React – A JavaScript Library

• React.js is a JavaScript library for building user interfaces.
• Developed by Facebook (now Meta) in 2013.
• It is component-based, declarative, and allows for efficient UI updates.

Key Features of React

• Component-Based Architecture: UI is built using reusable components.
• Virtual DOM: Updates only the changed parts, making React fast.
• Unidirectional Data Flow: Props and state maintain a predictable data structure.
• JSX (JavaScript XML): A syntax extension that lets you write HTML inside JavaScript.
• Hooks: Allow functional components to have state and other features.

Mutable vs Immutable

Mutable (Changeable)

• Mutable data types can be changed after creation.
• Example: Objects & Arrays are mutable.

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let arr = [1, 2, 3];
arr.push(4); // Modifies the original array
console.log(arr); // [1, 2, 3, 4]

Immutable (Unchangeable)

• Immutable data cannot be changed directly. Instead, you create a new copy.
• Example: Strings & Numbers are immutable.

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let str = "Hello";
str[0] = "M"; // This won't change the string
console.log(str); // "Hello"

Example of immutable object handling (creating a new object instead of modifying the original):

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const person = { name: "Ahmad", age: 19 };
const updatedPerson = { ...person, age: 20 };
console.log(updatedPerson); // { name: "Ahmad", age: 20 }

Arrays & Objects Destructuring

Array Destructuring

• Extract values from an array and assign them to variables.

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const numbers = [10, 20, 30];
const [a, b, c] = numbers;
console.log(a, b, c); // 10 20 30

Object Destructuring

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const person = { name: "Ahmad", age: 19, city: "Multan" };
const { name, age } = person;
console.log(name, age); // Ahmad 19

Import & Export in JavaScript

Exporting in JavaScript

Named Export (Multiple Exports)

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export const name = "Ahmad";
export const age = 19;
export function greet() {
  return "Hello!";
}

Default Export (Single Export per File)

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export default function greet() {
  return "Hello!";
}

Importing in JavaScript

Named Import

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import { name, age, greet } from "./module.js";
console.log(name, age); // Ahmad 19
console.log(greet()); // Hello!

Default Import

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import greet from "./module.js";
console.log(greet()); // Hello!

Import Everything

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import * as data from "./module.js";
console.log(data.name); // Ahmad
console.log(data.greet()); // Hello!

Map & Filter in JavaScript for React

map() Method

• map() is used to iterate over an array and return a new array with modified elements.
• It does not modify the original array.

Syntax:

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const newArray = array.map((element, index, array) => {
  return modifiedElement;
});

Example:

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const numbers = [1, 2, 3, 4];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6, 8]

filter() Method

• filter() is used to filter elements based on a condition.
• It returns a new array with only the elements that satisfy the condition.
• The original array remains unchanged.

Syntax:

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const newArray = array.filter((element, index, array) => {
  return condition;
});

Example:

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const numbers = [1, 2, 3, 4, 5, 6];
const evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers); // [2, 4, 6]

✅ Use map() to modify each element in an array.
✅ Use filter() to remove unwanted elements from an array.
✅ Always use a key prop in React when rendering lists.

Difference Between Library & Framework

Key Difference

• React (Library): You decide how to structure the app.
• Angular (Framework): It dictates the structure.

Real DOM vs Virtual DOM

What is the DOM (Document Object Model)?

• The DOM represents the structure of an HTML document as a tree.
• JavaScript can manipulate the DOM to update elements dynamically.

What is Real DOM?

• The actual structure of an HTML document in the browser.
• When a change is made, the entire page re-renders.
• Slow performance for frequent updates.

What is Virtual DOM?

• A lightweight copy of the Real DOM.
• React updates the Virtual DOM first, then compares it with the previous version using diffing.
• Only the changed parts of the Real DOM get updated, making React much faster.

Difference Between Real DOM & Virtual DOM

Why Virtual DOM is Important in React?

✅ Faster updates compared to Real DOM.
✅ Better performance for dynamic content.
✅ Efficient UI rendering with minimal reflows.

Vite vs Create React App (CRA) in React.js

1. Vite 🚀

• Modern, fast, and optimized build tool for React.
• Uses ES modules (ESM) and Hot Module Replacement (HMR) for instant updates.
• Super fast startup (no bundling in development).
• Supports TypeScript, JSX, and more out-of-the-box.
• Tree-shaking & optimized builds for production.

