frontend interview questions

Top 20 Frontend Developer Interview Questions (2025) Himanshu Nigam April 4, 2025 Agar aap frontend development ka interview dene ja rahe ho, to yeh 20 questions zaroor padho. In mein basic HTML, CSS, JavaScript aur responsive design ke important concepts cover kiye gaye hain. Chaliye shuru karte hain:- 1. What is HTML ? Answer: HTML ka full form hai HyperText Markup Language. Ye web pages ka structure define karta hai jaise headings, paragraphs, images, forms, etc. 2. What is CSS ? Answer: CSS ka matlab hai Cascading Style Sheets. Ye HTML elements ko style dene ke liye use hota hai – jaise colors, font, spacing, layout. 3. What is JavaScript ? Answer: JavaScript ek scripting language hai jo web page ko interactive banata hai. Jaise button click, animations, dynamic updates bina page reload kiye. 4. Difference between ID and Class in CSS? Answer: ID unique hoti hai (#idname), ek page mein sirf ek baar use hoti hai. Class (.classname) multiple elements mein use ho sakti hai. 5. What is Box Model ? Answer: Box Model web elements ke 4 parts define karta hai – content, padding, border, aur margin. Ye samajhna zaroori hai layout ke liye. 6. What is Responsive Design ? Answer: Responsive design ka matlab hai ki website har device pe (mobile, tablet, desktop) achhi dikhe aur sahi tarike se kaam kare. 7. What is Media Queries ? Answer: CSS Media Queries responsive design ke liye use hoti hain. Ye screen size ke hisaab se alag styles apply karti hain. @media (max-width: 600px) {  body { background-color: lightblue; }} 8. Difference between == and === in JavaScript? Answer: == values compare karta hai (type convert karke). === strict comparison karta hai (value aur type dono match hone chahiye). 9. What is Semantic ? Answer: Semantic HTML wo elements hote hain jo content ka meaning batate hain. Jaise <article>, <section>, <header>, <footer> etc. 10. What is the use of ‘this’ keyword in JavaScript? Answer:  this keyword current object ko refer karta hai. Iska meaning depend karta hai function kis context mein call hua. 11. JavaScript mein DOM kya hota hai? Answer: DOM (Document Object Model) ek tree structure hai jo HTML elements ko represent karta hai. JavaScript se hum DOM ko access aur manipulate karte hain. 12. Difference between let, var and const? Answer: var function-scoped hota hai. let block-scoped hota hai. const block-scoped hota hai but ismein value reassign nahi kar sakte. 13. What is Event Delegation? Answer: Event Delegation mein parent element pe event listener lagaya jata hai, aur woh child elements ke events ko handle karta hai. 14. Forms mein GET aur POST ka difference? Answer: GET: data URL ke through send hota hai, visible hota hai. POST: data hidden hota hai, secure hota hai (sensitive data ke liye better). 15. What are pseudo-classes in CSS? Answer: Pseudo-classes jaise :hover, :focus, etc. user interaction ke basis par element ka style change karti hain. 16. What is the difference between inline, block, and inline-block elements? Answer: Inline: width, height set nahi hota (e.g. span) Block: full width le leta hai (e.g. div) Inline-block: inline bhi hai aur width/height bhi set ho sakta hai 17. LocalStorage vs SessionStorage? Answer: LocalStorage: data permanently store hota hai jab tak manually delete na karein. SessionStorage: data tab tak store hota hai jab tak browser tab open ho. 18. What is Flexbox in CSS? Answer: Flexbox ek layout model hai jo responsive layouts banane mein help karta hai. Ye elements ko easily align aur space karta hai. display: flex;justify-content: center;align-items: center;ge: data tab tak store hota hai jab tak browser tab open ho. 19. How do you optimize frontend performance? Answer: Images compress karo CSS & JS minify karo Lazy loading use karo CDN se files serve karo FAQ How to Prepare for Frontend Interview? HTML, CSS, JS basics clear rakho Projects banao GitHub pe code share karo Mock interviews do Frameworks jaise React ka basic idea rakho