Top 40 NodeJS Developer Interview Questions and Answers

Due to its effective scalability and high performance in developing real-time applications, Node.JS stays strong as the web development tool. Since it is also applied in full-stack, MEAN stack, and MERN stack development, Node.JS professionals are in demand in the job  market. Prepare for jobs with our Node.JS developer interview questions and answers.

Node.JS Interview Questions for Freshers

Here are the frequently asked interview questions for nodejs developer freshers.

1. What is Node.js and how does it work?

A cross-platform, open-source JavaScript runtime environment called Node.js allows JavaScript code to be executed outside of a web browser. It is very effective at managing concurrent requests because it has an event-driven, non-blocking I/O approach.

How it works:

• An event-driven architecture is used by Node.js.
• Requests from clients are received and added to an event queue.
• The event loop calls callbacks and retrieves the events.
• To fulfill client requests, Node.js contains a small thread pool. 

2. Describe the Node.js event loop.

The core of Node.js is the event loop. It is a single-threaded process that runs the appropriate callbacks after continuously monitoring for events (such as file system actions or network requests). Node.js’s asynchronous nature enables it to manage numerous concurrent requests without causing the main thread to stall.

3. What are the key features of Node.js?

The key features of Node.JS are as follows:

• Asynchronous and Event-Driven: Multiple requests can be handled simultaneously due to asynchronous and event-driven programming.
• Non-Blocking I/O: This keeps the main thread from waiting for I/O operations to finish.
• Cross-Platform: Compatible with Windows, macOS, and Linux, among other operating systems.
• Large and Active Community: A sizable and vibrant community that offers a wealth of frameworks, libraries, and support.
• JavaScript Everywhere: Front-end and back-end development both make use of JavaScript.

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

JavaScript is a programming language that is mostly used in browsers for front-end web development.

Node.js: A runtime environment that facilitates server-side development by permitting JavaScript code execution outside of the browser.

5. What is the role of the npm command in Node.js?

For Node.js, the default package manager is called npm (Node Package Manager). Reusable code packages (modules) can be installed, managed, and shared via the npm registry.

6. Explain the concept of modules in Node.js.

Reusable code chunks that contain functionality are called modules. Every file is handled as a module in Node.js’s CommonJS module system. Modules can use exports and require import and export values, functions, and objects.

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7. In Node.js, how do you make and utilize a custom module?

• Use module.exports to export the desired functions or data from a JavaScript file (such as myModule.js).
• Use const myModule = require(‘./myModule’); to import the module into a different file.

8. What is the Node.js global object used for?

In a Node.js application, the global object is a unique object that grants access to global variables and functions.

A built-in object called the Node.js Global Object makes global variables and functions available across the Node.js runtime environment. Though designed specifically for server-side JavaScript, it is comparable to the window object found in browsers.

9. Explain the concept of streams in Node.js.

Streams are Node.js objects that handle data efficiently by processing it in parts. When working with big data sets, such files, network requests, and real-time data, they are helpful. 

Data flows continuously across streams. For effective data handling, including reading from files, writing to the network, and processing data in chunks, Node.js offers a variety of streams (readable, writable, duplex, and transform).

• Rather than putting the complete dataset into memory. 
• Streams can handle data in short, consecutive pieces.
• Streams emit events at several stages of data processing, including data, end, and error. They can be readable, writable, or both. 

10. What distinguishes Node.js’s synchronous and asynchronous operations?

Synchronous: Until an operation is finished, the main thread’s execution is blocked.

Asynchronous: Callbacks are triggered after an action is completed, and operations run in the background without halting the main thread.

11. What is Express.js, and what are its key features?

By offering a simple API for routing, middleware support, and HTTP utilities, Express.js is a minimal and adaptable Node.js web application framework that makes it easier to create server-side applications. Its main features include error handling, middleware handling, routing, and templating engine integration. 

