react-development

What this skill does

# React Development Skill

This skill provides comprehensive guidance for building modern React applications using hooks, components, state management, context, effects, and performance optimization techniques based on official React documentation from react.dev.

## When to Use This Skill

Use this skill when:
- Building single-page applications (SPAs) with React
- Creating reusable UI components and component libraries
- Managing complex application state with hooks and context
- Implementing forms, data fetching, and side effects
- Optimizing React application performance
- Building interactive user interfaces with dynamic data
- Migrating class components to functional components with hooks
- Implementing global state management without external libraries
- Creating custom hooks for reusable logic
- Building accessible and performant web applications

## Core Concepts

### Components

Components are the building blocks of React applications. They let you split the UI into independent, reusable pieces.

**Functional Components (Modern Approach):**
```jsx
function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

// Arrow function syntax
const Greeting = ({ name, age }) => {
  return (
    <div>
      <h2>{name}</h2>
      <p>Age: {age}</p>
    </div>
  );
};
```

**Component Composition:**
```jsx
function App() {
  return (
    <div>
      <Welcome name="Sara" />
      <Welcome name="Cahal" />
      <Welcome name="Edite" />
    </div>
  );
}
```

### JSX

JSX is a syntax extension for JavaScript that looks similar to HTML. It produces React elements.

**JSX Fundamentals:**
```jsx
// Embedding expressions
const name = 'Josh Perez';
const element = <h1>Hello, {name}</h1>;

// JSX attributes
const image = <img src={user.avatarUrl} alt={user.name} />;

// JSX children
const container = (
  <div>
    <h1>Welcome</h1>
    <p>Get started with React</p>
  </div>
);

// Conditional rendering
const greeting = (
  <div>
    {isLoggedIn ? <UserGreeting /> : <GuestGreeting />}
  </div>
);

// Lists and keys
const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
  <li key={number.toString()}>{number}</li>
);
```

### Props

Props are arguments passed into React components. They are passed to components via HTML attributes.

**Passing and Using Props:**
```jsx
function Product({ name, price, inStock }) {
  return (
    <div className="product">
      <h3>{name}</h3>
      <p>${price}</p>
      {inStock ? <span>In Stock</span> : <span>Out of Stock</span>}
    </div>
  );
}

// Usage
<Product name="Laptop" price={999} inStock={true} />
```

**Props with Children:**
```jsx
function Card({ title, children }) {
  return (
    <div className="card">
      <h2>{title}</h2>
      <div className="card-content">
        {children}
      </div>
    </div>
  );
}

// Usage
<Card title="Welcome">
  <p>This is the card content</p>
  <button>Click me</button>
</Card>
```

**Default Props:**
```jsx
function Button({ text = 'Click me', variant = 'primary' }) {
  return <button className={variant}>{text}</button>;
}
```

### State

State is a component's memory. It lets components remember information and respond to user interactions.

**Local Component State:**
```jsx
import { useState } from 'react';

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

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setCount(count - 1)}>Decrement</button>
      <button onClick={() => setCount(0)}>Reset</button>
    </div>
  );
}
```

## React Hooks

Hooks let you use state and other React features in functional components.

### useState

The useState hook lets you add state to functional components.

**Basic Usage:**
```jsx
import { useState } from 'react';

function Form() {
  const [name, setName] = useState('');
  const [email, setEmail] = useState('');

  const handleSubmit = (e) => {
    e.preventDefault();
    console.log({ name, email });
  };

  return (
    <form onSubmit={handleSubmit}>
      <input
        value={name}
        onChange={(e) => setName(e.target.value)}
        placeholder="Name"
      />
      <input
        value={email}
        onChange={(e) => setEmail(e.target.value)}
        placeholder="Email"
        type="email"
      />
      <button type="submit">Submit</button>
    </form>
  );
}
```

**State with Objects:**
```jsx
function UserProfile() {
  const [user, setUser] = useState({
    name: '',
    age: 0,
    email: ''
  });

  const updateField = (field, value) => {
    setUser(prev => ({
      ...prev,
      [field]: value
    }));
  };

  return (
    <div>
      <input
        value={user.name}
        onChange={(e) => updateField('name', e.target.value)}
      />
      <input
        type="number"
        value={user.age}
        onChange={(e) => updateField('age', parseInt(e.target.value))}
      />
      <input
        type="email"
        value={user.email}
        onChange={(e) => updateField('email', e.target.value)}
      />
    </div>
  );
}
```

**State with Arrays:**
```jsx
function TodoList() {
  const [todos, setTodos] = useState([]);
  const [input, setInput] = useState('');

  const addTodo = () => {
    setTodos(prev => [...prev, { id: Date.now(), text: input }]);
    setInput('');
  };

  const removeTodo = (id) => {
    setTodos(prev => prev.filter(todo => todo.id !== id));
  };

  return (
    <div>
      <input value={input} onChange={(e) => setInput(e.target.value)} />
      <button onClick={addTodo}>Add</button>
      <ul>
        {todos.map(todo => (
          <li key={todo.id}>
            {todo.text}
            <button onClick={() => removeTodo(todo.id)}>Delete</button>
          </li>
        ))}
      </ul>
    </div>
  );
}
```

### useEffect

The useEffect hook lets you perform side effects in functional components.

**Basic Side Effects:**
```jsx
import { useState, useEffect } from 'react';

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

  useEffect(() => {
    document.title = `Count: ${count}`;
  }, [count]);

  return (
    <button onClick={() => setCount(count + 1)}>
      Clicked {count} times
    </button>
  );
}
```

**Data Fetching:**
```jsx
function UserData({ userId }) {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    let cancelled = false;

    async function fetchUser() {
      try {
        setLoading(true);
        const response = await fetch(`/api/users/${userId}`);
        const data = await response.json();

        if (!cancelled) {
          setUser(data);
          setError(null);
        }
      } catch (err) {
        if (!cancelled) {
          setError(err.message);
        }
      } finally {
        if (!cancelled) {
          setLoading(false);
        }
      }
    }

    fetchUser();

    return () => {
      cancelled = true;
    };
  }, [userId]);

  if (loading) return <div>Loading...</div>;
  if (error) return <div>Error: {error}</div>;

  return <div>{user?.name}</div>;
}
```

**Event Listeners and Cleanup:**
```jsx
function WindowSize() {
  const [size, setSize] = useState({
    width: window.innerWidth,
    height: window.innerHeight
  });

  useEffect(() => {
    function handleResize() {
      setSize({
        width: window.innerWidth,
        height: window.innerHeight
      });
    }

    window.addEventListener('resize', handleResize);

    // Cleanup function
    return () => {
      window.removeEventListener('resize', handleResize);
    };
  }, []); // Empty dependency array = run once on mount

  return <div>{size.width} x {size.height}</div>;
}
```

**Timers and Intervals:**
```jsx
function Timer() {
  const [seconds, setSeconds] = useState(0);
  const [isRunning, setIsRunning] = useState(false);

  useEffect(() => {
    if (!isRunning) return;

    const interval = setInterval(() => {
      setSeconds(s => s +