Install React with Vite

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npm create vite@latest
npm install
npm run dev

2. Create React App (CRA) 🏗️

• Traditional way to set up React projects.
• Uses Webpack (slower than Vite).
• Slower builds and HMR compared to Vite.
• Good for beginners, but outdated for large apps.

Install React with CRA

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npx create-react-app my-app
cd my-app
npm start

JSX (JavaScript XML) & React Vite Folder Structure

1. JSX (JavaScript XML)

• JSX is a syntax extension for JavaScript used in React.
• Looks like HTML inside JavaScript but gets converted to JavaScript functions or React.createElement.
• Allows embedding JavaScript inside {}.
• Must return a single parent element (use <>...</> if needed).

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const App = () => {
  const name = "Ahmad";
  return <h1>Hello, {name}!</h1>;
};
export default App;

2. React Vite Folder Structure

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.
├── eslint.config.js     # ESLint configuration (optional)
├── index.html           # Main HTML file (root of the app)
├── node_modules/        # Installed dependencies
├── package.json         # Project metadata & dependencies
├── package-lock.json    # Exact versions of installed packages
├── public/              # Static assets (images, fonts, etc.)
├── README.md            # Project documentation
├── src/                 # Main source code directory
│   ├── App.jsx          # Main React component
│   ├── main.jsx         # React entry file (renders App)
│   ├── assets/          # Images, styles, etc.
│   ├── components/      # Reusable UI components
│   ├── pages/           # Page components (if using routing)
│   └── styles/          # Global CSS files (if needed)
└── vite.config.js       # Vite configuration file

Key Points

✅ src/ contains all React components & logic.
✅ index.html is the main HTML template.
✅ Vite uses main.jsx to mount the React app into #root.
✅ Public files are served as-is from /public/.

React Components & How They Work

1. Component in React

• A component is a reusable, independent UI piece in React.
• Two types:
  • Functional Components (recommended, use hooks).
  • Class Components (older, uses this.state).
• Components return JSX and manage their own logic & state.

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const MyComponent = () => {
  return <h1>Hello, React!</h1>;
};
export default MyComponent;

2. How They Work

✅ We create components inside the src/components/ folder (e.g., MyComponent.jsx).
✅ Components are imported into App.jsx, which acts as the main component.
✅ main.jsx is responsible for rendering App.jsx inside the root <div> in index.html.
✅ ReactDOM.createRoot(document.getElementById('root')) creates the Virtual DOM and syncs

Flow of Code in React

1. Create a component (MyComponent.jsx):

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const MyComponent = () => <h1>Hello, React!</h1>;
export default MyComponent;

2. import it inside App.jsx:

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import MyComponent from "./components/MyComponent";

const App = () => {
  return (
    <div>
      <MyComponent />
    </div>
  );
};
export default App;

3. Render App.jsx inside main.jsx:

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import React from "react";
import ReactDOM from "react-dom/client";
import App from "./App";

ReactDOM.createRoot(document.getElementById("root")).render(<App />);

4. index.html contains root div:

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<body>
  <div id="root"></div>
</body>

Key Points

✅ Components are modular & reusable in React.
✅ main.jsx renders App.jsx inside the root div.
✅ React Virtual DOM updates only changed parts for better performance. 🚀

Conditional Rendering in React

Conditional rendering in React allows components to render dynamically based on conditions.

Using if-else Statements

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function Greeting(props) {
  if (props.isLoggedIn) {
    return <h1>Welcome Back!</h1>;
  } else {
    return <h1>Please Sign In</h1>;
  }
}

Using Ternary Operator (? :)

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function Greeting({ isLoggedIn }) {
  return <h1>{isLoggedIn ? "Welcome Back!" : "Please Sign In"}</h1>;
}

Event Handling in React

React follows the Synthetic Event system, making event handling efficient across different browsers.

Handling Click Events

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function Button() {
  function handleClick() {
    alert("Button Clicked!");
  }

  return <button onClick={handleClick}>Click Me</button>;
}

Passing Arguments in Event Handlers

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function Greeting({ name }) {
  function sayHello(name) {
    alert(`Hello, ${name}!`);
  }

  return <button onClick={() => sayHello(name)}>Greet</button>;
}

Handling Keyboard Events

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function InputBox() {
  function handleKeyPress(event) {
    if (event.key === "Enter") {
      alert("Enter key pressed!");
    }
  }

  return <input type="text" onKeyPress={handleKeyPress} />;
}

✅ Conditional Rendering: Use if-else, ternary (? :), &&, or switch-case for rendering components based on conditions.
✅ Event Handling: Use event handlers like onClick, onKeyPress, onSubmit, etc., with functions.
✅ Pass Parameters: Use arrow functions for passing arguments in event handlers.
✅ Prevent Default Behavior: Use event.preventDefault() to stop default browser actions like form submissions.

