Javascript Developer

40+ Javascript Developer Interview Questions and Answers

Updated 6 Jul 2025
search-icon
3d ago

Q. If you have 3 nested for loops, how many times will the innermost loop run?

Ans.

Nested for loops multiply their iterations, leading to exponential growth in total runs based on their ranges.

  • A single for loop runs 'n' times if it iterates from 0 to n-1.

  • For nested loops, the total runs are the product of their individual iterations.

  • Example: 3 loops, each running 5 times: 5 * 5 * 5 = 125 total runs.

  • If the outer loop runs 'a' times and the inner loops run 'b' and 'c' times, total runs = a * b * c.

Asked in Forvia Hella

6d ago

Q. Can you tell me about promise, async in javascript

Ans.

Promises and async/await are used in JavaScript to handle asynchronous operations and avoid callback hell.

  • Promises are objects representing the eventual completion or failure of an asynchronous operation.

  • Async/await is a syntactic sugar built on top of promises, making asynchronous code look synchronous.

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

  • Async functions return a promise, allowing you to use await to pause execution until the p...read more

Javascript Developer Interview Questions and Answers for Freshers

illustration image

Asked in Deloitte

3d ago

Q. 1. Is .map and .filter pure functions ? 2. What should not be blocked in Node.js ? 3. Is For.. in.. loop for iterating through object or arrays ?

Ans.

1. Yes, .map and .filter are pure functions. 2. Network calls should not be blocked in Node.js. 3. For..in.. loop is for iterating through objects.

  • 1. Yes, .map and .filter are pure functions as they do not modify the original array and always return a new array with the desired transformations or filtered elements.

  • 2. Network calls should not be blocked in Node.js as it can lead to performance issues and hinder the event-driven nature of Node.js.

  • 3. For..in.. loop is used for i...read more

4d ago

Q. difference between var, let and const, es6 concepts, state, props in react, arrow function, funciton key word, scope in javascript

Ans.

var, let, and const are used for variable declaration in JavaScript. ES6 concepts include arrow functions and block-scoped variables.

  • var is function-scoped, let and const are block-scoped

  • let and const support block-level scoping, reducing the risk of variable hoisting

  • const variables cannot be reassigned, but their properties can be modified

  • ES6 introduces arrow functions for concise syntax and lexical scoping

  • State and props are key concepts in React for managing component data...read more

Are these interview questions helpful?

Asked in Uplers

6d ago

Q. Map reduce and filter in JavaScript

Ans.

Map, reduce and filter are array methods in JavaScript used for manipulating and transforming data.

  • Map creates a new array by applying a function to each element of the original array.

  • Reduce reduces an array to a single value by applying a function to each element.

  • Filter creates a new array with all elements that pass the test implemented by the provided function.

Q. What is the difference between the forEach and map methods?

Ans.

forEach and map are array methods in JavaScript. forEach executes a provided function once for each array element. Map creates a new array with the results of calling a provided function on every element in the calling array.

  • forEach does not return anything, it just iterates over the array and executes the provided function for each element.

  • Map returns a new array with the results of calling a provided function on every element in the calling array.

  • Map is useful when you want...read more

Javascript Developer Jobs

Infosys logo
React JS Developer-STG(P) 14-17 years
Infosys
3.6
Kolkata
Infosys logo
React JS Developer -STG (P) 8-13 years
Infosys
3.6
Hyderabad / Secunderabad
HCLTech logo
Job Opening | React JS Developer | HCL Tech | 5-7 years
HCLTech
3.5
₹ 5 L/yr - ₹ 12 L/yr
Bangalore Rural
1d ago

Q. How do you check if a number is even in JavaScript?

Ans.

Check if a number is even by using the modulo operator.

  • Use the modulo operator (%) to check if the number divided by 2 has a remainder of 0.

  • If the remainder is 0, then the number is even.

  • Example: if(num % 2 === 0) { // number is even }

1d ago

Q. How would you design an NLP engine?

Ans.

Designing a NLP engine involves defining the problem, selecting appropriate algorithms, and training the model.

  • Define the problem and identify the data sources

  • Select appropriate algorithms for tasks such as tokenization, part-of-speech tagging, and named entity recognition

  • Train the model using labeled data and evaluate its performance

  • Continuously improve the model by incorporating feedback and updating the algorithms

Share interview questions and help millions of jobseekers 🌟

man-with-laptop

Asked in Nofinite

4d ago

Q. How do you declare a variable in JavaScript?

Ans.

Variables in JavaScript are declared using the 'var', 'let', or 'const' keywords.

  • Use 'var' to declare a variable with function scope.

  • Use 'let' to declare a variable with block scope.

  • Use 'const' to declare a constant variable.

