Question 1

What is the difference between var, let, and const in JavaScript?

Accepted Answer

var: Function-scoped (or global if declared outside a function). Hoisted to the top of its scope and initialized with undefined. Can be re-declared and re-assigned. No temporal dead zone. Can lead to bugs due to unexpected scoping and hoisting behavior.

let: Block-scoped (limited to the enclosing {} block). Hoisted but NOT initialized (accessing before declaration throws ReferenceError - temporal dead zone). Cannot be re-declared in the same scope. Can be re-assigned. Preferred for variables th

Question 2

Explain the event loop in JavaScript. What are microtasks and macrotasks?

Accepted Answer

JavaScript is single-threaded but handles asynchronous operations through the event loop. The event loop continuously checks if the call stack is empty and, if so, moves tasks from the task queues to the call stack for execution.

Components:
1. Call Stack: Executes synchronous code. Functions are pushed/popped as they are called/return.
2. Web APIs/Node APIs: Handle async operations (setTimeout, fetch, DOM events). Run in separate threads provided by the runtime.
3. Callback Queue (Task Queue /

Question 3

What is the Virtual DOM? How does React use it for performance?

Accepted Answer

The Virtual DOM is a lightweight JavaScript representation (object tree) of the actual DOM. Instead of manipulating the real DOM directly (which is expensive), React maintains a virtual copy in memory and uses it to calculate the minimal set of changes needed.

Why direct DOM manipulation is slow: The real DOM is a complex tree of objects with many properties. Accessing and modifying DOM triggers layout recalculation (reflow), style recalculation, and repainting. These are expensive browser oper

Question 4

Explain closures in JavaScript. Give practical examples.

Accepted Answer

A closure is a function that has access to variables from its outer (enclosing) function scope, even after the outer function has returned. The inner function closes over the variables it references, keeping them alive in memory.

How closures work: When a function is created, it captures a reference to its surrounding lexical environment (the variables in scope at creation time). This environment persists as long as the closure exists, even if the outer function has completed execution.

Simple

Question 5

What is CORS? How does it work and why is it needed?

Accepted Answer

CORS (Cross-Origin Resource Sharing) is a security mechanism implemented by browsers that restricts web pages from making requests to a different domain (origin) than the one that served the page. An origin is defined by the combination of protocol, domain, and port.

Why CORS exists (Same-Origin Policy): Without it, a malicious website could make requests to your bank API using your authenticated session cookies, stealing data or performing actions on your behalf (CSRF attacks). The browser blo

Question 6

What is the difference between cookies, localStorage, and sessionStorage?

Accepted Answer

All three are client-side storage mechanisms in web browsers, but they differ in capacity, lifetime, accessibility, and intended use.

Cookies: Small data (max 4KB per cookie, ~50 per domain). Sent automatically with every HTTP request to the matching domain (adds overhead). Can be set by server (Set-Cookie header) or client (document.cookie). Have expiration dates (persistent) or session lifetime (deleted on browser close). Accessible across subdomains with proper domain attribute. Support secu

Question 7

Explain REST API design principles. What makes an API RESTful?

Accepted Answer

REST (Representational State Transfer) is an architectural style for designing networked applications. A RESTful API adheres to specific constraints that make it scalable, simple, and maintainable.

REST Constraints:

1. Client-Server: Separation of concerns. Client handles UI, server handles data storage and processing. They evolve independently.

2. Stateless: Each request contains all information needed to process it. Server does not store client session state between requests. Improves scala

Question 8

What are Promises in JavaScript? Explain async/await.

Accepted Answer

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation. It provides a cleaner way to handle async code compared to callbacks, avoiding callback hell (deeply nested callbacks).

Promise States:
1. Pending: Initial state, neither fulfilled nor rejected.
2. Fulfilled: Operation completed successfully, promise has a result value.
3. Rejected: Operation failed, promise has a reason (error).

Promise API:
- new Promise((resolve, reject) => { ... }): Creat

Question 9

What is XSS (Cross-Site Scripting)? How can it be prevented?

Accepted Answer

XSS (Cross-Site Scripting) is a security vulnerability where attackers inject malicious scripts into web pages viewed by other users. The injected script executes in the victim browser with the same privileges as the legitimate page, allowing theft of session cookies, personal data, or performing actions on behalf of the user.

Types of XSS:

1. Stored (Persistent) XSS: Malicious script is permanently stored on the target server (database, forum post, comment). Every user who views the affected

Question 10

What is the CSS Box Model? Explain positioning in CSS.

Accepted Answer

The CSS Box Model describes how every HTML element is rendered as a rectangular box with four layers (from inside out):

1. Content: The actual content (text, images). Sized by width/height properties.
2. Padding: Space between content and border. Transparent, shows background color. Adds to element total size (unless box-sizing: border-box).
3. Border: Surrounds the padding. Has width, style, and color.
4. Margin: Space outside the border. Transparent, creates space between elements. Margins co

Web Technologies Interview Questions

What is the difference between var, let, and const in JavaScript?

Explain the event loop in JavaScript. What are microtasks and macrotasks?

What is the Virtual DOM? How does React use it for performance?

Explain closures in JavaScript. Give practical examples.

What is CORS? How does it work and why is it needed?

What is the difference between cookies, localStorage, and sessionStorage?

Explain REST API design principles. What makes an API RESTful?

What are Promises in JavaScript? Explain async/await.

What is XSS (Cross-Site Scripting)? How can it be prevented?

What is the CSS Box Model? Explain positioning in CSS.

Explore More Interview Questions

Web Technologies Interview Questions

What is the difference between var, let, and const in JavaScript?

Explain the event loop in JavaScript. What are microtasks and macrotasks?

What is the Virtual DOM? How does React use it for performance?

Explain closures in JavaScript. Give practical examples.

What is CORS? How does it work and why is it needed?

What is the difference between cookies, localStorage, and sessionStorage?

Explain REST API design principles. What makes an API RESTful?

What are Promises in JavaScript? Explain async/await.

What is XSS (Cross-Site Scripting)? How can it be prevented?

What is the CSS Box Model? Explain positioning in CSS.

Explore More Interview Questions