already executes a callback function.

46) What is an Event Loop in Node.js?

In Node.js, event loops handle asynchronous callbacks. Node.js is the key component of non-blocking input/output, making it one of the most important environments.

47) What is the package.json file?

Package.json is the heart of a Node.js system. The file contains the metadata for a particular project. Normally, package.json is found in the root directory of a Node application or module.

This is what a package.json file looks like immediately after you create a Node.js project using the command: npm init

When you create a Node.js project, you can modify the parameters.

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

Node.js's URL module provides utilities for parsing and resolving URLs. This is a built-in module that helps break up web addresses into readable parts.

49) What is the buffer class in Node.js?

The buffer class stores raw data similar to an array of integers, but it corresponds to a raw memory allocation outside the V8 heap. Due to pure JavaScript's incompatibility with binary data, the buffer class is used.

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

Follow these simple steps to connect a MongoDB database to Node.js:

• Install Node.js
• Install the MongoDB Node.js Driver
• Create a free MongoDB Atlas cluster and load the sample data
• Get your cluster’s connection info
• Import MongoClient
• Create our main function
• List the databases in our cluster
• Save Your File

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If you have made it this far, then certainly you are willing to learn more about JavaScript. Here are some more resources related to JavaScript that we think will be useful to you.

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