  • Example: var x = 5; let y = 'hello'; const PI = 3.14;

Asked in Accenture

2d ago

Q. What is the difference between == and ===?

Ans.

The '==' operator checks for equality after type coercion, while '===' checks for equality without type coercion.

  • The '==' operator compares two values after converting them to a common type.

  • The '===' operator compares two values without type conversion, so they must be of the same type to be considered equal.

  • Example: 1 == '1' will return true because the values are equal after type coercion, but 1 === '1' will return false because they are of different types.

Q. What is the map method?

Ans.

The map method creates a new array with the results of calling a provided function on every element in the original array.

  • The map method does not modify the original array.

  • The provided function takes three arguments: currentValue, index, and array.

  • The map method returns a new array with the same length as the original array.

  • Example: const numbers = [1, 2, 3]; const doubledNumbers = numbers.map(num => num * 2); // [2, 4, 6]

1d ago

Q. What is event delegation?

Ans.

Event delegation is a technique where a single event listener is added to a parent element to handle events on its child elements.

  • Improves performance by reducing the number of event listeners

  • Useful for dynamically added elements

  • Event bubbling is utilized

  • Example: Click event on a list of items

Asked in Wingify

2d ago

Q. What is callback hell in JavaScript?

Ans.

Callback hell refers to the situation where multiple nested callbacks make the code difficult to read and maintain.

  • Occurs when multiple asynchronous operations are nested within each other

  • Can lead to deeply nested code structure which is hard to follow

  • Can be avoided by using promises or async/await

Q. What is the useState hook used for in React?

Ans.

useState is a hook in React that allows functional components to have state.

  • useState is used to manage state in functional components

  • It returns an array with two elements: the current state value and a function to update it

  • Example: const [count, setCount] = useState(0);

  • The initial state value is passed as an argument to useState

Asked in Accenture

5d ago

Q. What is event bubbling?

Ans.

Event bubbling is the propagation of an event from the innermost element to the outermost element in the DOM tree.

  • When an event occurs on an element, it first runs the handlers on that element, then on its parent, and so on up to the document object.

  • It allows for event delegation, where a single event handler can be attached to a parent element to handle events for all its children.

  • Event.stopPropagation() can be used to stop the event from bubbling up the DOM tree.

Q. Do you have experience with ServiceNow?

Ans.

ServiceNow is a cloud-based platform that provides IT service management, IT operations management, and IT business management solutions.

  • ServiceNow is commonly used for IT service management, helping organizations streamline their IT processes.

  • It offers features such as incident management, change management, problem management, and asset management.

  • ServiceNow also provides IT operations management capabilities, including monitoring, event management, and automation.

  • The platf...read more

6d ago

Q. What are the differences between HTML and HTML5?

Ans.

HTML5 is the latest version of HTML, introducing new features, elements, and APIs for modern web development.

  • New semantic elements: HTML5 introduced elements like <header>, <footer>, <article>, and <section> for better structure.

  • Multimedia support: HTML5 provides native support for audio and video with <audio> and <video> tags, eliminating the need for plugins.

  • Canvas element: HTML5 includes the <canvas> element for dynamic, scriptable rendering of 2D shapes and bitmap images....read more

Q. Promises , differences between var let and const

Ans.

Promises are used for asynchronous programming in JavaScript. var is function-scoped, let is block-scoped, and const is block-scoped and cannot be reassigned.

  • Promises are used to handle asynchronous operations in JavaScript.

  • var is function-scoped, let is block-scoped, and const is block-scoped and cannot be reassigned.

  • Example: var x = 10; let y = 20; const z = 30;

Asked in Infosys

5d ago

Q. Are you familiar with agile methodologies?

Ans.

Yes, Agile is a project management methodology that emphasizes iterative development and collaboration.

  • Agile is a project management methodology that focuses on iterative development, collaboration, and flexibility.

  • It involves breaking down projects into smaller tasks and completing them in short iterations called sprints.

  • Agile promotes frequent communication and feedback among team members to adapt to changing requirements.

  • Common Agile frameworks include Scrum, Kanban, and E...read more

Asked in Bloowatch

1d ago

Q. Dynamic forms using JavaScript and emberJS

Ans.

Dynamic forms can be created using JavaScript and emberJS to allow for interactive and customizable user input.

  • Use JavaScript to dynamically generate form elements based on user input or predefined conditions

  • Leverage emberJS to handle form data binding and validation

  • Implement event listeners and callbacks to update form elements and perform actions based on user input

2d ago

Q. What is a component in React?

Ans.

A reusable piece of UI that can be composed with other components to form a complete UI.

  • Components are the building blocks of React applications.

  • They can be either functional or class-based.

  • They can have state, props, and lifecycle methods.

  • They can be reused and composed to create complex UIs.

  • Examples include buttons, forms, and navigation menus.

