Node Js Backend Developer

40+ Node Js Backend Developer Interview Questions and Answers

Updated 5 Feb 2025
search-icon

Q1. complete the following program: class MyEventEmitter { /// ..... } const emitterInst = new MyEventEmitter(); emitterInst.on("message", () => console.log("message event | callback 1")); emitterInst.on("message",...

read more
Ans.

Implement a custom event emitter class in Node.js

  • Create a class MyEventEmitter with methods 'on' and 'emit'

  • Store event listeners in an object with event names as keys and arrays of callbacks as values

  • Implement 'on' method to add event listeners to the object

  • Implement 'emit' method to trigger all callbacks for a given event

Q2. what is closures? Problem of promise with settimeout

Ans.

Closures are functions that have access to variables from their outer scope, even after the outer function has finished executing.

  • Closures are created when a function is defined inside another function.

  • The inner function has access to the outer function's variables, even after the outer function has returned.

  • Closures are useful for creating private variables and data encapsulation.

  • They can be used to implement callback functions and maintain state in asynchronous operations.

Node Js Backend Developer Interview Questions and Answers for Freshers

illustration image

Q3. What are the steps to create a basic POST API that handles all possible errors? What is marshalling? Also write test cases for api. Discuss all http status codes you know.

Ans.

Creating a basic POST API with error handling, marshalling, test cases, and HTTP status codes.

  • Create a Node.js server using Express framework

  • Define a POST route that handles incoming requests

  • Implement error handling using try-catch blocks

  • Use marshalling to convert data between different formats (e.g. JSON to object)

  • Write test cases using a testing framework like Mocha and Chai

  • HTTP status codes: 200 (OK), 400 (Bad Request), 401 (Unauthorized), 404 (Not Found), 500 (Internal Se...read more

Q4. What is node js and how does it handle concurrency?

Ans.

Node.js is a runtime environment that allows you to run JavaScript on the server side.

  • Node.js is built on the V8 JavaScript engine from Google Chrome.

  • It uses an event-driven, non-blocking I/O model that makes it lightweight and efficient for handling concurrent operations.

  • Node.js uses the libuv library to handle asynchronous operations and manage event loops.

  • Concurrency in Node.js is achieved through event loops and callbacks, allowing multiple operations to be processed simu...read more

Are these interview questions helpful?

Q5. Given an array, how can you determine the frequency of a specific element and subsequently delete all occurrences of that element?

Ans.

Use JavaScript methods to determine frequency and delete all occurrences of a specific element in an array of strings.

  • Use the reduce method to count the frequency of the specific element in the array.

  • Use the filter method to remove all occurrences of the specific element from the array.

Q6. How many years of experience with Node.js do you have?

Ans.

I have 5 years of experience working with Node.js in various projects and environments.

  • 5 years of hands-on experience with Node.js

  • Developed multiple backend applications using Node.js

  • Familiar with popular Node.js frameworks like Express.js

  • Experience in optimizing performance and scalability of Node.js applications

  • Worked on integrating Node.js with databases like MongoDB and MySQL

Share interview questions and help millions of jobseekers 🌟

man-with-laptop

Q7. How can we make use of multiple threads with node.js

Ans.

Node.js is single-threaded, but we can utilize multiple threads using worker threads module.

  • Node.js is designed to be single-threaded for better performance and scalability.

  • However, we can make use of the worker threads module to run JavaScript code in separate threads.

  • Worker threads allow us to perform CPU-intensive tasks without blocking the event loop.

  • We can create new worker threads using the Worker class from the worker_threads module.

  • Communication between the main threa...read more

Q8. What is an Event Emitter in Node.js?
Ans.

Event Emitter is a class in Node.js that allows objects to emit and listen for events.

  • Event Emitter is a core module in Node.js that provides an implementation of the observer pattern.

  • It allows multiple functions to be attached to a single event, which can be triggered synchronously or asynchronously.

  • Example: const EventEmitter = require('events');

  • Example: const myEmitter = new EventEmitter();

  • Example: myEmitter.on('event', () => { console.log('an event occurred!'); });

  • Example...read more

Node Js Backend Developer Jobs

Node Js Backend Developer 3-5 years
Livguard
4.0
Gurgaon / Gurugram
Node Js Backend Developer 3-6 years
Vlink
3.9
₹ 4 L/yr - ₹ 9 L/yr
Gurgaon / Gurugram
Sr. Node Js Backend Developer 5-10 years
OrangeMantra
4.3
Gurgaon / Gurugram
Q9. How does Node.js work?
Ans.

