Preparing for a frontend developer interview? This guide covers 50 essential frontend interview questions and answers to help you get hired. Whether you’re a beginner or an experienced developer, these questions will boost your confidence and improve your chances of success.
1. What is the difference between div and span ?
Answer:
- <div> is a block-level element that takes up the full width of its parent container, while <span> is an inline element that only takes up as much width as needed.
2. What is the difference between id and class attributes in HTML?
Answer:
- id is unique and can only be used once per page, whereas class can be assigned to multiple elements to apply common styles.
3. What are semantic elements in HTML?
Answer:
- Semantic elements, like <header>, <section>, and <article>, give meaning to the content, improving SEO and accessibility.
4. What is the difference between strong and b?
Answer:
- <strong> indicates importance and affects SEO, while <b> only makes text bold without adding meaning.
5. How do you optimize HTML for SEO?
Answer:
- Use semantic tags, meta tags, proper headings, alt attributes for images, and structured URLs.
6. What are the different types of CSS?
Answer:
- CSS can be applied using inline styles, internal styles (inside <style>), and external stylesheets (.css file).
7. What is the difference between absolute, relative, fixed, and sticky positioning?
Answer:
- Relative: Positioned relative to its normal place.
- Absolute: Positioned relative to the nearest positioned ancestor.
- Fixed: Stays at a fixed position on the screen.
- Sticky: Acts as relative until scrolling reaches a specific position, then sticks.
8. What is the difference between px, em, rem, and %?
Answer:
- px is fixed.
- em is relative to the parent’s font size.
- rem is relative to the root element’s font size.
- % is relative to the parent’s dimensions.
9. What is the difference between Flexbox and Grid?
Answer:
- Flexbox is best for one-dimensional layouts (rows or columns), while Grid is better for two-dimensional layouts (both rows and columns).
10. What is the difference between pseudo-classes and pseudo-elements?
Answer:
- Pseudo-classes (like :hover, :nth-child) apply styles based on state or position.
- Pseudo-elements (like ::before, ::after) create elements without adding extra HTML.
11. What are JavaScript data types?
Answer:
- JavaScript has primitive types (string, number, boolean, null, undefined, symbol, bigint) and non-primitive types (object, array, function).
12. What is the difference between var, let, and const?
Answer:
- var is function-scoped and can be redeclared.
- let is block-scoped and can be reassigned.
- const is block-scoped and cannot be reassigned.
13. What is a closure in JavaScript?
Answer:
- A closure is when a function remembers variables from its outer function even after the outer function has finished executing.
14. What is the difference between == and ===?
Answer:
- == checks only values (allows type conversion), while === checks both values and types (strict comparison).
15. What is event delegation in JavaScript?
Answer:
- Event delegation allows handling events efficiently by attaching a single event listener to a parent element instead of multiple child elements.
16. What is the difference between synchronous and asynchronous JavaScript?
Answer:
- Synchronous: Executes line by line.
- Asynchronous: Executes independently, allowing non-blocking operations (e.g., setTimeout, fetch).
17. What are Promises in JavaScript?
Answer:
- A Promise represents an asynchronous operation that can be in pending, fulfilled, or rejected states.
18. What is the difference between map(), filter(), and reduce()?
Answer:
- map(): Transforms each item in an array.
- filter(): Returns items that meet a condition.
- reduce(): Accumulates values into a single result.
19. What is debouncing and throttling?
Answer:
- Debouncing: Delays execution until after a pause in events (e.g., typing in a search box).
- Throttling: Ensures a function runs only once within a fixed time interval.
20. What is async/await?
Answer:
- async/await is a modern way to handle asynchronous code, making it look like synchronous code while still being non-blocking.
21. What is async/await?
Answer:
- React is a JavaScript library for building reusable and dynamic UI components.
22. What is the Virtual DOM in React?
Answer:
- A Virtual DOM is a lightweight copy of the real DOM that helps React update only the necessary parts of the UI efficiently.
23. What are props in React?
Answer:
- Props (short for “properties”) are used to pass data from parent components to child components.
24. What is the difference between state and props?
Answer:
- Props: Passed from parent to child and cannot be changed by the child.