Q. What types of loops are available in JavaScript?

Ans.

Types of loops in JavaScript include for, while, and do-while loops.

  • For loop: Executes a block of code a specified number of times.

  • While loop: Executes a block of code while a specified condition is true.

  • Do-while loop: Executes a block of code once, and then repeats the loop as long as a specified condition is true.

Asked in Aerosol IO

2d ago

Q. UI stuff for report / load large stuff data

Ans.

Use pagination, lazy loading, virtual scrolling, and server-side processing for efficient loading of large data in UI reports.

  • Implement pagination to display data in smaller chunks

  • Utilize lazy loading to load data only when needed

  • Implement virtual scrolling to render only the visible portion of data

  • Use server-side processing to offload heavy data processing to the server

4d ago

Q. What are callback functions?

Ans.

Callback functions are functions passed as arguments to other functions to be executed later.

  • Callback functions are commonly used in asynchronous programming.

  • They are executed after a certain task is completed.

  • Example: setTimeout function in JavaScript takes a callback function as an argument.

3d ago

Q. What are JavaScript libraries?

Ans.

JS libraries are pre-written code that can be used to perform specific tasks in JavaScript.

  • Libraries can save time and effort by providing pre-written code for common tasks

  • Popular JS libraries include jQuery, React, and Angular

  • Libraries can be installed using package managers like npm or yarn

Q. What is the forEach method?

Ans.

The forEach method executes a provided function once for each array element.

  • Used to iterate over elements in an array.

  • Syntax: array.forEach(callback(currentValue, index, array))

  • Does not return a new array; returns undefined.

  • Example: [1, 2, 3].forEach(num => console.log(num)); // Logs 1, 2, 3

  • Cannot be stopped or broken; continues until all elements are processed.

Q. Do you know JavaScript?

Ans.

Yes, I am proficient in JavaScript and have experience in developing web applications using it.

  • Proficient in JavaScript programming language

  • Experience in developing web applications using JavaScript

  • Familiar with popular JavaScript libraries and frameworks like React, Angular, and Node.js

3d ago

Q. What is multiple inheritance and what type is it?

Ans.

Multiple inheritance is when a class can inherit attributes and methods from more than one parent class.

  • Allows a class to inherit from multiple parent classes

  • Can lead to the diamond problem where ambiguity arises if two parent classes have a method with the same name

  • Languages like JavaScript do not support multiple inheritance but can achieve similar functionality using mixins

Asked in Bloowatch

6d ago

Q. How is an Ember app deployed?

Ans.

Ember apps can be deployed using various methods such as hosting on a server, using a cloud platform, or deploying to a content delivery network (CDN).

  • Ember apps can be deployed by hosting them on a server, such as Apache or Nginx.

  • They can also be deployed using cloud platforms like Heroku or AWS.

  • Another option is to deploy the app to a content delivery network (CDN) for faster and more efficient distribution.

  • Deployment can be automated using tools like Ember CLI or continuou...read more

Asked in Paytm

2d ago

Q. Write a polyfill for Promise.all.

Ans.

A polyfill is a code that adds a feature which the browser may not support. Promise.all() polyfill can be used for older browsers.

  • Create a function that takes an array of promises as input

  • Return a new promise that resolves when all promises in the input array have resolved

  • If any promise in the input array rejects, reject the new promise with the reason of the first promise that rejects

  • Use setTimeout and recursion to check if all promises have resolved

  • If the browser supports P...read more

1
2
Next

Interview Experiences of Popular Companies

TCS Logo
3.6
 • 11.1k Interviews
Accenture Logo
3.7
 • 8.7k Interviews
Infosys Logo
3.6
 • 7.9k Interviews
HCLTech Logo
3.5
 • 4.1k Interviews
Deloitte Logo
3.7
 • 3k Interviews
View all

Top Interview Questions for Javascript Developer Related Skills

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

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

Javascript Developer Interview Questions
Share an Interview
Stay ahead in your career. Get AmbitionBox app
play-icon
play-icon
qr-code
Trusted by over 1.5 Crore job seekers to find their right fit company
80 L+

Reviews

10L+

Interviews

4 Cr+

Salaries

1.5 Cr+

Users

Contribute to help millions

Made with ❤️ in India. Trademarks belong to their respective owners. All rights reserved © 2025 Info Edge (India) Ltd.

Follow Us
  • Youtube
  • Instagram
  • LinkedIn
  • Facebook
  • Twitter
Profile Image
Hello, Guest
AmbitionBox Employee Choice Awards 2025
Winners announced!
awards-icon
Contribute to help millions!
Write a review
Write a review
Share interview
Share interview
Contribute salary
Contribute salary
Add office photos
Add office photos
Add office benefits
Add office benefits