Node.js is a runtime environment that allows JavaScript code to run on the server side.

  • Node.js uses an event-driven, non-blocking I/O model, making it lightweight and efficient.

  • It uses the V8 JavaScript engine from Google Chrome to execute code.

  • Node.js allows developers to use JavaScript for both client-side and server-side development.

  • It has a large ecosystem of libraries and frameworks, such as Express.js for building web applications.

  • Node.js can be used to build scalable n...read more

Q10. Is Node.js Single threaded or Multi threaded?

Ans.

Node.js is single threaded, but it can support concurrency through event loop and non-blocking I/O operations.

  • Node.js runs on a single thread and uses an event-driven, non-blocking I/O model.

  • It can handle multiple requests concurrently by leveraging the event loop.

  • Node.js uses callbacks or promises to handle asynchronous operations.

  • The event loop allows Node.js to efficiently handle I/O operations without blocking the execution of other tasks.

  • Node.js can scale well for handli...read more

Q11. What is function overriding, and how is it implemented in JavaScript?

Ans.

Function overriding is a concept where a subclass provides a specific implementation of a method that is already provided by its superclass.

  • In JavaScript, function overriding can be achieved by defining a method in a subclass with the same name as a method in its superclass.

  • When an object of the subclass calls the overridden method, the subclass's implementation is executed instead of the superclass's.

  • This allows for customization and extension of behavior in subclasses while...read more

Q12. What is the process to create a basic connection to MongoDB?

Ans.

To create a basic connection to MongoDB, you need to install the MongoDB driver, set up a connection string, and use the MongoClient to connect to the database.

  • Install the MongoDB driver using npm install mongodb

  • Set up a connection string with the MongoDB URI

  • Use the MongoClient to connect to the database

Q13. 3rd Highest number in an array without using any sorting method/algo.

Ans.

Find the 3rd highest number in an array without sorting.

  • Iterate through the array to find the highest, second highest, and third highest numbers.

  • Keep track of these numbers as you iterate.

  • Return the third highest number once the iteration is complete.

Q14. How is MySQL Different from MongoDB? What would you prefer MongoDb or MySQL

Ans.

MySQL is a relational database management system, while MongoDB is a NoSQL database. Preference depends on project requirements.

  • MySQL is a relational database, while MongoDB is a NoSQL database.

  • MySQL uses tables and rows to store data, while MongoDB uses collections and documents.

  • MySQL is better suited for complex queries and transactions, while MongoDB is more flexible for unstructured data.

  • MySQL is ACID-compliant, ensuring data integrity, while MongoDB sacrifices some of th...read more

Q15. Do you have experience with GraphQL?

Ans.

Yes, I have experience with GraphQL.

  • I have worked on implementing GraphQL APIs in various projects.

  • I am familiar with creating schemas, resolvers, and queries in GraphQL.

  • I have used tools like Apollo Server and GraphQL Playground for development.

Q16. Do you have experience with PostgreSQL?

Ans.

Yes, I have experience with PostgreSQL.

  • I have worked on multiple projects where PostgreSQL was used as the database.

  • I am proficient in writing complex SQL queries and optimizing database performance.

  • I have experience in setting up and maintaining PostgreSQL databases.

  • I have used PostgreSQL in conjunction with Node.js for backend development.

Q17. What is a closure in Javascript?

Ans.

A closure in Javascript is a function that has access to its own scope, as well as the outer function's scope.

  • A closure allows a function to access variables from an outer function even after the outer function has finished executing.

  • Closures are commonly used to create private variables and functions in Javascript.

  • Example: function outerFunction() { let outerVar = 'I am outer'; return function innerFunction() { console.log(outerVar); }; }

Q18. What is MongoDB Aggregation? Which Aggregations have you used?

Ans.

MongoDB Aggregation is a framework for performing data processing operations on MongoDB documents.

  • Aggregation pipeline stages like $match, $group, $project, $sort, $limit, $lookup, etc.

