React components are independent, reusable building blocks in a React application that define what gets displayed on the UI. They accept inputs called props and return React elements describing the UI.

• Components can be reused across different parts of the application to maintain consistency and reduce code duplication.
• Components manage how their output is rendered in the DOM based on their state and props.
• React loads only the necessary components, ensuring optimized performance.
• Only the specific Component updates instead of the whole page.

There are two primary types of React components:

1. Functional Components

Functional components are simpler and preferred for most use cases. They are JavaScript functions that return React elements. With the introduction of React Hooks, functional components can also manage state and lifecycle events.

• Stateless or Stateful: Can manage state using React Hooks.
• Simpler Syntax: Ideal for small and reusable components.
• Performance: Generally faster since they don’t require a ‘this’ keyword.

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

2. Class Components

Class components are ES6 classes that extend React Component. They include additional features like state management and lifecycle methods.

• State Management: State is managed using the this.state property.
• Lifecycle Methods: Includes methods like componentDidMount, componentDidUpdate, etc.

class Greet extends React.Component {
    render() {
        return <h1>Hello, {this.props.name}!</h1>;
  }
}

Props in React Components

Props (short for properties) are read-only inputs passed from a parent component to a child component. They enable dynamic data flow and reusability.

• Props are immutable.
• They enable communication between components.

function Greet(props) {
    return <h2>Welcome, {props.username}!</h2>;
}

// Usage
<Greet username="Anil" />;

State in React Components

The state is a JavaScript object managed within a component, allowing it to maintain and update its own data over time. Unlike props, state is mutable and controlled entirely by the component.

• State updates trigger re-renders.
• Functional components use the useState hook to manage state.

function Counter() {
    const [count, setCount] = React.useState(0);

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

Rendering a Component

Rendering a component refers to displaying it on the browser. React components can be rendered using the ReactDOM.render() method or by embedding them inside other components.

• Ensure the component is imported before rendering.
• The ReactDOM.render method is generally used in the root file.

ReactDOM.render(<Greeting name="Pooja" />, document.getElementById('root'));

Components in Components

In React, you can nest components inside other components to build a modular and hierarchical structure.

• Components can be reused multiple times within the same or different components.
• Props can be passed to nested components for dynamic content.

function Header() {
    return <h1>Welcome to My Site</h1>;
}

function Footer() {
    return <p>© 2024 My Company</p>;
}

function App() {
    return (
        <div>
            <Header />
            <p>This is the main content.</p>
            <Footer />
        </div>
    );
}

export default App;