40+ 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.

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

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

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.

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' } } }])

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

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

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

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

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

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.

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

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

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.

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

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.

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

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

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.

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

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

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.

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.

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); });

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

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

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

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.