State in React

• State is an object that holds dynamic data that affects a component’s rendering.
• Unlike props, state is mutable and can be changed within a component.
• When state changes, React re-renders the component to reflect the updates.

Types of State in React

1. Local State:

• Managed within a single component using useState().
• Example: Form inputs, modals, toggles.

2. Global State:

• Shared across multiple components.
• Managed using Context API, Redux, or other state management libraries.

3. Server State:

• Data fetched from an API that needs to be integrated into UI.
• Managed using tools like React Query, SWR, or Redux Thunk.

4. URL State:

• State stored in the URL (query parameters, path, etc.).
• Managed using React Router.

Understanding useState() Hook

• useState() is a React Hook that allows functional components to manage state.
• It returns an array with two values:
  1. The current state value.
  2. A function to update the state.

Syntax:

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const [state, setState] = useState(initialValue);

Example: Counter App

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import { useState } from "react";

function Counter() {
  const [count, setCount] = useState(0); // Initial state is 0

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

export default Counter;

• useState(0): Initializes count to 0.
• setCount(count + 1): Updates the state, causing a re-render.

Updating State Correctly

1. Using Functional Updates (Best Practice)

• When the new state depends on the previous state, use a function inside setState().

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setCount((prevCount) => prevCount + 1);

• This ensures correct updates, especially in async operations.

2. Updating Objects in State

• Use spread operator (...) to avoid modifying the original state.

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const [user, setUser] = useState({ name: "Ahmad", age: 19 });
setUser(prevUser => ({ ...prevUser, age: prevUser.age + 1 }));

3. Updating Arrays in State

• Use spread operator (...) to update arrays.

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const [items, setItems] = useState([1, 2, 3]);
setItems(prevItems => [...prevItems, 4]); // Adds new item

✅ State is a way to store data inside a component.
✅ useState() is used to manage local state in functional components.
✅ Always use the updater function when updating state based on previous values.
✅ Never modify state directly, always use setState().

Props (Properties) in React

What are Props?

• Props stand for Properties.
• Props are read-only data passed from a parent component to a child component.
• They allow components to be dynamic and reusable by passing different data to them.
• Think of props as function arguments for components.

How Props Work

• Parent component passes props to a child component.
• Child component receives them as a parameter (usually an object).
• The child accesses props using props.propName.

Syntax

1. Passing Props

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<ChildComponent name="Ahmad" age={19} />

2. Receiving Props

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function ChildComponent(props) {
  return (
    <div>
      <p>Name: {props.name}</p>
      <p>Age: {props.age}</p>
    </div>
  );
}

✅ OR Using Destructuring

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function ChildComponent({ name, age }) {
  return (
    <div>
      <p>Name: {name}</p>
      <p>Age: {age}</p>
    </div>
  );
}

🧩 Key Points

• Props are immutable: You cannot change props inside the child component.
• Unidirectional flow: Data flows from parent to child (one-way data binding).
• Can be any data type: String, number, boolean, array, object, function, etc.
• Used to customize components and make them reusable.

CSS Modules

• A CSS Module is a CSS file in which all class and animation names are scoped locally by default.
• This means that styles defined in one component won’t affect other components, preventing style conflicts.
• Used in React.js to maintain modular, reusable, and conflict-free CSS

How It Works

• A CSS file is treated as a module when named with the .module.css extension.
  • Example: MyComponent.module.css
• In React, you import it as an object where each CSS class becomes a property of that object.

📄 Example File: Button.module.css

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.button {
  background-color: blue;
  color: white;
}

Example Usage in Component

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import styles from './Button.module.css';

function Button() {
  return <button className={styles.button}>Click me</button>;
}

Behind the Scenes

• CSS Modules use a build tool like Webpack to automatically transform class names into unique identifiers.
  • Example:
    • .button in the source might become .Button_button__3hWkj in the final DOM.
• The mapping ensures local scoping and uniqueness across components.