Key Features of Express.JS

• Routing: Specifies how the server reacts, based on URLs, to various HTTP requests (GET, POST, etc.).
• Middleware: Makes it possible to handle requests and answers in a flexible and modular manner.
• Template Engines: Rendering dynamic HTML content is supported by templating engines.
• Error Handling: It offers tools, such as custom error handling middleware, to gracefully manage errors that arise during request processing.
• Flexibility: Provides a modular architecture that lets programmers alter the structure of their applications and select the features they require.
• Minimalist Approach: The minimalist approach seeks to make it lightweight and efficient by supplying vital features without extraneous costs.
• Scalability: Because it can handle multiple requests at once efficiently, it can be used to create both small and large applications. 

12. How do you define routes and handle requests in Express.js?

To accommodate various HTTP methods and URL patterns, Express.js routers provide a range of ways to define routes: 

• get(path, callback): It defines a route that responds to GET requests to the given path using the get(path, callback) function. 
• post(path, callback): A route that responds to POST requests to the given path is defined by the expression post(path, callback).
• To define routes for various HTTP methods, use the app.get(), app.post(), app.put(), and app.delete() methods.

To process the request and provide a response, access the request and response objects inside the route handler function.

13. What are some popular Node.js frameworks besides Express.js?

Some popular Node.JS frameworks besides Express.JS:

• Koa.js: Next Generation Node.js Framework.
• Meteor.js: One Application, One Language.
• Socket.io: Chat Apps Made Easy with Socket.io.
• Nest.js: A Nestling of Code.
• Sails.js: Modernized Data-Oriented MVC Framework.
• Total.js: A Complete Framework.
• Hapi.js: Secure than Ever.

14. What is the purpose of using a template engine in Node.js?

We can separate the presentation logic (HTML structure) from the business logic by using a template engine in Node.js to create dynamic HTML pages by injecting data from your application into pre-defined templates. 

This makes it easier to manage complex layouts and create dynamic web pages with reusable components without having to write raw HTML code directly in your server-side JavaScript. 

15. What are the key advantages of template engines?

Principal advantages of utilizing a template engine:

• Dynamic Content: Using information obtained from a database, you may quickly add dynamic data, such as user names, product specifications, or other variables, into HTML templates.
• Code Reusability: To make code easier to maintain, provide reusable elements (such as headers and footers) that may be used on different pages.
• Clean Separation: Code that is cleaner and better organized results from keeping the presentation logic (HTML structure) apart from the application logic.
• Improved Development Speed: Create intricate web pages quickly by concentrating on the data that will be shown rather than adjusting HTML strings by hand.  

16. How does a template engine work?

• Template Files: You create HTML templates with unique placeholders for dynamic data (such as curly braces).
• Data Context: You create an object in your Node.js application that has the information required to fill in the template.
• Rendering: The template engine takes the template file and creates the final HTML output by replacing the placeholders with the appropriate data from the context when a page has to be generated.

17. Explain the concept of Object-Document Mapping (ODM) in Node.js.

Instead of working directly with database-specific structures, ODM libraries (such as Mongoose for MongoDB) offer a means to communicate with the database using JavaScript objects.

18. In Node.js, what are promises and how do they operate?

Promises stand for an asynchronous operation’s ultimate success (or failure) and the value that results. Compared to callbacks, they offer a more streamlined method of managing asynchronous actions.

19. Describe the Node.js async/await concept.

• async/wait: Writing asynchronous code is made more synchronous with async/await.
• async keyword: A function that returns a promise is declared via the async keyword.
• await: The async function’s execution is halted by the await keyword until the promised value is resolved.

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Node js Interview Questions for Experienced Professionals

Here are the node js interview questions for senior developer:

1. What are the popular Node.JS template engines?

Well-known template engines for Node.js:

• Embedded JavaScript, or EJS: HTML with a straightforward syntax and direct JavaScript embedding.
• Pug (Jade): Simple syntax with a structure based on indentation.
• Handlebars: A well-liked option with facilities for complicated logic and a strong templating language.

2. What function does middleware serve in Express.js?

Before requests and answers reach route handlers, you can intercept and modify them using middleware, a request handler. These are the methods that the Express.js routing layer calls.

A series of middleware operations are carried out before the last request handler. They can carry out a variety of duties, including:

• Logging requests.
• Parsing request bodies, such as form data or JSON.
• Authorization and authentication.
• Error handling.

3. How do you connect to a database (e.g., MongoDB) in Node.js?

Installing the MongoDB Node.js driver with npm is the first step in connecting to a database like MongoDB in Node.js. 

