Top 250 JavaScript Interview Questions and Answers

Slice method returns a shallow copy of a portion of an array into a new array, while splice method changes the contents of an array by removing or replacing existing elements.

• Slice method does not modify the original array, while splice method does

• Slice method takes two arguments - start and end index, while splice method takes three arguments - start index, number of elements to remove, and optional elements to add

• Example of slice method: let arr = [1, 2, 3, 4, 5]; let newAr...read more

diff var let const

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

• var can be redeclared and reassigned, let can be reassigned but not redeclared, const cannot be reassigned or redeclared

• const must be initialized during declaration, let and var can be declared without initialization

Hosting in JavaScript refers to the process of declaring a variable or function before it is used in the code.

• Hosting allows you to use variables and functions before they are declared in the code.

• Variables are hoisted to the top of their scope, while functions are fully hoisted.

• Example: console.log(myVar); var myVar = 10; This will not throw an error because 'myVar' is hoisted to the top of the scope.

Creating a table in JavaScript involves using HTML elements like

JS is a scripting language used for web development. React JS is a JavaScript library used for building user interfaces.

• JS is a client-side scripting language used for web development.

• React JS is a JavaScript library used for building user interfaces.

• React JS allows for the creation of reusable UI components.

• React JS uses a virtual DOM to optimize performance.

• React JS is maintained by Facebook and has a large community of developers.

JavaScript event loop manages the execution of multiple tasks in a single-threaded environment.

• JavaScript is single-threaded, meaning it can only execute one task at a time.

• The event loop continuously checks the call stack for tasks to execute.

• Tasks are added to the call stack from the callback queue once the call stack is empty.

Web development using JavaScript and Python involves using both languages to create dynamic and interactive websites.

• JavaScript is used for client-side scripting to make web pages interactive.

• Python can be used for server-side scripting to handle backend logic and data processing.

• Both languages can be used together in a full-stack web development project.

• Examples: Using JavaScript for front-end validation and Python for backend API development.

One of the most commonly used design patterns in JavaScript is the Module pattern.

• Encapsulates code into a single object, providing a way to create private and public methods and variables

• Helps in organizing code, reducing global scope pollution, and promoting reusability

• Example: var Module = (function() { var privateVar = 'private'; return { publicMethod: function() { console.log(privateVar); } }; })();

JS basics and React JS are essential for web development.

• JS basics include variables, data types, functions, and control structures.

• React JS is a popular JavaScript library for building user interfaces.

• React components, state, and props are important concepts to understand.

• JSX is a syntax extension used in React to write HTML-like code.

• React Router is a library used for client-side routing in React applications.

Polyfills of bind method provide a way to use the bind method in older browsers that do not support it natively.

• Polyfill code typically checks if the bind method is available and if not, it creates a new function that mimics the behavior of bind.

• One common polyfill for the bind method is the following: Function.prototype.bind = Function.prototype.bind || function() { // polyfill code here };

• Polyfills are often used to ensure compatibility with older browsers that do not suppo...read more

State polyfill is a tool used to provide support for the latest JavaScript features in older browsers.

• State polyfills are used to fill in the gaps in browser support for newer JavaScript features.

• They can be included in a project using a package manager like npm or yarn.

• Examples of state polyfills include core-js and babel-polyfill.

Shadowing in JavaScript is when a variable declared within a certain scope has the same name as a variable in an outer scope.

• Occurs when a variable in a local scope has the same name as a variable in a parent scope

• The local variable takes precedence over the parent variable within the local scope

• Can cause confusion and unexpected behavior if not handled properly

You can add a Javascript file to HTML by using the <script> tag with the src attribute pointing to the file's location.

• Use the <script> tag in the HTML file.

• Add the src attribute to the <script> tag with the path to the Javascript file.

• Place the <script> tag in the <head> or <body> section of the HTML file.

Yes, I have worked on JavaScript.

• I have experience in using JavaScript for front-end development.

• I have worked with jQuery library for DOM manipulation and AJAX requests.

• I have also used JavaScript for form validation and creating interactive UI elements.

• I am familiar with ES6 features like arrow functions, template literals, and let/const declarations.

Yes, it is possible to create a slideshow without using JavaScript.

• Use CSS animations and transitions to create the slideshow.

• Use HTML and CSS to structure and style the slideshow.

• Utilize keyframe animations to create the slide transitions.

• Use radio buttons or checkboxes to control the slideshow navigation.

• Apply the :target pseudo-class to create a simple slideshow without JavaScript.

Server side rendering is implemented by pre-rendering the initial state of a web page on the server before sending it to the client.