Comparison with Other Styling Methods

Form Handling in React.js

Form handling refers to how input data is captured from HTML forms (like <input>, <textarea>, <select>) and managed using state or references in React.

Key Concepts:

• React forms are usually managed through state (controlled) or refs (uncontrolled).
• You handle form submissions via an onSubmit event handler.
• You use onChange to capture input updates.

Uncontrolled Components & useRef()

An uncontrolled component lets the DOM manage the input value, and React accesses it using a ref.

• Used to directly access a DOM element
• Doesn’t re-render the component on value change

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const inputRef = useRef();
const handleSubmit = () => {
  console.log(inputRef.current.value);
};

<input type="text" ref={inputRef} />

Controlled Components

A controlled component is an input element whose value is controlled by React state.

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const [name, setName] = useState("");

<input type="text" value={name} onChange={(e) => setName(e.target.value)} />

Full Example:

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import React, { useState } from "react";

function Form() {
  const [val, setVal] = useState({ name: "", email: "" });

  const handleSubmit = (event) => {
    event.preventDefault();
    console.log(val);
  };
  return (
    <>
      <form action="" onSubmit={handleSubmit}>
        <input
          onChange={(event) => setVal({ ...val, name: event.target.value })}
          type="text"
          placeholder="name"
        />
        <input
          type="email"
          placeholder="Email"
          onChange={(event) => setVal({ ...val, email: event.target.value })}
        />
        <input type="submit" />
      </form>
    </>
  );
}
export default Form;

React Hook Form (Library)

React Hook Form is a popular library for building performant and flexible forms in React.

Why Use It?

• Reduces boilerplate code
• Built-in validation
• Integrates well with UI libraries like Material UI, Chakra UI, etc.

Basic Example:

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import { useForm } from "react-hook-form";

const { register, handleSubmit } = useForm();

const onSubmit = (data) => console.log(data);

<form onSubmit={handleSubmit(onSubmit)}>
  <input {...register("username")} />
  <input type="submit" />
</form>

Full Example

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import React from "react";
import { useForm } from "react-hook-form";

function Form() {
  const { register, handleSubmit } = useForm();

  return (
    <>
      <form action="" onSubmit={handleSubmit((data) => console.log(data))}>
        <input {...register("name")} type="text" placeholder="name" />
        <input {...register("age")} type="age" placeholder="age" />
        <input {...register("email")} type="email" placeholder="email" />
        <input type="submit" />
      </form>
    </>
  );
}

export default Form;

Dynamic Routing in React.js

Dynamic Routing means that the routing is handled at runtime (not at compile time like in traditional HTML websites).

In React.js, dynamic routing is implemented using React Router, a library that enables navigation among views of various components.

Unlike traditional routing where routes are hardcoded in a file (like routes.php), React dynamically loads components based on the current URL.

Installation of react-router-dom

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npm install react-router-dom

Core Components of React Router

• BrowserRouter – Wraps the app and enables routing.
• Routes – A container for all the Route components.
• Route – Defines the path and component to render.
• Link / NavLink – Used for navigation without reloading the page.
• useParams – Hook to access URL parameters (useful for dynamic routes).

Basic Routing Setup

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

function App() {
  return (
    <BrowserRouter>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/about" element={<About />} />
      </Routes>
    </BrowserRouter>
  );
}

What is Dynamic Routing?

Dynamic routing allows us to create routes where parts of the URL are variables, e.g., /user/1, /user/2, etc.

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<Route path="/user/:id" element={<UserProfile />} />

In this case, :id is a route parameter.

Accessing Dynamic Parameters: useParams

• useParams() is a React Router hook used to access dynamic parameters from the current URL.
• It is most useful with dynamic routes like /user/:id, /post/:slug, etc.

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import { useParams } from "react-router-dom";

function UserProfile() {
  const { id } = useParams();

  return <h1>User ID: {id}</h1>;
}

useNavigate() in React Router

• useNavigate() is a hook provided by react-router-dom.
• It is used to programmatically navigate to different routes (without clicking a <Link> or <NavLink>).
• It replaces the older useHistory() from React Router v5.

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const navigate = useNavigate();

function handleClick() {
  navigate("/about");
}

🔁 Navigate with Parameters

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navigate(`/user/${userId}`);

⏮️ Navigate Back or Forward

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navigate(-1); // Go back
navigate(1);  // Go forward

NavLink in React Router

NavLink is a special version of the <Link> component in react-router-dom that adds styling attributes to the rendered element when it matches the current URL.

