💻Technical Questions
Q1What is the difference between == and === in JavaScript?
💡Type coercion with ==. Strict equality with ===. Always use === unless you explicitly need coercion.
Q2Explain the JavaScript event loop and how async/await works under the hood.
💡Call stack, task queue, microtask queue. How Promises go to microtask queue (higher priority). Practical implications for UI rendering.
Q3What are React hooks and why were they introduced?
💡useState, useEffect, useCallback, useMemo, useRef. Hooks replaced class components to share stateful logic without HOCs/render props.
Q4Explain virtual DOM and reconciliation in React.
💡Diffing algorithm, fiber architecture, keys in lists. Why keys must be stable IDs, not array indexes.
Q5What is the difference between useMemo and useCallback?
💡useMemo memoizes a value; useCallback memoizes a function. When to use: expensive computations and stable function references for child components.
Q6How do you optimize the performance of a React application?
💡Code splitting (lazy/Suspense), React.memo, useMemo/useCallback, virtualization (react-window), image optimization, bundle analysis.
Q7What is CSS specificity and how is it calculated?
💡Inline styles (1000) > IDs (100) > classes/attributes (10) > elements (1). !important overrides all. Common gotchas in large codebases.
Q8What are Web Vitals and why do they matter?
💡LCP, FID (now INP), CLS. Google ranking signal. How to measure (Lighthouse, CrUX) and what affects each metric.
🧠Behavioral Questions
B1Tell me about a time you improved the performance of a web application.
💡Specific metric (LCP, bundle size, TTI). What you measured, changed, and the before/after numbers.
B2Describe a complex UI you built from scratch. What were the challenges?
💡Architecture decisions, state management, accessibility, responsive design, cross-browser compatibility.
B3How do you stay updated with the rapidly changing frontend ecosystem?
💡Blogs, Twitter/X, GitHub releases, side projects. Show genuine curiosity. Mention specific resources you follow.
🎯Situational Questions
S1Your React app has a memory leak causing slowdowns after extended use. How do you debug it?
💡Chrome DevTools Memory tab, heap snapshots, useEffect cleanup functions, event listener removal, setInterval clearance.
S2Design a real-time collaborative editor (like Google Docs) for the frontend.
💡Operational transforms or CRDTs, WebSocket connection, cursor tracking, conflict resolution, offline sync.
Must-Know Topics
- ✓JavaScript (closures, prototypes, async)
- ✓React (hooks, reconciliation, patterns)
- ✓HTML5 Semantic elements, accessibility
- ✓CSS (Flexbox, Grid, animations)
- ✓Browser APIs (Storage, Fetch, WebSockets)
- ✓Web Performance (Core Web Vitals)
- ✓Testing (Jest, RTL, Cypress)
- ✓Build tools (Webpack/Vite, Babel)
Common Interview Mistakes to Avoid
- ✗Using array index as key in React lists
- ✗Missing useEffect dependency arrays (infinite loops)
- ✗Not handling loading/error states in async operations
- ✗Ignoring accessibility (ARIA, keyboard navigation)
- ✗Not considering mobile/responsive in component design
Frequently Asked Questions
What React concepts are most commonly tested in frontend interviews?▼
Hooks (useState, useEffect, useMemo, useCallback), virtual DOM/reconciliation, component lifecycle, state management patterns, and performance optimization (React.memo, code splitting).
Do frontend developers need to know DSA for interviews?▼
At product companies (Swiggy, Razorpay, Atlassian), a basic DSA round is common — arrays, strings, trees. You won't need DP or graphs typically. Focus on JS-specific problems (closures, async, event loop) over pure algorithms.
Is TypeScript required for frontend developer interviews?▼
TypeScript is strongly preferred at most product companies in 2025. If you know JavaScript well, TypeScript is a 2–3 week learning curve. Most companies will test your React skills in TypeScript during live coding.
What system design topics should a frontend developer know?▼
Frontend-specific: designing a news feed, infinite scroll, real-time chat, autocomplete, image carousel, drag-and-drop. Focus on component architecture, state management, API design, and performance trade-offs.
How important is CSS knowledge in 2025 frontend interviews?▼
CSS is tested at most companies — especially specificity, Flexbox/Grid, animations, and responsive design. Some companies (Figma, Canva, design-tool companies) test CSS extensively. Know the box model, positioning, and media queries cold.
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