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ReactJS: The Ultimate Beginner-To-Expert Guide On How It Works Behind The Scenes

Introduction

ReactJS is a powerful, efficient, and flexible JavaScript library developed by Facebook for building user interfaces, especially for single-page applications (SPAs). It allows developers to create large web applications that can update and render efficiently in response to data changes. In this comprehensive guide, we will walk you through everything you need to know about React — from how to create a project using modern tooling to what happens under the hood.

Getting Started With React in 2025

Why Not create-react-app?

create-react-app is now deprecated. The React team recommends using modern alternatives such as Vite, Next.js, or Parcel for better development experience.

Create a React Project Using Vite

Step 1: Prerequisites

Ensure Node.js (v16+) is installed:

node -v
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Step 2: Create Project with Vite 

npm create vite@latest my-react-app -- --template react
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For TypeScript:

npm create vite@latest my-react-app -- --template react-ts
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Step 3: Install Dependencies 

cd my-react-app
npm install
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Step 4: Start Development Server 

npm run dev
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Open http://localhost:5173 in your browser.

Folder Structure 

my-react-app/
├── index.html
├── package.json
├── vite.config.js
└── src/
    ├── App.jsx
    ├── main.jsx
    └── ...
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Adding React Router 

npm install react-router-dom
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Example usage:

import { BrowserRouter, Routes, Route, Link } from 'react-router-dom';
import Home from './pages/Home';
import About from './pages/About';

function App() {
  return (
    <BrowserRouter>
      <nav>
        <Link to="/">Home</Link>
        <Link to="/about">About</Link>
      </nav>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/about" element={<About />} />
      </Routes>
    </BrowserRouter>
  );
}
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Adding Tailwind CSS (Optional) 

npm install -D tailwindcss postcss autoprefixer
npx tailwindcss init -p
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Update tailwind.config.js and include Tailwind in your main CSS.

How React Works Behind the Scenes

Virtual DOM: Efficient UI Representation

React creates a lightweight copy of the real DOM called the Virtual DOM. When the state of a component changes, React updates the Virtual DOM first. Then it compares the new Virtual DOM with the previous one using a process called reconciliation.

Reconciliation and Diffing

React uses a diffing algorithm to determine the minimal number of changes between the current and previous Virtual DOM. It then updates only those parts in the actual DOM, greatly improving performance.

  • Uses key props to identify changed elements in lists
  • Applies minimal updates to the real DOM

Fiber Architecture

React Fiber is the internal engine introduced in React 16:

  • Allows incremental rendering
  • Supports concurrency
  • Breaks rendering into units of work
  • Improves error boundaries

Batching and Scheduling

React groups multiple state updates into a single render pass, a technique called batching. It also uses a scheduler to prioritize urgent tasks like user interactions.

Writing Efficient React Code

Use React.memo

Prevents unnecessary re-renders of components when props haven't changed.

Use useCallback and useMemo

These hooks help you memoize functions and values between renders to improve performance.

Avoid Excessive State

Minimize state and lift it only when necessary. Prefer local component state unless multiple components need to share it.

Lazy Loading

Use React.lazy and Suspense to defer loading components until they’re needed:

const LazyComponent = React.lazy(() => import('./LazyComponent'));
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Frequently Asked Questions (FAQs)

What Is the Virtual DOM?

A virtual representation of the real DOM in memory. React uses it to determine what has changed and updates only the necessary parts of the real DOM.

Why Is create-react-app Deprecated?

It is outdated and doesn't support the latest JavaScript and tooling features. Vite or Next.js are faster and more flexible alternatives.

What Is Reconciliation in React?

It is the process where React compares the updated Virtual DOM with the previous one and makes the smallest number of changes required to the real DOM.

What Is the Purpose of key Props?

They help React identify which elements in a list have changed, been added, or been removed. This improves the performance of re-renders.

How Does React Handle Rendering Performance?

With the help of batching updates, Fiber architecture, memoization, and Virtual DOM diffing, React ensures that your UI remains fast and responsive.

Conclusion

ReactJS is more than just a library for building UI — it’s a performance-focused architecture for building dynamic, scalable applications. With the deprecation of CRA, tools like Vite offer a modern approach to development. Understanding concepts like the Virtual DOM, reconciliation, Fiber architecture, and efficient coding practices gives you an edge in creating responsive and powerful web applications.

Take the time to explore and practice, and you'll go from zero to hero in no time!

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