It’s perfect when you want to highlight the active page in a navigation menu (e.g., changing the color or adding underline).

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<NavLink to="/home">Home</NavLink>
<NavLink to="/about">About</NavLink>

Active Styling with NavLink

By default, NavLink applies an active class when the route matches.

You can also provide your own styles or class names based on the active state using:

✅ className as a Function

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<NavLink 
  to="/home"
  className={({ isActive }) => isActive ? "active-link" : "inactive-link"}
>
  Home
</NavLink>

✅ style as a Function

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<NavLink 
  to="/about"
  style={({ isActive }) => ({
    color: isActive ? "green" : "gray",
    textDecoration: isActive ? "underline" : "none"
  })}
>
  About
</NavLink>

Example with Full NavBar

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import { NavLink } from "react-router-dom";
import "./styles.css"; // Assume you have .active-link class here

function Navbar() {
  return (
    <nav>
      <NavLink to="/" end className={({ isActive }) => isActive ? "active-link" : ""}>
        Home
      </NavLink>
      <NavLink to="/about" className={({ isActive }) => isActive ? "active-link" : ""}>
        About
      </NavLink>
      <NavLink to="/contact" className={({ isActive }) => isActive ? "active-link" : ""}>
        Contact
      </NavLink>
    </nav>
  );
}

Summary of NavLink

<Outlet /> in React Router

• <Outlet /> is a placeholder component used to render child routes inside a parent route.
• It’s like saying: “Here is where the nested component will appear.”

✅ How It Works

• Visiting /dashboard/profile will render:
  • Dashboard component
  • Profile component inside the <Outlet />
• Visiting /dashboard/settings will render:
  • Dashboard component
  • Settings component inside the <Outlet />

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import { Outlet } from "react-router-dom";

function Dashboard() {
  return (
    <div>
      <h1>Dashboard Layout</h1>
      {/* Nested route component will be rendered here */}
      <Outlet />
    </div>
  );
}

API Integration in React.js using AJAX (fetch / axios)

API integration in React lets your app communicate with backends, external services, or databases. AJAX (Asynchronous JavaScript and XML) is used to make asynchronous requests to APIs.

1. Using fetch

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useEffect(() => {
  fetch('https://api.example.com/data')
    .then(res => res.json())
    .then(data => setData(data))
    .catch(err => console.error(err));
}, []);

• fetch() returns a Promise.
• Must manually handle JSON parsing (res.json()).

2. Using axios

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npm install axios // install this package to use axios

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import axios from "axios";
import React from "react";

const App = () => {
  const api = "https://fakestoreapi.com/products";

  // 📥 Function to GET products from the API
  const getProducts = () => {
    axios
      .get(api)
      .then((res) => console.log("GET:", res))
      .catch((err) => console.error("GET Error:", err));
  };

  // 📤 Function to POST (add) a new product to the API
  const addProducts = () => {
    axios
      .post(api, {
        id: 0,
        title: "Sample Product",
        price: 0.1,
        description: "This is a test product.",
        category: "sample-category",
        image: "http://example.com/image.jpg",
      })
      .then((res) => console.log("POST:", res))
      .catch((err) => console.error("POST Error:", err));
  };

  return (
    <div className="p-5">
      {/* 🟢 Button to trigger GET request */}
      <button
        onClick={getProducts}
        className="px-3 py-2 m-2 bg-blue-500 rounded-lg text-white"
      >
        Get Data
      </button>

      {/* 🔵 Button to trigger POST request */}
      <button
        onClick={addProducts}
        className="px-3 py-2 m-2 bg-green-600 rounded-lg text-white"
      >
        Send Data
      </button>
    </div>
  );
};

export default App;

Example (with async/await)

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useEffect(() => {
  const fetchData = async () => {
    try {
      const response = await axios.get('https://api.example.com/data');
      setData(response.data);
    } catch (err) {
      console.error(err);
    } finally {
      setLoading(false);
    }
  };

  fetchData();
}, []);

React Component Lifecycle

The component lifecycle refers to the sequence of methods/functions that are invoked during the life of a React component — from its creation to its removal from the DOM.

🧱 3 Main Phases:

1. Mounting

Component is being created and inserted into the DOM.

• constructor() → (Class component only)
• render()
• componentDidMount() ✅ (ideal for API calls, subscriptions)

2. Updating

When props or state change.