  • Used for data transformation, filtering, grouping, sorting, joining, and more.

  • Allows for complex data analysis and reporting.

  • Example: db.collection.aggregate([{ $match: { status: 'A' } }, { $group: { _id: '$category', total: { $sum: '$quantity' } } }])

Q19. Can you demonstrate inheritance in JavaScript?

Ans.

Yes, inheritance in JavaScript is achieved through prototype chaining.

  • In JavaScript, objects can inherit properties and methods from other objects through prototype chaining.

  • The prototype property allows objects to inherit properties and methods from another object.

  • Child objects can access properties and methods of their parent objects through the prototype chain.

  • Example: function Animal() {} Animal.prototype.eat = function() { console.log('Eating'); }; function Dog() {} Dog....read more

Q20. What are middlewares?
Ans.

Middlewares are functions that have access to the request and response objects in Node.js, allowing for additional functionality to be added to the server.

  • Middlewares can be used to perform tasks like authentication, logging, error handling, etc.

  • They are executed in the order they are defined in the code.

  • Examples of middlewares include body-parser for parsing incoming request bodies, morgan for logging HTTP requests, and express-validator for validating request data.

Q21. Difference between find and filter method in js

Ans.

find method returns the first element that satisfies the condition, while filter method returns all elements that satisfy the condition.

  • find method returns the first element that satisfies the condition

  • filter method returns all elements that satisfy the condition

  • find method stops iterating once it finds a match

  • filter method iterates through the entire array

  • find method returns undefined if no element satisfies the condition

  • filter method returns an empty array if no element sat...read more

Q22. what is closure? what is hoisting?. find the second largest number without sorting.

Ans.

Closure is a function that has access to its own scope, as well as the scope of its outer function.

  • Closure allows a function to access variables from its outer function even after the outer function has finished executing.

  • Example: function outerFunction() { let outerVar = 'I am outer'; return function innerFunction() { console.log(outerVar); }; }

  • Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope before c...read more

Q23. What is a pipe in Node.js

Ans.

A pipe in Node.js is a mechanism that allows you to connect the output of one stream to the input of another stream.

  • Pipes are used to read data from a readable stream and write it to a writable stream.

  • They are created using the pipe() method.

  • Pipes help in simplifying the code by avoiding nested callbacks.

  • Example: readableStream.pipe(writableStream);

Q24. How event loop works?

Ans.

Event loop is a mechanism in Node.js that allows non-blocking I/O operations to be performed asynchronously.

  • Event loop continuously checks the call stack for any functions that need to be executed.

  • If the call stack is empty, event loop checks the callback queue for any pending tasks.

  • Event loop moves tasks from the callback queue to the call stack for execution.

  • This allows Node.js to handle multiple I/O operations concurrently without blocking the main thread.

Q25. What is temporal dead zone?

Ans.

Temporal dead zone is the period between entering scope and being able to access a variable.

  • Occurs when trying to access a variable before it is declared

  • Caused by hoisting in JavaScript

  • Example: accessing a let or const variable before its declaration will result in a ReferenceError

Q26. What is NodeJS? Why do we use it?

Ans.

NodeJS is a runtime environment that allows you to run JavaScript on the server side.

  • NodeJS is built on Chrome's V8 JavaScript engine.

  • It is used for building scalable network applications.

  • NodeJS uses an event-driven, non-blocking I/O model which makes it lightweight and efficient.

  • It is commonly used for building web servers, APIs, and real-time applications.

  • NodeJS has a large ecosystem of libraries and frameworks such as Express, Socket.io, and Sequelize.

Q27. Difference between SQL and NoSQL.

Ans.

SQL is a relational database management system, while NoSQL is a non-relational database management system.

  • SQL databases are table-based and have a predefined schema, while NoSQL databases are document-based, key-value pairs, graph databases, or wide-column stores.

  • SQL databases are good for complex queries and transactions, while NoSQL databases are better for hierarchical data storage and real-time web applications.

  • Examples of SQL databases include MySQL, PostgreSQL, and Ora...read more

Q28. Do you know the difference between put and patch

Ans.

PUT is used to update or replace an entire resource, while PATCH is used to update or modify a part of a resource.

