React Js

• React.js is a JavaScript library for building fast, interactive user interfaces. It uses reusable components to create modern web apps and updates only the parts of a webpage that change, making it fast and efficient.

Why we use React Js?

• We use React because it’s fast, reusable, and efficient for building dynamic, user-friendly web apps. It updates only what’s needed, saves time, and is supported by a large community.

How it is Work?

• React works by using components to build the UI and a Virtual DOM to update only the changed parts, making it fast and efficient.

Why we install npm.

• We install npm to manage React’s tools, libraries, and scripts, making it easy to set up and run React projects.

What is NPM ?

• npm (Node Package Manager) is a tool that helps manage JavaScript packages and libraries. It lets you easily install, update, and manage the dependencies (tools and libraries) needed for your projects. It’s commonly used in React and other JavaScript frameworks.

What is NPX ?

• npx lets you run npm packages without installing them globally. It’s useful for one-time uses, like creating a React app, saving time and space.

Difference B/t NPM and NPX

Feature	npm	npx
Purpose	Installs and manages packages	Runs packages without installing them
Usage	npm install <package>	npx <command>
Example	npm install react	npx create-react-app my-app
Installation	Installs packages globally or locally	Runs packages without installation
Use Case	Managing dependencies for your project	Running one-time commands or tools

Create-react-app

• create-react-app is a command-line tool that sets up a new React project with a ready-to-use development environment. It configures all the necessary files, tools, and settings (like Webpack, Babel, etc.) so you can start building a React app immediately without manual setup.

Webpack:

• What it is: A module bundler for JavaScript.
• What it does: It bundles your JavaScript files (and other assets like images, CSS) into one or more optimized files that can be loaded in the browser.
• Why it's important: Webpack helps in splitting code, optimizing load times, and managing dependencies in a project.

Babel:

• What it is: A JavaScript compiler.
• What it does: It converts modern JavaScript (like ES6/ES7) code into backward-compatible JavaScript that works on older browsers.
• Why it's important: Babel allows you to use the latest JavaScript features without worrying about browser compatibility.

Vite

• Vite is a fast development tool for building JavaScript apps. It starts quickly, updates changes instantly, and creates smaller, optimized production builds. It’s designed to be faster and more efficient than older tools like create-react-app.

Why We Use Vite Instead of create-react-app:

• Faster Builds: Vite is much faster in development, with instant server start and quick updates.
• Smaller Bundles: Vite produces smaller production bundles, improving performance.
• Modern Features: Vite supports modern tools like TypeScript and JSX right out of the box.
• Better for Large Projects: Vite handles large apps more efficiently.

DOM

• The DOM (Document Object Model) is a way for browsers to represent the structure of an HTML or XML document as a tree. Each part of the page (like elements, attributes, or text) is a "node" in this tree.

With JavaScript, you can interact with the DOM to:

• Change the content of the page.
• Add or remove elements.
• Modify styles and respond to user actions (like clicks).

Virtual DOM

• The Virtual DOM (VR DOM) is a copy of the real DOM. It helps make web apps faster by updating the virtual version first instead of the real DOM directly.

Here’s how it works:

• React updates the Virtual DOM when there's a change in the app state or data.
• React then compares the updated Virtual DOM with the previous version of the Virtual DOM.
• React calculates the differences (called "diffing") between the two Virtual DOMs.
• Finally, React updates the real DOM with only the differences (the minimal changes), rather than re-rendering the entire DOM.

Diffing

• Diffing is the process React uses to compare the old and new versions of the Virtual DOM to find out what has changed. React then applies only the necessary changes to the real DOM.

Reconciliation

• Reconciliation is the broader process in React that includes diffing. It’s the algorithm that decides how to update the real DOM based on the differences found between the old and new Virtual DOM.

Concept	Diffing	Reconciliation
What it is	The process of comparing the old and new Virtual DOM.	The overall process that includes diffing and updating the real DOM.
What it does	Finds the differences between the two Virtual DOMs.	Uses the diffing results to efficiently update the real DOM.
Goal	Identify what changed in the Virtual DOM.	Apply the minimal updates to the real DOM based on the diffing results.

React Fiber

• React Fiber is the new reconciliation algorithm that was introduced in React 16. It improves the rendering performance by breaking the work into chunks and allowing React to work asynchronously.