• Use a server-side rendering framework like Next.js or Nuxt.js

• Create components that can be rendered on the server using Node.js or other server-side technologies

• Optimize the server-side rendering process to improve performance

Use useState hook to manage state in functional components

• Import useState hook from React

• Declare state variable using useState hook

• Use state variable and setState function to update state

• Pass state variable and setState function as props to child components if needed

Create a profile using CSS and JavaScript

• Use CSS to style the profile layout

• Use JavaScript to add interactive elements like buttons or animations

• Include a profile picture, name, bio, and contact information

• Add hover effects or transitions for a polished look

The .live() method attaches an event handler to the current and future elements, while .delegate() attaches an event handler to a parent element.

• The .live() method is deprecated in newer versions of jQuery.

• The .delegate() method is preferred over .live() for performance reasons.

• Both methods are used to handle events for dynamically added elements.

• Example: $(document).live('click', 'button', function() { ... });

• Example: $(document).delegate('button', 'click', function() { ... ...read more

Traditional functions are defined using the function keyword, while arrow functions are defined using a concise syntax with =>.

• Traditional functions are hoisted, while arrow functions are not.

• Arrow functions do not have their own 'this' keyword, they inherit it from the parent scope.

• Arrow functions are more concise and easier to read compared to traditional functions.

• Traditional functions are better for methods in objects, while arrow functions are better for callbacks or eve...read more

Angular is a popular front-end framework for building dynamic web applications, while JavaScript is a programming language commonly used for web development.

• Angular is a front-end framework developed by Google for building single-page applications

• JavaScript is a programming language used for adding interactivity to web pages

• Angular uses TypeScript, a superset of JavaScript, for development

• JavaScript is widely used for client-side scripting and server-side development with Nod...read more

Java is a programming language used for building applications, while JavaScript is a scripting language primarily used for web development.

• Java is a statically typed language, while JavaScript is dynamically typed.

• Java is compiled and runs on the Java Virtual Machine (JVM), while JavaScript is interpreted by the browser.

• Java is used for building standalone applications, server-side applications, and Android apps, while JavaScript is mainly used for client-side web development...read more

Node.js is a runtime environment for executing JavaScript code outside of a web browser.

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

• Node.js allows JavaScript to be run on the server-side, enabling backend development.

• Node.js has a built-in module system and package manager called npm.

• Traditional JavaScript is typically used for client-side scripting within web browsers.

Promises are used in Node to handle asynchronous operations and avoid callback hell.

• Promises simplify error handling and make code more readable.

• Promises allow for chaining multiple asynchronous operations.

• Promises can be used with async/await to write synchronous-looking code.

• Example: fetching data from an API using axios library with promises.

• Example: reading a file with promises in Node's fs module.

Arrow functions are a shorthand way of writing functions in JavaScript.

• Arrow functions have a shorter syntax than traditional functions.

• They do not have their own 'this' keyword.

• They are best suited for non-method functions.

• They are commonly used with array methods like map, filter, and reduce.

• Example: const add = (a, b) => a + b;

ES6 features are new functionalities added to JavaScript in ECMAScript 6 version.

• Arrow functions for concise syntax

• Let and const for block-scoped variables

• Classes for object-oriented programming

• Template literals for string interpolation

• Destructuring for easy assignment of variables

Arrow functions are concise syntax for writing functions in ES6. They do not have their own 'this' keyword.

• Arrow functions do not have their own 'this' keyword, they inherit 'this' from the parent scope.

• Normal function expressions have their own 'this' keyword, which is determined by how the function is called.

• Arrow functions are more concise and have implicit return, while normal functions require explicit return statement.

JavaScript is a dynamically typed language, while TypeScript is a statically typed superset of JavaScript.

• JavaScript is dynamically typed, meaning variable types are determined at runtime.

• TypeScript is statically typed, allowing for type checking at compile time.

• TypeScript supports features like interfaces, enums, and type annotations that are not present in JavaScript.

• TypeScript code needs to be transpiled to JavaScript before it can be executed in a browser.

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

let, var, and const are all used to declare variables in JavaScript, but they have different scopes and behaviors.

• let: block-scoped variable, can be reassigned

• var: function-scoped variable, can be reassigned

• const: block-scoped variable, cannot be reassigned, but its properties can be modified

You can change the class of an element using JavaScript by accessing the element and modifying its class attribute.

• Access the element using document.getElementById(), document.getElementsByClassName(), document.querySelector(), etc.

• Use the element's classList property to add, remove, or toggle classes.

• Example: document.getElementById('myElement').classList.add('newClass');

Arrow functions are used for concise syntax, lexical scoping of 'this', and implicit return of single expressions.

• Arrow functions have a more concise syntax compared to traditional function expressions.

• Arrow functions do not bind their own 'this' value, instead they inherit 'this' from the containing scope.

• Arrow functions automatically return the result of a single expression without needing the 'return' keyword.

ReactJs is a JavaScript library for building user interfaces.

