React Interview Questions | Crucial to crack any React Interview

Essential React Interview Concepts and Tooling Fundamentals
Executive Summary

This briefing outlines the must-know React concepts and tooling every candidate should prepare before attending a React developer interview. The core message is clear: do not appear for a React interview without mastering these topics. Interviewers expect strong command over React fundamentals, modern frontend tooling, and application architecture & performance optimization. Knowledge should go beyond syntax into why and how React works internally.

Core React Concepts
State Management & Data Flow

React interviews heavily test how well you understand state and component communication.

Lifting State Up: Sharing state between sibling components by moving it to the nearest common parent.

React Context: Passing data through the component tree without prop drilling.

Parent–Child State Interaction: Understanding that state updates are isolated unless explicitly shared.

Controlled vs Uncontrolled Components: Whether form state is managed by React state or the DOM.

Component Lifecycle & Hooks

Modern React relies on Hooks, especially useEffect.

useEffect Dependency Array:

No array → runs after every render

Empty array → runs once on mount

With dependencies → runs when dependencies change

Cleanup Function: Returned function in useEffect runs on unmount or before re-execution.

React.Fragment: Groups multiple elements without adding extra DOM nodes.

Rendering & Reconciliation

Reconciliation: React’s algorithm for comparing Virtual DOM trees and efficiently updating the real DOM.

Component Invocation vs Logging:

console.log(Component) logs the function

console.log(Component()) executes it and logs the returned JSX structure

Advanced Patterns

Higher-Order Components (HOC): Functions that take a component and return an enhanced component to reuse logic.

Development Ecosystem & Tooling
Build Tools & Bundlers

Webpack / Vite / Parcel: Bundle, optimize, and serve React applications.

Hot Module Replacement (HMR): Updates modules in real time without full page reload.

create-react-app (CRA): Abstracts complex build configurations using Webpack under the hood.

Package Management

npm vs npx: Installing packages vs executing them without installation.

dependencies vs devDependencies: Runtime vs development-only packages.

package.json vs package-lock.json:

package.json defines dependencies

package-lock.json locks exact versions for consistent installs

Optimization

Tree Shaking: Removes unused code from bundles to reduce size.

Application Architecture & Performance
Routing

React Routing: Client-side navigation using libraries like React Router.

Client-Side vs Server-Side Routing:

Client-side: Faster navigation, SPA behavior

Server-side: New HTML page per route, better initial SEO

Styling Approaches

Global CSS

CSS Modules

CSS-in-JS solutions

Performance Optimization

Code Splitting: Loads parts of the app on demand to improve initial load time.

This checklist reflects real interview expectations. Strong understanding of these areas significantly improves React interview performance.