React Шпаргалка

// Basic function component
function Welcome({ name }) {
  return <h1>Hello, {name}!</h1>;
}

// Arrow function component
const Greeting = ({ name = 'Guest' }) => (
  <p>Welcome, {name}!</p>
);

// Composing components
function App() {
  return (
    <div>
      <Welcome name="Alice" />
      <Welcome name="Bob" />
      <Greeting />
    </div>
  );
}

function Expression({ user, items }) {
  const fullName = user.first + ' ' + user.last;
  const itemCount = items.length;

  return (
    <div>
      {/* Expressions in curly braces */}
      <h1>{fullName}</h1>
      <p>{itemCount} items</p>

      {/* Conditional */}
      <p>{itemCount > 0 ? 'In stock' : 'Sold out'}</p>

      {/* Method calls */}
      <p>{fullName.toUpperCase()}</p>

      {/* Numbers and booleans render as nothing */}
      <p>{false}{null}{undefined}</p>
    </div>
  );
}

// Fragment: group without extra DOM node
function App() {
  return (
    <>
      <header>Header</header>
      <main>Content</main>
      <footer>Footer</footer>
    </>
  );
}

// Array of elements (needs keys)
function List() {
  const fruits = ['Apple', 'Banana', 'Cherry'];
  return (
    <ul>
      {fruits.map((fruit, i) => (
        <li key={i}>{fruit}</li>
      ))}
    </ul>
  );
}

function Greeting({ isLoggedIn, user }) {
  // 1. If/else (use early return)
  if (!isLoggedIn) return <Login />;

  // 2. Ternary operator
  return (
    <div>
      {user ? <Dashboard user={user} /> : <Loading />}

      {/* 3. Logical AND (render if truthy) */}
      {user.isAdmin && <AdminPanel />}

      {/* 4. IIFE for complex logic */}
      {(() => {
        if (user.role === 'admin') return <Admin />;
        if (user.role === 'mod') return <Mod />;
        return <User />;
      })()}
    </div>
  );
}

// children prop: content between tags
function Card({ title, children }) {
  return (
    <div className="card">
      <h2>{title}</h2>
      <div className="card-body">{children}</div>
    </div>
  );
}

// Usage
<Card title="Profile">
  <p>Name: Alice</p>
  <p>Age: 30</p>
</Card>

// Render prop pattern
function DataProvider({ render }) {
  const data = fetchData();
  return <div>{render(data)}</div>;
}

// Parent passes props to child
function App() {
  return (
    <User
      name="Alice"
      age={30}
      isActive={true}
      tags={['admin', 'dev']}
      onClick={() => console.log('clicked')}
    />
  );
}

// Child receives props
function User({ name, age, isActive, tags, onClick }) {
  return (
    <div onClick={onClick}>
      <h1>{name}</h1>
      <p>Age: {age}</p>
      <p>Status: {isActive ? 'Active' : 'Inactive'}</p>
    </div>
  );
}

// Default values via destructuring
function Button({ color = 'blue', size = 'md', children }) {
  return (
    <button className={'btn btn-' + color + ' btn-' + size}>
      {children}
    </button>
  );
}

// Optional props (undefined if not passed)
function Profile({ name, bio }) {
  return (
    <div>
      <h1>{name}</h1>
      {bio && <p>{bio}</p>}
    </div>
  );
}

// Usage
<Button>Click</Button>           {/* color='blue', size='md' */}
<Profile name="Alice" />         {/* bio is undefined */}

// Spread: pass all props to child
function Input(props) {
  return <input {...props} className="input" />;
}

// Usage
<Input type="text" placeholder="Name" value="Alice" />

// Rest: collect remaining props
function Button({ label, ...rest }) {
  return <button {...rest}>{label}</button>;
}

// Selective spreading
function Card({ title, children, ...divProps }) {
  return (
    <div {...divProps}>
      <h2>{title}</h2>
      {children}
    </div>
  );
}

// TypeScript interface (recommended)
interface UserProps {
  name: string;
  age: number;
  isActive?: boolean;       // optional
  onClick: (id: number) => void;
}

function User({ name, age, isActive = true, onClick }: UserProps) {
  return <div onClick={() => onClick(1)}>{name}, {age}</div>;
}

// PropTypes (runtime checking, legacy)
import PropTypes from 'prop-types';
User.propTypes = {
  name: PropTypes.string.isRequired,
  age: PropTypes.number,
  isActive: PropTypes.bool,
};

