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

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.

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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.

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Core React Concepts and Questions

1. 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.
2. 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.
3. 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).
4. 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).
5. 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.
6. What are Props?
   • Props are read-only data passed from parent to child components.
   • Used to trigger actions or display data in child components.
7. Difference between State and Props
   • State: mutable, private to the component, stores dynamic data.
   • Props: immutable, controlled by parent, passed down to children.
8. 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.
9. Purpose of the Key Attribute in Lists
   • Keys uniquely identify list items to optimize rendering and avoid re-rendering unchanged items.
10. What are Fragments?
    • Fragments group multiple elements without adding extra nodes to the DOM (avoids unnecessary wrappers like <div>).

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Advanced React Concepts

11. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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.
17. Explain Reconciliation in React
    • The process React uses to update the DOM efficiently.

Notable Quotes

— 21:24 — « Props drilling is when data has to be passed through many layers of components, even if intermediate components don't need to use that data, just so that a deeply nested child can access it. This can become hectic and confusing as the nesting grows. »

— 26:39 — « Context API is a way to solve the problem of props drilling by creating a global space where variables are stored, allowing any child component to access them directly without passing props through every intermediate component. »

— 32:42 — « A higher order component is a function that takes a component as input and returns a new component with additional functionality or modified behavior, allowing you to reuse logic like authentication across multiple pages without repeating code. »

— 48:31 — « React Router handles navigation in a single page application by allowing you to switch between different user pages without reloading the entire page, maintaining a seamless user experience. »

— 52:36 — « React Strict Mode helps developers identify potential problems by intentionally invoking warnings and behaviors, acting as a safety net to ensure safe coding practices and prevent errors later on. »

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Educational

Video