Then, using the MongoClient class, you can connect using your database URI, which contains your hostname, port, database name, and authentication information if necessary. 

In other words, you import the driver, create a connection string, and use the connect method to start the connection.  

Steps to Connect to a Database in Node.JS:

• Install the MongoDB Driver: npm install mongodb
• Import the MongoClient Class: const MongoClient = require(‘mongodb’).MongoClient;
• Create a connection string: 

const uri = “mongodb://<username>:<password>@<hostname>:<port>/<database_name>”; 

• Connect to the database:

MongoClient.connect(uri, { useNewUrlParser: true, useUnifiedTopology: true })

        .then(client => {

            console.log(“Connected to MongoDB”);

            const db = client.db(“<database_name>”);

            // Perform database operations using the ‘db’ object        })

        .catch(err => console.error(err));

4. What are some common database choices for Node.js applications?

Some of the common database choices for Node.JS are MongoDB, PostgreSQL, MySQL, Redis.

• MongoDB: MongoDB is a NoSQL database solution that employs documents that resemble JSON and optional schemas.
• PostgreSQL: “Postgres” or PostgreSQL is an open-source database system that supports JSON and SQL. It is employed to arrange data and comprehend how data relates to one another.  
• MySQL: MySQL is a relational database management system (RDBMS) that uses SQL to store and manage data. It is open source, so anyone can use and modify it.  
• Redis: An open-source in-memory data store called Redis (REmote DIctionary Server) serves as a message broker, cache, and key-value database. It is renowned for its performance, dependability, and quickness.  

5. What are callbacks in Node.JS?

Callbacks are functions in Node.js that are invoked after another function has completed running. They are employed to carry out asynchronous tasks like processing requests or reading files.

It is another function that receives a callback function as an input. The callback is invoked when the function containing it completes. This enables the execution of other programs during the callback.

Common uses of callback:

• Node.js can handle numerous requests without waiting for a function to complete due to callbacks.
• Node.js is scalable as a result.
• Callbacks make sure that some code doesn’t execute until a particular task is completed.

Examples of Callback

• A callback is exemplified by Node.js’s setTimeout() method.
• Callbacks are useful for handling requests, reading files, and interacting with databases. 

6. What are some common testing frameworks for Node.js

The most popular testing frameworks for Node.js are Mocha, Jest, Jasmine, Cypress, Chai, and Sinon. Mocha and Jest are frequently used in conjunction with assertion libraries like Chai to improve the readability and flexibility of tests.  

Some of them are:

• Jest: Facebook created Jest, which is renowned for its ease of use and unconventional features, such as snapshot testing and an integrated mocking library.
• Mocha: A versatile framework that lets users select their favorite assertion library, such as Chai, and supports both synchronous and asynchronous testing.
• Chai: Mostly utilized as an assertion library that offers a variety of assertion styles when paired with other testing frameworks, such as Mocha.  

7. What are unit tests, and why are they important?

Testing the smallest components of a program to make sure they function as intended is known as unit testing in software development. Because it aids in the early detection and correction of errors, it is a crucial component of software development.  

The importance of Unit Tests:

• Early bug detection: Unit testing assists in finding bugs before they become more complicated and challenging to resolve. 
• Code quality: Clean, modular, and well-structured code is ensured via unit testing. 
• Security: Finding flaws in code through security unit testing can help safeguard user information and privacy. 
• Documentation: To help new developers better understand the code, unit tests can be used as documentation for the codebase. 
• Refactoring: With the assurance that any errors will be promptly identified, unit tests can assist developers in refactoring code.

8. How can you deploy a Node.js application?

In order to deploy a Node.js application, you usually need to: 

• Clone your project repository on a server 
• Use npm to install the required dependencies
• Set up a process manager like PM2 to start and monitor your application
• Run the “start” script to launch your Node.js application. 

Depending on your needs, you can do this on a dedicated server or on a cloud platform like AWS, Heroku, or Azure.  

Important Steps to Deploy a Node.JS Application:

Step 1: Set up your project:

• Make sure the required dependencies are specified in your package and that your Node.js application is appropriately organized.JSON file.
• Your package should include a start script.json to indicate how your application should be launched. 