// Prop drilling: passing through multiple levels
function App() {
  const [user, setUser] = useState(null);
  return <Layout user={user} />;
}

function Layout({ user }) {
  return <Sidebar user={user} />;
}

function Sidebar({ user }) {
  return <UserInfo user={user} />;
}

// Solution: Context API (see State Management section)
// Avoid drilling more than 2-3 levels

import { useState } from 'react';

function Counter() {
  // [currentValue, setterFunction] = useState(initialValue)
  const [count, setCount] = useState(0);

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

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

  // BAD: may not work correctly with rapid updates
  const incrementBad = () => setCount(count + 1);

  // GOOD: functional update uses previous state
  const incrementGood = () => setCount(prev => prev + 1);

  // Batch updates
  const addThree = () => {
    setCount(prev => prev + 1);
    setCount(prev => prev + 1);
    setCount(prev => prev + 1);
  };

  return <button onClick={addThree}>Count: {count}</button>;
}

function Form() {
  const [form, setForm] = useState({ name: '', email: '', age: 0 });

  // BAD: mutates state directly
  // form.name = 'Alice'; setForm(form);

  // GOOD: spread to create new object
  const updateField = (field, value) => {
    setForm(prev => ({ ...prev, [field]: value }));
  };

  return (
    <input
      value={form.name}
      onChange={e => updateField('name', e.target.value)}
    />
  );
}

// Array state
function TodoList() {
  const [todos, setTodos] = useState([]);
  const addTodo = (text) => setTodos(prev => [...prev, { id: Date.now(), text }]);
  const removeTodo = (id) => setTodos(prev => prev.filter(t => t.id !== id));
}

import { useState } from 'react';

function ExpensiveInit() {
  // BAD: runs on every render (even though result is ignored)
  const [data, setData] = useState(computeExpensiveValue());

  // GOOD: lazy initialization — function runs only once
  const [data2, setData2] = useState(() => computeExpensiveValue());

  // Reading from localStorage
  const [user, setUser] = useState(() => {
    const saved = localStorage.getItem('user');
    return saved ? JSON.parse(saved) : null;
  });

  return <div>{data2}</div>;
}

function LoginForm() {
  // Multiple independent state variables
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');
  const [errors, setErrors] = useState({});
  const [isSubmitting, setIsSubmitting] = useState(false);
  const [rememberMe, setRememberMe] = useState(false);

  const handleSubmit = (e) => {
    e.preventDefault();
    setIsSubmitting(true);
    // ... validation and submission
  };

  return (
    <form onSubmit={handleSubmit}>
      <input value={email} onChange={e => setEmail(e.target.value)} />
      <input type="password" value={password}
        onChange={e => setPassword(e.target.value)} />
      <button disabled={isSubmitting}>Submit</button>
    </form>
  );
}

import { useState, useEffect } from 'react';

function Timer() {
  const [seconds, setSeconds] = useState(0);

  // Runs after every render
  useEffect(() => {
    const interval = setInterval(() => {
      setSeconds(s => s + 1);
    }, 1000);

    // Cleanup function runs before next effect or unmount
    return () => clearInterval(interval);
  }, []); // empty array = run once on mount

  return <p>Seconds: {seconds}</p>;
}

function UserProfile({ userId }) {
  const [user, setUser] = useState(null);

  // Runs once on mount (empty deps)
  useEffect(() => {
    console.log('Component mounted');
  }, []);

  // Runs when userId changes
  useEffect(() => {
    fetch('/api/users/' + userId)
      .then(r => r.json())
      .then(setUser);
  }, [userId]); // re-run when userId changes

  // Runs on every render (no deps) - rarely needed
  useEffect(() => {
    console.log('Every render');
  });

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

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    const ws = new WebSocket('wss://chat.example.com/' + roomId);

    ws.onmessage = (event) => {
      setMessages(prev => [...prev, JSON.parse(event.data)]);
    };

    // Cleanup: close connection when roomId changes or unmount
    return () => {
      ws.close();
      setMessages([]); // reset for new room
    };
  }, [roomId]);

  // Window event listener
  useEffect(() => {
    const handleResize = () => console.log(window.innerWidth);
    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, []);

  return <div>{messages.length} messages</div>;
}

function UserList() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

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

    const fetchData = async () => {
      try {
        setLoading(true);
        const res = await fetch('/api/users');
        const data = await res.json();
        if (!cancelled) setUsers(data);
      } catch (err) {
        if (!cancelled) setError(err.message);
      } finally {
        if (!cancelled) setLoading(false);
      }
    };

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

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;
  return <ul>{users.map(u => <li key={u.id}>{u.name}</li>)}</ul>;
}

import { useState, useEffect, useLayoutEffect } from 'react';

function Tooltip({ text }) {
  const [position, setPosition] = useState({ x: 0, y: 0 });

