50+ Node Js Backend Developer Interview Questions and Answers

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

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.

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

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

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

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

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

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.

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

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

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

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.

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.

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); }; }

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' } } }])

TypeScript is a superset of JavaScript that adds static typing and other features for improved development and maintainability.

• TypeScript is statically typed, while JavaScript is dynamically typed. Example: In TypeScript, you can define a variable as 'let num: number = 5;'

• TypeScript supports interfaces and enums, which help in defining contracts and enumerations. Example: 'interface User { name: string; age: number; }'

• TypeScript provides better tooling and IDE support, includ...read more

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

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.

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

Node.js supports shallow and deep copy methods for object duplication, each with distinct behaviors and use cases.

• Shallow Copy: Copies the object's properties but not nested objects. Example: Object.assign({}, obj).

• Deep Copy: Recursively copies all properties and nested objects. Example: JSON.parse(JSON.stringify(obj)).

• Spread Operator: A modern way to create a shallow copy. Example: const newObj = { ...oldObj };

• Array Methods: Methods like slice() and concat() can create shall...read more

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

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);

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.

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

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

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.

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