Step 2: Version Control: Keep your project code in a Git repository, such as GitHub, to facilitate deployment and version control.  

Step 3: Configuring the server:

• Select a hosting platform, such as DigitalOcean, Heroku, or AWS.
• Install Node.js and set up a server instance.  

Step 4: Deployment Process:

• Clone your repository: To clone the code for your project onto the server, use Git. 
• Install dependencies: To install the necessary packages, navigate to your project directory and execute npm install. 
• Configure a process manager (optional): To handle restarts and logging in your Node.js process, use a solution such as PM2. 
• Launch your application: To start your Node.js application, run npm start. 

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9. What are some common security vulnerabilities in Node.js applications?

Some of the common security flaws in Node.js applications that an attacker could use to obtain unauthorized access or alter data inside the application: 

• SQL Injection: When untrusted user input is directly injected into SQL queries, allowing attackers to control database operations.
• Cross-Site Scripting (XSS): Attackers can run arbitrary scripts on a user’s browser when malicious JavaScript code is introduced into a website.
• Broken Authentication: Weak password hashing, improper session management, or predictable session IDs permitting unwanted access.
• Insecure Direct Object References: When a user manipulates URL parameters without the necessary authorization checks, allowing them to access resources directly. 
• Brute Force Attacks: Attempting various password combinations repeatedly in an attempt to access an account. 
• Command Injection: Hackers can run arbitrary code on the server by inserting malicious commands inside a system command. 
• Inadequate Input Validation: Not adequately verifying user input, which exposes the program to many types of attacks. 
• Vulnerable and Out-of-Date Components: It is using dependencies and libraries that are out-of-date or unsafe and have known vulnerabilities. 
• Insecure Deserialization: Untrusted data deserialization without enough validation could result in remote code execution. 
• Improper Access Control: Insufficiently controlling user permissions, which permits unwanted access to private information or features. 

10. How can you protect your Node.js application from common security threats?

Some of the ways to protect a Node.JS application:

• Implementing robust authentication and authorization procedures.
• Rigorously validating user input. 
• Encrypting data using HTTPS. 
• Updating dependencies frequently. 
• Utilizing security headers such as Helmet. 
• Implementing CSRF protection. 
• Using security linters to find vulnerabilities.
• Conducting regular security audits.

In essence, these measures focus on input validation, secure coding practices, and maintaining your dependencies up to date to reduce potential risks.  

11. What is clustering in Node.js?

In Node.js, clustering is a mechanism that handles requests concurrently by using many worker processes. This increases performance and decreases downtime by enabling the application to utilize all of the CPU cores that are available.

How Clustering Works:

• Worker processes that operate on different CPU cores are created using the cluster module.
• A master process controls the worker processes, which share a single port.
• Incoming requests can be handled independently by each worker process.

12. What are the advantages of Clustering?

The benefits of clustering are as follows:

• Improved Performance: The application may process more requests at once by dividing the load among several CPU cores.
• Reduced Downtime: The program can lessen outages and slowdowns by dividing incoming connections among all worker processes.
• Fault Tolerance: The program can continue to function even if one worker process crashes.
• Horizontal Scaling: Without complicating the codebase, the application can grow horizontally. 

13. Explain the concept of a worker pool in Node.js.

When dealing with multiple concurrent requests that require heavy computations, a “worker pool” in Node.js refers to a group of pre-spawned “worker threads” that are managed as a pool. 

• This allows the main thread to offload CPU-intensive tasks to these worker threads. 
• Preventing the main event loop from being blocked and ensuring smooth application performance. 

In essence, tasks are queued and assigned to available workers within the pool, who then execute them independently and return results to the main thread once they are finished.  

14. What are the key features of Worker Pool in Node.JS?

Some of the important features of Worker Pool in Node.JS:

Efficient Task Distribution: By reusing a fixed set of worker threads rather than generating new ones for each job, a pool lowers the overhead associated with thread generation and destruction.

Non-Blocking I/O: The main event loop is able to manage various I/O-bound operations, such as network requests, without being stalled by shifting complex computations to worker threads.