• shouldComponentUpdate() → (optional)
• render()
• componentDidUpdate() ✅ (runs after update)

3. Unmounting

Component is removed from the DOM.

• componentWillUnmount() ✅ (cleanup tasks: timers, subscriptions, etc.)

Functional Components use useEffect for all of this!

• A React Hook for handling side effects (e.g. API calls, DOM updates, subscriptions).
• Runs after the component renders.

Basic Syntax

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useEffect(() => {
  // effect logic
  return () => {
    // optional cleanup
  };
}, [dependencies]);

Dependency Array

• []: runs once after initial render (like componentDidMount)
• [var]: runs when var changes
• No array: runs after every render

Note: When we [] square bracket in useEffect() component only update the changed part and if we don’t add square bracket component re-render means first it deleted and then again created.

💡 Common Uses

• API calls
• Event listeners
• Timers
• Cleanups (return function)

Full Example

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useEffect(() => {
    console.log("About Component is Created");

return () => {
      console.log("About Comonent is Deleted");
};

Example: Cleanup (like componentWillUnmount)

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useEffect(() => {
  const timer = setInterval(() => {
    console.log("Running interval...");
  }, 1000);

  return () => {
    clearInterval(timer); // 🧹 cleanup
    console.log("Interval cleared");
  };
}, []);

What is React Reconciliation

React Reconciliation is the process React uses to update the DOM efficiently when the state or props of a component change.

This makes updates fast and efficient, instead of redrawing the whole UI.

Note: 🔍 Think of reconciliation as React saying: “Okay, what exactly changed? Let me only update that part.”

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  const [val, setVal] = useState("This is Normal Data");
  const [val2, setVal2] = useState("This is Very Large Data");

  useEffect(() => {
    console.log("About Component is Created");

    return () => {
      console.log("About Comonent is Deleted");
    };
  }, [val2]); // component only re-render when val2 is changed otherwise it update only changed part.

Axios for Session Management & Separation

1. Why Use a Separate Axios File?

• Centralizes config (base URL, headers, interceptors)
• Cleaner code, reusable instance
• Easy to maintain sessions (cookies, tokens)
• Can add error handlers globally

2. Basic Axios Setup (Separate File)

🗂️ File: axiosInstance.js

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import axios from "axios";

// Create an Axios instance
const instance = axios.create({
  baseURL: "https://fakestoreapi.com/",  // 🏠 Your backend URL
  withCredentials: true,                 // ✅ Important for session cookies
});

export default instance;

3. What withCredentials: true Does

• Allows Axios to send cookies and authentication headers
• Required when working with sessions (esp. JWT or Express sessions)
• Must also be enabled on backend (CORS settings)

4. Using It in Components

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import axios from "../axiosInstance";

const getProducts = async () => {
  try {
    const res = await axios.get("/products");
    console.log(res.data);
  } catch (err) {
    console.error(err);
  }
};

ReactJS Context API

• A way to share data between components without passing props manually at every level.
• Solves prop drilling (passing data from parent to child repeatedly).

When to Use Context API?

• When you need to share state across many components (e.g., theme, user authentication, language, etc.)
• Good for global state management in small to medium-sized apps.

Key Components of Context API

1. React.createContext()

• Creates a context object.

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const MyContext = React.createContext();

2. <Context.Provider>

• Wraps components to provide the context value.

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<MyContext.Provider value={someData}>
  <ChildComponent />
</MyContext.Provider>

3. useContext(Context)

• A hook to access the context value in a functional component.

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const value = useContext(MyContext);

Example: Basic Usage

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// 1. Create Context
const ThemeContext = React.createContext();

// 2. Create a Provider Component
const ThemeProvider = ({ children }) => {
  const [theme, setTheme] = React.useState("light");

  return (
    <ThemeContext.Provider value={{ theme, setTheme }}>
      {children}
    </ThemeContext.Provider>
  );
};

// 3. Use Context in a Child Component
const ThemeToggler = () => {
  const { theme, setTheme } = React.useContext(ThemeContext);

  return (
    <button onClick={() => setTheme(theme === "light" ? "dark" : "light")}>
      Switch to {theme === "light" ? "dark" : "light"} mode
    </button>
  );
};

// 4. Wrap in App
const App = () => (
  <ThemeProvider>
    <ThemeToggler />
  </ThemeProvider>
);

Redux Toolkit (RTK)

What is Redux

Redux is a state management library used with JavaScript apps (especially React) to manage and share application-wide state in a predictable way.

