Full-Stack Developer Interview Questions India 2026: Complete Guide

React fundamentals every full-stack interview tests: Component lifecycle with hooks. useEffect: when does it run (after every render by default), how the dependency array changes this behaviour (run only when dependencies change), the cleanup function (returned from useEffect, runs before the next effect or on unmount — critical for clearing timers, subscriptions, event listeners). useState vs useRef: useState triggers re-render on change, useRef does not — use useRef for values you want to persist across renders without triggering re-render (timer IDs, DOM element references, previous prop values). React performance: React.memo (skip re-render if props unchanged), useMemo (memoize expensive calculation, recompute only when dependencies change), useCallback (memoize function reference, prevent child re-renders caused by new function instances on every parent render). Code splitting: React.lazy + Suspense for route-level code splitting, reducing initial bundle size. Accessibility (a11y): ARIA roles and attributes, keyboard navigation (all interactive elements must be focusable and operable with Enter/Space), screen reader compatibility (semantic HTML: use button not div for clickable elements).

REST API design: resource naming (nouns not verbs: /orders not /getOrders), HTTP method semantics (GET for reads, POST for creation, PUT for full update, PATCH for partial update, DELETE), HTTP status codes (200 OK, 201 Created, 400 Bad Request, 401 Unauthorized vs 403 Forbidden vs 404 Not Found, 422 Unprocessable Entity for validation errors, 429 Too Many Requests, 500 Internal Server Error), pagination (cursor-based pagination for large datasets vs offset pagination: why cursor is preferred for sorted, frequently-updated datasets). Node.js: the event loop (single-threaded, non-blocking I/O — explain why a CPU-intensive function blocks the event loop while an async I/O operation does not), Promise chaining vs async/await (both compile to the same execution model, async/await is more readable), worker threads for CPU-intensive work. Database: SQL joins (INNER, LEFT, RIGHT, FULL OUTER — write a query that finds users who have never placed an order), indexing strategy (which columns to index: high-cardinality columns used in WHERE clauses and JOIN conditions), transactions (ACID: atomicity, consistency, isolation, durability — give an example where each matters), N+1 query problem (when an ORM makes one query to fetch N records, then N queries to fetch related data — how to solve with eager loading or a JOIN).

Full-stack system design is distinct from pure backend system design: interviewers want to see how you think about a feature from database to UI, including client-server interaction. Common question format: 'Design the comment system for an e-commerce product page with real-time updates.' Expected coverage: Database schema (comments table with productid, userid, body, createdat, parentid for threads; index on product_id for fast retrieval). REST API (GET /products/:id/comments with cursor pagination, POST /products/:id/comments for creation, WebSocket or Server-Sent Events channel for real-time new comment delivery). Backend (Node.js + Express or Next.js API routes, PostgreSQL via Prisma ORM, Redis pub/sub for real-time fan-out). Frontend (React with optimistic UI — immediately show the new comment on submit before the server confirms, revert if the server returns an error). Caching (cache the first page of comments in Redis, invalidate on new comment). Moderation (content policy filter before persistence, report endpoint for user-flagged comments). Covering all these layers without prompting signals strong full-stack thinking.

Three full-stack projects that impress interviewers at Indian startups and scale-ups: (1) A multi-user SaaS with authentication: notes app or project management tool with user accounts, team workspaces, and basic permissions. Tech: Next.js (App Router), PostgreSQL (via Supabase or Neon), Clerk or Auth.js for auth, deployed on Vercel. This demonstrates full-stack ownership of a realistic product. (2) An e-commerce platform with UPI/Razorpay payment integration: shows checkout flow, inventory management, order state machine, and payment webhook handling. Real payment integration is rare in portfolios and highly memorable to interviewers. (3) A real-time collaborative feature: a shared document editor (ProseMirror + Y.js for CRDT-based collaboration), a chat system (Socket.io + Redis pub/sub), or a live dashboard (Server-Sent Events). Demonstrates WebSocket knowledge, optimistic UI, and real-time database patterns. Full-stack salary benchmarks 2026: junior (0-2 years): Rs 8-20 LPA at product companies. Mid-level (2-5 years): Rs 20-50 LPA. Senior full-stack (5-8 years): Rs 45-90 LPA. Full-stack premium over single-layer specialists: 10-20% at the same experience level.