Key Features:

• More responsive UI by prioritizing updates.
• Enabling features like Concurrent Mode and Suspense.

React DevTools

• React DevTools is a Chrome and Firefox extension that allows you to inspect the React component tree, state, props, and hooks.

Key Features:

• Visualizes the component tree for easier debugging.
• Inspect and modify the state/props of components directly in the browser.
• Offers hooks support, helping you debug custom hooks and useState/useEffect behavior.

Concept	Reconciliation (Pre-Fiber)	React Fiber
Rendering	Synchronous: All work is done in one go, blocking the thread.	Asynchronous: Work is split into smaller units and can be paused/resumed.
Performance	Slower for complex updates or large applications.	Faster due to prioritization, time-slicing, and concurrency.
Update Prioritization	No prioritization (all updates are handled the same).	Updates are prioritized based on urgency (e.g., animations, user inputs).
Interruptible Updates	Updates can't be interrupted.	Updates can be interrupted, allowing React to work on more urgent updates first.
Handling Large Apps	Poor handling of large or complex applications with many updates.	Optimized for large apps with efficient handling of complex updates.
UI Responsiveness	Poor UI responsiveness in case of heavy updates.	Improved responsiveness, even during heavy updates.

Thread

A thread is the smallest unit of execution within a process in computer science. It is a sequence of instructions that can be executed independently by a computer’s central processing unit (CPU). Threads are the building blocks of multitasking in modern operating systems, allowing multiple operations to run concurrently.

• Thread = Small task or instruction.
• Process = A larger task that can contain many threads.
• CPU = The brain that handles these tasks.

Types of Threads:

• Main Thread: The primary thread where the program starts. Most of the time, user interaction and other major tasks happen here.
• Worker Threads: Additional threads that can be created to perform tasks concurrently, such as background work, without blocking the main thread.

Hook

• A hook is a special function in React that allows functional components to use state, lifecycle methods, and other features that were previously only available in class components. Hooks enable functional components to be more powerful and manage state and side effects without requiring class-based components.

Key Points:

• useState: Manages state in functional components.
• useEffect: Handles side effects (e.g., data fetching, timers).
• useContext: Accesses context without prop drilling.
• useRef: References DOM elements or persistent values without causing re-renders.
• useMemo & useCallback: Optimizes performance by memoizing values or functions.

Props

• In React, props are used to pass data from a parent component to a child component.
• Props help in creating flexible and reusable components by passing data between them.

Key Points:

• Immutable: Props cannot be changed by the child component.
• Dynamic: Allows components to be reusable with different data.
• Default Props: You can set default values if props are not passed.

State

• In React, state is a built-in object used to store data or information about a component. Unlike props, state is managed within the component and can be changed, allowing the component to react and re-render dynamically when the state updates.

Differences Between State and Props

Feature	State	Props
Definition	Data managed within the component.	Data passed from parent to child.
Mutable	Yes, it can be updated.	No, it is read-only.
Purpose	Used for dynamic and local data.	Used to pass external data.

JSX (JavaScript XML)

What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript used in React to create UI components. It allows writing HTML-like code inside JavaScript, making it easier to structure the UI.

Key Features:

• Looks Like HTML: JSX allows writing familiar HTML-like syntax.
• Embeds JavaScript: JavaScript expressions can be used inside JSX using {}.
• Transpiled by Babel: JSX is converted to standard JavaScript before execution.

Example:

function Greeting() {
  const name = "Alice";
  return <h1>Hello, {name}!</h1>; // Embedding JavaScript inside JSX
}

Why Use JSX?

• Improves readability and maintainability of UI code.
• Prevents XSS attacks by escaping injected values.
• Optimized performance as React compiles JSX to JavaScript efficiently.

JSX simplifies building React components by combining UI and logic in one place

Difference Between .jsx and .js Files

Feature	.jsx (JavaScript XML)	.js (JavaScript)
Purpose	Used for React components with JSX syntax.	General JavaScript code (functions, logic, APIs, etc.).
Syntax	Supports JSX (HTML-like syntax inside JS).	Standard JavaScript syntax.
Compilation	Needs Babel to transpile JSX into JS.	No additional compilation required.
Usage	Mostly used for React UI components.	Used for utility functions, API calls, or business logic.
File Extension	.jsx is conventionally used for JSX-based components.	.js is used for JavaScript logic and functions.