What is React-Redux?

React-Redux is the official binding library that allows React components to interact with the Redux store using hooks like:

• useSelector() – read state
• useDispatch() – send actions

It acts as a bridge between Redux and React.

Why Redux Toolkit is Useful

• ✅ Reduces boilerplate code (less code, same result)
• ✅ Built-in support for async actions using createAsyncThunk
• ✅ Uses Immer.js internally so you can write mutable-style logic safely
• ✅ Easy and clean setup via configureStore and createSlice
• ✅ Encourages best practices by default

Core Concepts

Redux Toolkit vs Context API

Use Context API:

• For theme, language, auth user – simple state

Use Redux Toolkit:

• For complex state logic, shared global data, multiple components, async logic

Redux Flow Diagram:

📦 Store – Global State Container

• The store is where all the application state lives.
• It combines different reducers (e.g., user, products, carts) into one centralized global state.
• The store is created using configureStore() in Redux Toolkit.

🧩 Reducers – Handle State Changes

• Reducers are functions that decide how the state should change in response to actions.
• Each reducer manages its own slice of state (e.g., user state, products state, carts state).
• Redux Toolkit allows you to define reducers inside createSlice().

🎯 Action – Event Trigger

• An action is a plain JavaScript object that describes what happened.
• Actions are sent from components using dispatch() and are automatically created via createSlice() in RTK.
• For example:
  { type: "cart/addToCart", payload: { id: 1, name: "Product A" } }

🧵 From App.jsx (Any Component)

• Any component (like App.jsx or its children) can:
  • Read data using useSelector() – it’s read-only, not mutable.
  • Send actions using useDispatch() to update the global state.

Installation and Usage

1. Install Redux Toolkit and React-Redux

Add the Redux Toolkit and React-Redux packages to your project:

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npm install @reduxjs/toolkit react-redux

2. Create a Redux Store

Create a file named src/app/store.js. Import the configureStore API from Redux Toolkit. We’ll start by creating an empty Redux store, and exporting it:

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import { configureStore } from '@reduxjs/toolkit'

export const store = configureStore({
  reducer: {},
})

3. Provide the Redux Store to React

Once the store is created, we can make it available to our React components by putting a React-Redux <Provider> around our application in src/index.js. Import the Redux store we just created, put a <Provider> around your <App>, and pass the store as a prop:

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createRoot(document.getElementById("root")).render(
<Provider store={store}>
<App />
</Provider>
);

Note: Redux installed and below is example usage

4. Create a Redux State Slice

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import { createSlice } from '@reduxjs/toolkit'

const initialState = {
  value: 0,
}

export const counterSlice = createSlice({
  name: 'counter',
  initialState,
  reducers: {
    increment: (state) => {
      // Redux Toolkit allows us to write "mutating" logic in reducers. It
      // doesn't actually mutate the state because it uses the Immer library,
      // which detects changes to a "draft state" and produces a brand new
      // immutable state based off those changes
      state.value += 1
    },
    decrement: (state) => {
      state.value -= 1
    },
    incrementByAmount: (state, action) => {
      state.value += action.payload
    },
  },
})

// Action creators are generated for each case reducer function
export const { increment, decrement, incrementByAmount } = counterSlice.actions

export default counterSlice.reducer

5. Add Slice Reducers to the Store

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import { configureStore } from '@reduxjs/toolkit'
import counterReducer from '../features/counter/counterSlice'

export const store = configureStore({
  reducer: {
    counter: counterReducer,
  },
})

6. Use Redux State and Actions in React Components

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import React from 'react'
import { useSelector, useDispatch } from 'react-redux'
import { decrement, increment } from './counterSlice'

export function Counter() {
  const count = useSelector((state) => state.counter.value)
  const dispatch = useDispatch()

  return (
    <div>
      <div>
        <button
          aria-label="Increment value"
          onClick={() => dispatch(increment())}
        >
          Increment
        </button>
        <span>{count}</span>
        <button
          aria-label="Decrement value"
          onClick={() => dispatch(decrement())}
        >
          Decrement
        </button>
      </div>
    </div>
  )
}

There is always room for improvement and continuous learning, but we’ll end this note/article here. We’ve gained valuable knowledge about React — congratulations!