Summary of "Top 20 React JS Interview Questions For 2025 | React Interviewer Questions & Answers | Intellipaat"

This video by Intellipaat covers the top 20 React JS interview questions compiled by industry experts with over 7 years of experience. It explains fundamental and advanced React concepts, practical examples, and coding demonstrations aimed at preparing candidates for React developer interviews in 2025.

Main Ideas, Concepts, and Lessons

Introduction

React is a powerful JavaScript library developed by Jordan Walke at Meta (Facebook) in 2013.
Despite initial mixed reactions, React is now the 2nd most used web framework globally (2024).
Features like Virtual DOM, Hooks, and Context API make React highly popular.
Over 8,000 React job opportunities with an average salary of 4 lakhs INR per year.
The video covers 20 key interview questions selected by top industry experts.

Core React Concepts and Questions

What is React and why is it popular?
- React is a JavaScript library for building component-based reusable UI.
- Uses Virtual DOM to optimize performance by minimizing direct DOM manipulation.
What is a Single Page Application (SPA)?
- SPA loads a single HTML page and updates parts dynamically without full reloads.
- Benefits: faster load times, smoother user experience.
- Drawback: SEO challenges due to single URL usage.
What is JSX and how does it differ from HTML?
- JSX is a syntax extension allowing HTML-like code inside JavaScript.
- Requires proper closing of self-closing tags (unlike regular HTML).
Difference between Functional and Class Components
- Class components: traditional, use this.state and lifecycle methods.
- Functional components: simpler, use React Hooks (useState, useEffect).
- Functional components can now manage state (after Hooks introduction).
Difference between Stateless and Stateful Components
- Stateless: only receive and display data via props, no internal state.
- Stateful: manage and update their own state; can handle logic and user interaction.
What are Props?
- Props are read-only data passed from parent to child components.
- Used to trigger actions or display data in child components.
Difference between State and Props
- State: mutable, private to the component, stores dynamic data.
- Props: immutable, controlled by parent, passed down to children.
Controlled vs Uncontrolled Components
- Controlled: form inputs managed via React state (e.g., form validation).
- Uncontrolled: form inputs managed by the DOM, accessed via refs.
Purpose of the Key Attribute in Lists
- Keys uniquely identify list items to optimize rendering and avoid re-rendering unchanged items.
What are Fragments?
- Fragments group multiple elements without adding extra nodes to the DOM (avoids unnecessary wrappers like <div>).

Advanced React Concepts

What is the Virtual DOM?
- A lightweight in-memory copy of the real DOM.
- React compares the Virtual DOM with the real DOM (diffing) and updates only the changed parts.
- Improves performance by avoiding full page re-renders.
React Lifecycle Methods and Their Phases
- Lifecycle methods allow hooking into component phases: Mounting, Updating, Unmounting.
- Key methods: constructor, getDerivedStateFromProps, render, componentDidMount, shouldComponentUpdate, getSnapshotBeforeUpdate, componentDidUpdate, componentWillUnmount.
- Used for initialization, fetching data, cleanup, etc.
Explain useState and useEffect Hooks with Examples
- useState: stores and updates state in functional components.
- useEffect: handles side effects like API calls, timers, running after render.
- Examples shown: counter app with useState, timer app with useEffect.
What is Props Drilling?
- Passing props through many nested components even if intermediate components don’t need them.
- Leads to cumbersome and hard-to-maintain code.
What is Context API and How Does It Solve Props Drilling?
- Context API provides a global state accessible by any component without prop drilling.
- Creates a context provider that wraps components and shares data directly.
- Example shown with user data accessed by deeply nested components without passing props manually.
What are Higher Order Components (HOCs)?
- Functions that take a component and return an enhanced component with additional functionality.
- Useful for cross-cutting concerns like authentication.
- Example: wrapping a component to add extra props or behavior.
Explain Reconciliation in React
- The process React uses to update the DOM efficiently.