React.js

React.js is a popular JavaScript library for building user interfaces (UI). Developed by Facebook (now Meta) in 2013, it is used to create dynamic, interactive, and high-performance web applications.

React has become a standard in modern frontend development and is the foundation of many major projects — from Instagram and Netflix to corporate dashboards and marketplaces.

What is React.js

React is a library that allows you to build interfaces as a collection of components — independent blocks that combine logic, structure, and styles. Each component can be reused, combined, and dynamically updated without reloading the page.

In other words: React helps create fast web applications where the page updates partially, not entirely.

Core Principles of React

• Component-Based Architecture. The interface is broken down into independent parts — components (buttons, forms, cards, etc.) — that can be reused multiple times.
• Virtual DOM. React does not modify the real DOM directly. It creates a virtual copy and updates only the elements that have actually changed — significantly speeding up performance.
• Unidirectional Data Flow. Data flows from top to bottom — from parent to child components — making the application predictable and stable.
• JSX (JavaScript + XML). React uses JSX — a syntax that allows writing HTML-like code directly within JavaScript.

Example of a Simple React Component

jsx

import React from ‘react’;

function Hello({ name }) {

return <h1>Hello, {name}!</h1>;

}

export default Hello;

The Hello component accepts a parameter (props.name) and returns markup.
In the browser, it will display:

text

Hello, Anna!

How React Works

1. Rendering — A component is transformed into a virtual tree (Virtual DOM).
2. Diffing — React compares the current tree with the previous one.
3. Reconciliation — Only the changed nodes in the real DOM are updated.

Thanks to this, React works significantly faster than traditional approaches.

Advantages of React.js

• High Performance. Virtual DOM and optimized updates make the interface fast even with large datasets.
• Component Reusability. A component written once can be used in different parts of the application.
• Flexibility. React can be integrated into an existing project without a complete overhaul.
• Community Support. React has one of the largest communities and a vast number of ready-made libraries.
• SEO-Friendly. When using SSR (e.g., Next.js), pages are indexed by search engines.
• Shared Skills with React Native. The same principles can be applied when creating mobile applications.

Disadvantages

• A Library, Not a Framework. React is responsible only for the “V” in MVC — the view layer. Routing, data handling, and state management need to be added separately.
• Learning Curve. Confident use requires knowledge of modern JavaScript (ES6+) and the ecosystem of tools.
• Frequent Updates. The library evolves very quickly, requiring constant learning of new features.

State Management

React components can store internal data (state). For this, the useState hook is used:

jsx

import React, { useState } from ‘react’;

function Counter() {

const [count, setCount] = useState(0);

return (

<div>

<p>Count: {count}</p>

<button onClick={() => setCount(count + 1)}>+</button>

</div>

);

}

With each button click, the count value updates, and React re-renders only the necessary part of the interface.

Hooks

Hooks were introduced in React 16.8 and allow using component features without classes.
Main hooks:

React Ecosystem

React vs. Other Solutions

Where React is Used

• Social networks (Facebook, Instagram)
• Streaming services (Netflix, Twitch)
• E-commerce (Shopify, Etsy)
• SaaS platforms and CRMs
• Single-Page Applications (SPA)

React is suitable for interactive interfaces that require fast response to user actions.

Developer Tools

• Create React App (CRA) — standard tool for quick project setup.
• Next.js — React-based framework for SEO and SSR.
• React DevTools — browser extension for analyzing components.
• Vite — ultra-fast alternative to Webpack.

Example React Project Structure

text

my-app/

├── public/

│   └── index.html

├── src/

│   ├── components/

│   │   └── Header.jsx

│   ├── pages/

│   │   └── Home.jsx

│   ├── App.jsx

│   └── index.js

└── package.json

Conclusion

React.js is the foundation of modern frontend development. It combines simplicity, flexibility, and performance, allowing the creation of interfaces of any complexity — from landing pages to complex SPAs and PWAs.