Concurrency: Using the system’s multi-core capabilities, many worker threads can carry out tasks simultaneously for improved performance.

Communication Mechanism: A messaging system is used by worker threads to communicate with the main thread. Usually, after a task is finished, results are sent back to the main thread.

15. How and when to use the worker pool?

Utilizing a worker pool:

• Node.js Worker Threads API: You can manually build and manage worker threads using the integrated worker_threads module.
• Third Party Libraries: By offering a higher-level abstraction, libraries such as workerpool or piscina make the process of setting up and maintaining a worker pool easier.  

When to use a pool of workers:

• CPU-intensive tasks: When intricate computations, picture processing, or other computationally demanding tasks must be completed by your program.
• Large data processing: Processing huge datasets in parallel without interrupting the main event loop is known as “large data processing.” 

Important Things in using Worker Pool:

• Pool Size: To prevent excessive context switching overhead, it is imperative to optimize the worker pool’s size. In general, a decent place to start is by setting the pool size to the number of available CPU cores.
• Task Queuing: To guarantee effective task distribution among available workers, a suitable task queue mechanism must be put in place.  

16. What is the difference between Node.js and Deno?

Although both Deno and Node.js are JavaScript runtimes, their functionality, ecosystem sizes, and security vary. The requirements of your project will determine the best option.  

Factor	Deno	Node.JS
Security	Deno is built using a permissions paradigm that necessitates express consent in order to access system resources, making it by default secure.	Node.JS has a less stringent security model that permits system resource access by default.
Ecosystem	Has a smaller ecosystem than Node.js.	Has a large ecosystem with extensive library support.
Features	It features an integrated toolbox for code formatting, linting, and other tasks, and supports TypeScript without the need for a compiler.	Node.js is the de facto standard for server-side JavaScript and has strong security features.

17. What is GraphQL, and how can you use it with Node.js?

Node.js gives developers the ability to implement a GraphQL server that can handle complex data requests from clients, making it a popular choice for building modern APIs with fine-grained data control. 

• GraphQL is a query language for APIs that allows clients to specify exactly which data they need from a server, providing a flexible way to retrieve data from a single endpoint. 
• Node.js serves as the runtime environment to build and serve a GraphQL API. 
• This allows developers to create effective and customizable backend services by defining data structures (schema) and writing functions (resolvers) to retrieve data based on client queries. 

18. What are the key features of GraphQL and Node.JS?

Important details about Node.js and GraphQL:

• Single endpoint: GraphQL streamlines client interaction by using a single endpoint for all data requests, in contrast to conventional REST APIs that have many endpoints.
• Data schema: To make it easier for clients to examine the data structure, developers provide a schema that outlines the various data kinds and relationships.
• Customizable queries: To avoid over-fetching and under-fetching, clients can create customized queries to only retrieve the data they require.
• Resolvers: They are operations that manage the retrieval of data for every field in the schema, enabling you to retrieve data from a variety of sources, including databases and external APIs.  

19. How to use GraphQL with Node.js?

The ways to use GraphQL with Node.JS:

• Install a GraphQL Library: To create a GraphQL server in your Node.js application, use a library such as graphql or apollo-server.
• Define a Schema: Using a schema definition language, create a GraphQL schema file with the data types, fields, and relationships specified.
• Implement Resolvers: For every field in the schema, write functions (resolvers) that, upon receiving a query, retrieve the real data from your data sources.
• Set up the Server: Build a Node.js server that receives incoming GraphQL queries, runs them using the resolvers and schema specified, and then returns the results to the client. 

20. What are the advantages of using GraphQL with Node.JS?

Advantages of integrating Node.js and GraphQL:

• Flexibility: By allowing clients to request precisely the data they require, network overhead is decreased.
• Developer Efficiency: As the schema explicitly defines the data structure, API development and maintenance are made easier.
• API Evolution: Adding new fields to the schema is simple and doesn’t interfere with already-existing clients. 

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Conclusion

We hope our Node.JS developers interview questions and answers. Developers may create reliable, effective, and maintainable apps by grasping the fundamentals of Node.js, comprehending asynchronous programming principles, and getting experience with well-known frameworks like Express.js. Secure your web developer career by enrolling in our .