Frontend Engineer Interview Questions India 2026: Complete React and JavaScript Guide

Five question types in Indian frontend engineer interviews: (1) JavaScript fundamentals (the most predictable category: 'Explain the event loop and call stack', 'What is the difference between == and ===?', 'Explain closures with a practical example', 'What is the difference between null and undefined?', 'How does Promise.all differ from Promise.allSettled, Promise.race, and Promise.any?'). (2) React depth ('Explain when a component re-renders and how to prevent unnecessary re-renders', 'What is the difference between useMemo, useCallback, and React.memo?', 'Implement a custom hook that debounces a search input', 'Explain what useEffect cleanup does and when you need it'). (3) CSS and layout ('Flex vs grid: when would you use each?', 'Explain CSS specificity with an example of a cascade conflict', 'What is the critical rendering path and how does CSS affect time-to-first-paint?'). (4) Performance ('A React page with 500 list items is slow: diagnose and fix', 'What is code splitting and how do React.lazy and Suspense implement it?', 'Explain Core Web Vitals: LCP, FID/INP, CLS and how to measure each'). (5) Frontend system design ('Design the state management for a multi-step checkout flow with 5 steps, validation, and server-side persistence at each step', 'Design the architecture for a real-time collaborative document editor in the browser').

JavaScript depth tested at Indian product companies: Closures (a function that retains access to its lexical scope even after the outer function has returned; the basis of module patterns, the factory function pattern, and memoisation). Prototype chain (JavaScript inheritance: every object has a [[Prototype]] pointing to its prototype object; class syntax is syntactic sugar over prototype-based inheritance; interviewers probe this by asking you to implement inheritance without the class keyword). Event loop mechanics (the call stack for synchronous code, the task queue for setTimeout/setInterval callbacks, the microtask queue for Promise resolutions and queueMicrotask; Promises go to the microtask queue and run before the next setTimeout callback, regardless of which was enqueued first). this binding (four binding rules: default, implicit, explicit via call/apply/bind, and new binding; arrow functions have no own this and inherit it lexically). TypeScript generics (function generics, class generics, conditional types, mapped types, utility types: Partial, Required, Readonly, Pick, Omit, ReturnType, InstanceType; TypeScript depth is increasingly a differentiator at senior level at Indian product companies).

React depth by level: Junior (0-2 years): component lifecycle understanding (componentDidMount = useEffect with empty deps, componentDidUpdate = useEffect with deps, componentWillUnmount = useEffect cleanup). useState and useEffect basics. Controlled vs uncontrolled components. Mid-level (2-5 years): reconciliation algorithm (how React diffs the virtual DOM to compute minimal DOM updates; keys: why they must be unique and stable, not array indices for dynamic lists). Custom hooks for reusable stateful logic. Context API for shared state and when it causes unnecessary re-renders (any context consumer re-renders when any value in the context changes). useMemo and useCallback: when they help (expensive computation, callback identity for child component props) vs when they hurt (overhead of memoisation exceeds the benefit for cheap computations). Senior (5+ years): Concurrent React (React 18: useTransition marks an update as non-urgent so React can interrupt and resume it; useDeferredValue defers a value until urgent updates are flushed; Suspense for data fetching with React's use() hook). React Server Components (components that render on the server without client JS; they cannot have useState, useEffect, or browser APIs; how Next.js App Router implements them with the 'use client' directive boundary).

Frontend engineer salary ranges: junior (0-2 years): Rs 5-18 LPA. Mid-level (2-5 years): Rs 15-45 LPA. Senior (5-8 years): Rs 40-90 LPA. Staff/Principal (8+ years): Rs 80-160 LPA. Top-paying frontend employers: CRED (premium consumer product with a very high UI quality bar; Rs 30-80 LPA mid-senior), Razorpay (payments product frontend; Rs 25-70 LPA), Swiggy (consumer delivery app; Rs 20-55 LPA), Flipkart (e-commerce scale frontend engineering; Rs 20-55 LPA), Groww (investment app with high data visualisation quality; Rs 20-55 LPA). Frontend engineers earn 10-20% less than backend engineers at equivalent levels at most Indian product companies; this gap is narrowing as companies recognise that frontend performance and quality directly impact user retention and conversion rates.