• React uses a virtual DOM to improve performance by updating only the necessary components.

• Components in React are reusable and can be composed together to build complex UIs.

• React allows for declarative programming, making it easier to understand and maintain code.

• State management in React can be handled using hooks or external libraries like Redux.

• React Router is used for client-side routing in single-page applicati...read more

Event loop in JavaScript manages asynchronous operations by executing callback functions in a queue.

• Event loop is responsible for handling asynchronous operations in JavaScript.

• It continuously checks the call stack and the callback queue to see if there are any functions that need to be executed.

• If the call stack is empty, the event loop will take a function from the callback queue and push it onto the call stack for execution.

• Example: setTimeout() function in JavaScript uses...read more

Data transfer in ReactJs involves passing data between components using props and state.

• Props are used to pass data from parent to child components

• State is used to manage data within a component and can be passed down as props

• Data can also be transferred using context API or Redux for global state management

A polyfill is a piece of code that provides the technology that you expect the browser to provide natively.

• Polyfills are used to fill in the gaps in browser support for certain features.

• They allow developers to use new features in older browsers.

• Examples of polyfills include Babel for ES6 features and HTML5 Shiv for HTML5 elements in older browsers.

Event loop is a mechanism in JavaScript that handles asynchronous operations.

• JavaScript is a single-threaded language, meaning it can only execute one task at a time.

• Asynchronous programming allows multiple tasks to be executed simultaneously.

• Event loop is responsible for managing the execution of asynchronous code.

• Callbacks, Promises, and Async/Await are common ways to handle asynchronous code in JavaScript.

let, var, and const are all used to declare variables in JavaScript, but they have different scopes and behaviors.

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

• Using let:

• let x = 10;

• x = 20; // valid

• Using var:

• var y = 5;

• y = 8; // valid

• Using const:

• const z = 15;

• z = 25; // error, cannot reassign const variable

Use window.location.href to get the URL of the current page in JavaScript

• Access the URL using window.location.href

• Alternatively, use document.URL to get the URL

• You can also use window.location.toString() to get the URL

Local storage is a web storage mechanism that allows data to be stored in the user's browser.

• Local storage is a key-value storage system that stores data with no expiration date.

• Data stored in local storage persists even after the browser is closed.

• It is limited to about 5MB of data per domain.

• Local storage is accessed using the localStorage object in JavaScript.

• Example: localStorage.setItem('key', 'value') to store data, localStorage.getItem('key') to retrieve data.

forwardRef is a function in React that allows you to access a child component's instance before it is rendered.

• Used for accessing child component's instance before it is rendered

• Helps in resolving circular dependencies in components

• Commonly used in higher-order components and custom hooks

Jquery is a JavaScript library that simplifies HTML document traversing, event handling, and animation.

• Jquery is a lightweight library that simplifies JavaScript programming.

• It allows easy manipulation of HTML documents and event handling.

• Jquery uses CSS selectors to select elements and perform actions on them.

• It has a wide range of plugins available for various functionalities.

• Jquery is compatible with all major browsers and versions.

Call, bind, and apply are methods used to manipulate the value of 'this' in JavaScript functions.

• Call - invokes a function with a specified 'this' value and arguments provided individually.

• Bind - creates a new function that, when called, has its 'this' keyword set to the provided value.

• Apply - invokes a function with a specified 'this' value and arguments provided as an array.

JS is a scripting language used for web development. Angular is a JS framework for building web applications.

• JS is used for adding interactivity to web pages

• JS has data types like string, number, boolean, etc.

• Angular is a framework for building single-page applications

• Angular uses components, services, and directives for building applications

• Angular has features like dependency injection, routing, and forms

JavaScript is a programming language commonly used for web development. It is different from Java in terms of syntax, purpose, and usage.

• JavaScript is a scripting language primarily used for client-side web development.

• Java is a general-purpose programming language often used for server-side applications.

• JavaScript code is executed on the client's browser, while Java code is executed on the server.

• JavaScript is dynamically typed, while Java is statically typed.

• JavaScript is o...read more

Nodejs is a runtime environment for executing JavaScript code outside of a web browser.

• Nodejs is built on top of the V8 JavaScript engine

• JavaScript is a programming language used for both front-end and back-end development

• Nodejs allows for server-side scripting and can be used to build scalable network applications

• JavaScript is primarily used for client-side scripting in web browsers

promise.all waits for all promises to resolve, while promise.race waits for the first promise to resolve or reject

• promise.all resolves when all promises in the iterable have resolved

• promise.race resolves or rejects as soon as one of the promises in the iterable resolves or rejects

• Example: Promise.all([promise1, promise2, promise3]) will wait for all three promises to resolve before resolving itself

• Example: Promise.race([promise1, promise2, promise3]) will resolve or reject as...read more