- State: A component’s own data, which can change over time.
25. What is useState in React?
Answer:
- useState is a Hook that allows functional components to manage state.
26. What is the difference between functional and class components?
Answer:
- Functional components: Simpler and use hooks.
- Class components: Use lifecycle methods and this.state.
27. What is useEffect in React?
Answer:
- useEffect is a Hook used for performing side effects (like fetching data or updating the DOM).
28. What is React Router?
Answer:
- React Router is a library that enables navigation between pages in a React app.
29. What is lazy loading in React?
Answer:
- Lazy loading loads components only when needed, improving performance.
30. What is the Context API?
Answer:
- The Context API allows global state management without passing props manually at every level.
31. How do you optimize a React app?
Answer:
- Use memoization (useMemo, useCallback), lazy loading, and avoid unnecessary re-renders.
32. What is a Pure Component in React?
Answer:
- A component that only re-renders when its props or state change.
33. How can you improve website performance?
Answer:
- Minimize CSS/JS files, enable lazy loading, optimize images, use caching, and reduce HTTP requests.
34. What is code splitting in React?
Answer:
- Breaking large JavaScript files into smaller chunks to improve loading speed.
35. What are Service Workers?
Answer:
- Service workers allow background tasks like caching and offline functionality in web applications.
36. What is the difference between LocalStorage, SessionStorage, and Cookies?
Answer:
- LocalStorage: Persistent storage.
- SessionStorage: Storage that clears when the session ends.
- Cookies: Small data sent with requests, used for authentication.
37. What is CORS?
Answer:
- Cross-Origin Resource Sharing allows a webpage to request resources from a different domain securely.
38. What is the difference between HTTP and HTTPS?
Answer:
- HTTPS is the secure version of HTTP, encrypting data using SSL/TLS.
39. What is CSRF?
Answer:
- Cross-Site Request Forgery is an attack that tricks a user into performing unwanted actions. It can be prevented using CSRF tokens.
40. What is XSS?
Answer:
- Cross-Site Scripting allows attackers to inject malicious scripts. Prevent it by sanitizing user input.
41. What are some popular testing libraries for React?
Answer:
Some common testing libraries include:
- Jest: A widely used JavaScript testing framework.
- React Testing Library: Helps test React components in a user-friendly way.
- Cypress: Used for end-to-end testing of web applications.
42. What is unit testing?
Answer:
- Unit testing is the process of testing individual functions or components in isolation to ensure they work correctly.
43. What is Continuous Integration (CI)?
Answer:
- CI is the practice of automatically testing and merging code changes using tools like GitHub Actions, Travis CI, or Jenkins to detect errors early.
44. How do you deploy a React app?
Answer:
A React app can be deployed using platforms like:
- Vercel (Simple, fast, supports Next.js)
- Netlify (Good for static websites)
- Firebase Hosting (Google’s cloud hosting)
- GitHub Pages (For simple, free hosting)
45. What is Lighthouse?
Answer:
- Lighthouse is a tool by Google that analyzes webpage performance, SEO, accessibility, and best practices, helping developers improve website quality.
46. What is TypeScript and why use it?
Answer:
- TypeScript is a superset of JavaScript that adds static typing, helping catch errors early and making code easier to maintain.
47. What is a Progressive Web App (PWA)?
Answer:
- A PWA is a web app that behaves like a native app, offering offline support, push notifications, and fast performance using Service Workers.
48. What is Webpack?
Answer:
- Webpack is a JavaScript module bundler that processes and optimizes JavaScript, CSS, and images for faster and more efficient loading.
49. What is Babel?
Answer:
- Babel is a JavaScript compiler that converts modern ES6+ code into older versions for compatibility with older browsers.
50. What is JAMstack?
Answer:
JAMstack is a modern web architecture based on:
- JavaScript (for dynamic features)
- APIs (to handle backend logic)
- Markup (pre-built HTML for fast loading)
It makes websites faster, more scalable, and secure.
Check out more frontend interview tips and resources on My Website DesignWithRehana and don’t forget to watch our in-depth tutorials on My YouTube Channel for better understanding!
Very helpful content, Thank You
Thankyou so much Arun 🤩