How does React work internally?

React maintains a virtual DOM that it compares with the real DOM to update only the parts of the UI that changed.

What is React and How Does It Work? | FullStackPrep.Dev

💡 Concept Name

React – A JavaScript library for building dynamic and responsive user interfaces using a component-based architecture and virtual DOM for efficient updates.

📘 Quick Intro

React helps developers build rich, interactive UIs by breaking the UI into components. These components manage their own state and render dynamically as data changes. React leverages a virtual DOM for faster rendering and promotes declarative, reusable code.

🧠 Analogy / Short Story

Imagine a Lego castle where each block is independent and can be replaced easily without rebuilding the whole structure. React works similarly—each UI piece (component) updates independently and efficiently using a smart blueprint called the virtual DOM to decide what to change on the real screen.

🔧 Technical Explanation

🧱 Built on reusable components that manage their own logic and UI.
⚛️ Uses a virtual DOM to track changes and optimize real DOM updates.
📦 Data flows from parent to child using props (unidirectional data flow).
🧠 Supports hooks like useState and useEffect to manage side effects and state.
📜 JSX syntax allows HTML-like code inside JavaScript for better readability.

💻 Real Code Example

// App.jsx
import React from 'react';

function Welcome(props) {
  return <h1>Hello, {props.name}!</h1>;
}

export default function App() {
  return <Welcome name="FullStackPrep" />;
}

❓ Interview Q&A

Q1: What is React?
A: React is a JavaScript library for building user interfaces using reusable components and virtual DOM for efficient updates.

Q2: What is JSX in React?
A: JSX is a syntax extension that allows writing HTML-like code inside JavaScript for easier UI development.

Q3: What is the virtual DOM?
A: It's an in-memory representation of the real DOM. React compares it with the previous version to update only the changed parts.

Q4: What are components in React?
A: Components are reusable pieces of code that define UI and behavior. They can be class-based or function-based.

Q5: What is unidirectional data flow in React?
A: It means data flows from parent to child via props, making the application easier to debug and manage.

Q6: How does React render updates efficiently?
A: React uses a virtual DOM and diffing algorithm to detect changes and update only what’s needed in the real DOM.

Q7: What is the purpose of useState?
A: useState is a hook that lets functional components maintain and update local state.

Q8: Why is React preferred for modern web apps?
A: Due to its reusable component architecture, declarative code, and virtual DOM for performance optimization.

Q9: What is the difference between class and function components?
A: Class components use lifecycle methods, while function components use hooks to manage logic and state.

Q10: Can React be used for mobile development?
A: Yes, using React Native, which allows you to build native mobile apps using React principles.

📝 MCQs

Q1. What is the core idea behind React?

Server-side scripting
Database manipulation
Component-based UI
File handling

Q2. What is JSX?

JavaScript compiler
CSS engine
Database syntax
HTML-like syntax in JavaScript

Q3. What does the virtual DOM do?

Stores state
Deletes cookies
Manages routes
Tracks changes before applying to real DOM

Q4. Which hook is used for state in function components?

useRouter
useData
useState
useClass

Q5. How does React update the UI efficiently?

Full page reload
Random updates
By comparing virtual DOMs
Direct rendering

Q6. What is a React component?

Database query
External file
A reusable UI element
Browser extension

Q7. How does data flow in React?

Two-way
From child to parent
Randomly
From parent to child

Q8. What is required to create a React app?

Apache
Python
Node.js and npm
MongoDB

Q9. Which of these is a valid React hook?

useApi
useStyle
useCall
useEffect

Q10. What is React mainly used for?

Data science
Server setup
Building user interfaces
Gaming logic

💡 Bonus Insight

React's ecosystem includes tools like React Router, Redux, and React Query that help build scalable apps. Its unidirectional data flow and component reuse reduce bugs and improve maintainability in large codebases.

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What is React and How Does It Work?