  • PUT is idempotent, meaning multiple identical requests will have the same effect as a single request

  • PATCH is not necessarily idempotent, as multiple identical requests may have different effects

  • PUT requires the client to send the entire updated resource, while PATCH only requires the client to send the specific changes

Q29. What is event loop?

Ans.

Event loop is a mechanism in Node.js that allows non-blocking I/O operations to be performed asynchronously.

  • Event loop is responsible for handling asynchronous operations in Node.js.

  • It allows Node.js to perform multiple operations concurrently without blocking the execution.

  • Event loop continuously checks the event queue for new events and executes them in a non-blocking manner.

  • Example: When a file is being read asynchronously in Node.js, the event loop allows other operations...read more

Q30. What is hoisting?

Ans.

Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during compilation.

  • Variable declarations are hoisted to the top of their scope but not their initializations.

  • Function declarations are fully hoisted, meaning they can be called before they are declared.

  • Hoisting can lead to unexpected behavior if not understood properly.

Frequently asked in,

Q31. Explain Event loop in node.js

Ans.

Event loop is the core of Node.js runtime that handles asynchronous operations and ensures non-blocking I/O.

  • Event loop is responsible for executing callbacks and handling I/O operations.

  • It allows Node.js to handle multiple requests concurrently.

  • Event loop uses an event-driven architecture to efficiently manage resources.

  • Callbacks are added to different queues based on their type and priority.

  • Event loop continuously checks the queues and executes the callbacks in a non-blockin...read more

Q32. What is the expected income?

Ans.

The expected income for a Node Js Backend Developer can vary depending on factors such as experience, location, and company size.

  • Income can range from $60,000 to $150,000+ per year.

  • Factors influencing income include years of experience, level of expertise, and the specific industry.

  • Location can also play a significant role in determining salary, with higher salaries typically found in tech hubs like Silicon Valley.

  • Working for a large tech company or a startup can also impact ...read more

Q33. What is Socket io?

Ans.

Socket.io is a JavaScript library for real-time web applications that enables bidirectional communication between clients and servers.

  • Enables real-time, bidirectional communication between clients and servers

  • Supports multiple transport protocols like WebSocket, AJAX long polling, etc.

  • Facilitates event-based communication between client and server

  • Used for building chat applications, online gaming, real-time analytics, etc.

Q34. Node js async/await vs promised

Ans.

Async/await simplifies asynchronous code in Node.js by using promises under the hood.

  • Async/await is syntactic sugar built on top of promises in Node.js

  • Async/await makes asynchronous code look synchronous, making it easier to read and write

  • Promises are still used under the hood with async/await, providing better error handling and chaining

Q35. what is node js ?

Ans.

Node.js is a runtime environment that allows you to run JavaScript on the server side.

  • Node.js is built on Chrome's V8 JavaScript engine.

  • It uses an event-driven, non-blocking I/O model.

  • Node.js is commonly used for building scalable network applications.

Q36. why nodejs is asynchronous

Ans.

Node.js is asynchronous to handle multiple requests efficiently without blocking the execution of other code.

  • Node.js uses non-blocking I/O operations to handle multiple requests concurrently.

  • Asynchronous programming allows Node.js to execute code while waiting for I/O operations to complete.

  • Node.js event loop helps in managing asynchronous operations efficiently.

  • Callbacks are commonly used in Node.js to handle asynchronous operations.

  • Example: Reading a file while processing o...read more

Q37. Explain Node Js architecture.

Ans.

Node Js architecture is based on event-driven, non-blocking I/O model.

  • Event loop handles all I/O operations asynchronously

  • Single-threaded event loop allows for high scalability

  • Modules and packages can be easily added through NPM

  • Uses V8 engine for efficient JavaScript execution

Q38. Middle-ware in node js

Ans.

Middleware is a software layer that sits between the application and the server, allowing for additional functionality to be added.

  • Middleware functions are executed sequentially, and can modify the request and response objects.

  • Common middleware includes body-parser, cookie-parser, and express-session.

  • Custom middleware can be created to add specific functionality to an application.

  • Middleware can be used for authentication, logging, error handling, and more.

Q39. Merge overlapping intervals

Ans.

Merge overlapping intervals in an array

  • Sort the intervals based on their start times

  • Initialize an empty result array

  • Iterate through the sorted intervals

  • If the current interval overlaps with the previous interval, merge them

  • If not, add the previous interval to the result array and update the previous interval

  • Add the last interval to the result array

  • Return the result array

Q40. Average nodejs api design

Ans.

The average Node.js API design involves creating modular and scalable code, using RESTful principles, and implementing error handling and authentication.

  • Use modular code structure to separate concerns and improve maintainability

  • Follow RESTful principles for designing API endpoints and handling HTTP methods

  • Implement error handling to provide meaningful error messages and handle exceptions

  • Include authentication mechanisms like JWT or OAuth for secure access to API endpoints

Q41. What is callback

Ans.

A callback is a function passed as an argument to another function to be executed later.

  • Callbacks are commonly used in asynchronous programming to handle tasks that take time to complete.

  • They are often used in event handling, timers, and AJAX requests.

  • Callbacks can be synchronous or asynchronous, depending on when they are executed.

  • Example: setTimeout(callback, 1000) will execute the callback function after 1 second.

Q42. What is middleware

Ans.

Middleware is a software component that acts as a bridge between an application's request and response.

  • Middleware functions have access to the request and response objects

  • They can modify the request and response objects

  • Middleware can be used for tasks like authentication, logging, error handling, etc.

Q43. What is promise

Ans.

A promise is an object representing the eventual completion or failure of an asynchronous operation.

  • Promises are used to handle asynchronous operations in JavaScript.

  • They can be in one of three states: pending, fulfilled, or rejected.

  • Promises can be chained using .then() to handle success and .catch() to handle errors.

  • Example: new Promise((resolve, reject) => { setTimeout(() => resolve('Done!'), 1000); });

Q44. What is promise.all

Ans.

promise.all is a method in JavaScript that takes an array of promises and returns a single promise that resolves when all of the input promises have resolved.

  • Used to handle multiple asynchronous operations at once

  • Returns a single promise that resolves when all input promises have resolved

  • If any of the input promises is rejected, the returned promise is rejected with the reason of the first rejected promise

Q45. code optimization techniques

Ans.

Code optimization techniques improve performance and efficiency of code.

  • Use efficient data structures and algorithms

  • Minimize the number of database queries

  • Avoid unnecessary loops and function calls

  • Optimize memory usage by avoiding memory leaks

  • Use caching to reduce redundant computations

Q46. Even loop in node

Ans.

Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient.

  • Node.js event loop is a single-threaded loop that listens for events and executes callbacks

  • It uses non-blocking I/O operations to handle multiple requests simultaneously

  • Event loop has phases like timers, I/O callbacks, idle, prepare, poll, check, and close callbacks

  • setImmediate() and process.nextTick() are used to schedule callbacks in the event loop

Q47. Destry an Array

Ans.

Use the splice method to remove elements from an array in JavaScript.

  • Use the splice method with the starting index and number of elements to remove.

  • Example: array.splice(2, 3) will remove 3 elements starting from index 2.

  • Make sure to assign the result back to the original array if you want to update it.

Interview Tips & Stories
Ace your next interview with expert advice and inspiring stories

Top Interview Questions for Node Js Backend Developer Related Skills

Interview experiences of popular companies

3.7
 • 10.4k Interviews
3.6
 • 7.5k Interviews
3.8
 • 5.6k Interviews
4.0
 • 519 Interviews
3.5
 • 362 Interviews
3.6
 • 281 Interviews
3.9
 • 261 Interviews
3.7
 • 260 Interviews
View all

Calculate your in-hand salary

Confused about how your in-hand salary is calculated? Enter your annual salary (CTC) and get your in-hand salary

Node Js Backend Developer Interview Questions
Share an Interview
Stay ahead in your career. Get AmbitionBox app
qr-code
Helping over 1 Crore job seekers every month in choosing their right fit company
65 L+

Reviews

4 L+

Interviews

4 Cr+

Salaries

1 Cr+

Users/Month

Contribute to help millions

Made with ❤️ in India. Trademarks belong to their respective owners. All rights reserved © 2024 Info Edge (India) Ltd.

Follow us
  • Youtube
  • Instagram
  • LinkedIn
  • Facebook
  • Twitter