  // useEffect: runs AFTER paint (user may see flash)
  useEffect(() => {
    const rect = document.getElementById('tip').getBoundingClientRect();
    setPosition({ x: rect.x, y: rect.y });
  }, [text]);

  // useLayoutEffect: runs BEFORE paint (no flash)
  useLayoutEffect(() => {
    const rect = document.getElementById('tip').getBoundingClientRect();
    setPosition({ x: rect.x, y: rect.y });
  }, [text]);

  return <div id="tip" style={{ left: position.x, top: position.y }}>{text}</div>;
}

import { useRef } from 'react';

function FocusInput() {
  // ref to access DOM element
  const inputRef = useRef(null);

  const focus = () => inputRef.current.focus();
  const clear = () => {
    inputRef.current.value = '';
    inputRef.current.focus();
  };

  return (
    <div>
      <input ref={inputRef} type="text" />
      <button onClick={focus}>Focus</button>
      <button onClick={clear}>Clear</button>
    </div>
  );
}

function Stopwatch() {
  const [seconds, setSeconds] = useState(0);
  const intervalRef = useRef(null);
  const renderCount = useRef(0);

  // Track render count (doesn't trigger re-render)
  renderCount.current++;

  const start = () => {
    if (intervalRef.current) return;
    intervalRef.current = setInterval(() => {
      setSeconds(s => s + 1);
    }, 1000);
  };

  const stop = () => {
    clearInterval(intervalRef.current);
    intervalRef.current = null;
  };

  return (
    <div>
      <p>{seconds}s (render #{renderCount.current})</p>
      <button onClick={start}>Start</button>
      <button onClick={stop}>Stop</button>
    </div>
  );
}

import { useState, useMemo } from 'react';

function ProductList({ products, filter }) {
  const [search, setSearch] = useState('');

  // Memoize expensive computation
  const filtered = useMemo(() => {
    console.log('Filtering...');
    return products
      .filter(p => p.category === filter)
      .filter(p => p.name.includes(search));
  }, [products, filter, search]); // recompute only when these change

  // Memoize a value
  const totalPrice = useMemo(() =>
    filtered.reduce((sum, p) => sum + p.price, 0),
  [filtered]);

  return (
    <div>
      <input value={search} onChange={e => setSearch(e.target.value)} />
      <p>Total: ${totalPrice}</p>
      {filtered.map(p => <div key={p.id}>{p.name}</div>)}
    </div>
  );
}

import { useState, useCallback, memo } from 'react';

// Memoized child component
const Button = memo(function Button({ onClick, label }) {
  console.log('Button rendered');
  return <button onClick={onClick}>{label}</button>;
});

function App() {
  const [count, setCount] = useState(0);
  const [text, setText] = useState('');

  // Without useCallback: new function every render
  // const handleClick = () => setCount(c => c + 1);

  // With useCallback: stable function reference
  const handleClick = useCallback(() => {
    setCount(c => c + 1);
  }, []); // empty deps = stable forever

  return (
    <div>
      <input value={text} onChange={e => setText(e.target.value)} />
      <Button onClick={handleClick} label="Click" />
      <p>Count: {count}</p>
    </div>
  );
}

import { useRef, forwardRef, useImperativeHandle } from 'react';

// forwardRef: pass ref to child component
const FancyInput = forwardRef(function FancyInput(props, ref) {
  return <input ref={ref} className="fancy" {...props} />;
});

// useImperativeHandle: expose specific methods
const CustomInput = forwardRef(function CustomInput(props, ref) {
  const inputRef = useRef(null);

  useImperativeHandle(ref, () => ({
    focus: () => inputRef.current.focus(),
    clear: () => { inputRef.current.value = ''; },
    getValue: () => inputRef.current.value,
  }));

  return <input ref={inputRef} />;
});

// Usage
function App() {
  const ref = useRef(null);
  return (
    <>
      <CustomInput ref={ref} />
      <button onClick={() => ref.current.focus()}>Focus</button>
      <button onClick={() => ref.current.clear()}>Clear</button>
    </>
  );
}

import { useReducer } from 'react';

// Reducer function: (state, action) => newState
function counterReducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    case 'reset':
      return { count: 0 };
    case 'set':
      return { count: action.payload };
    default:
      throw new Error('Unknown action: ' + action.type);
  }
}

function Counter() {
  const [state, dispatch] = useReducer(counterReducer, { count: 0 });

  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>+</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
      <button onClick={() => dispatch({ type: 'reset' })}>Reset</button>
      <button onClick={() => dispatch({ type: 'set', payload: 10 })}>Set 10</button>
    </div>
  );
}

const initialState = {
  todos: [],
  filter: 'all',
  loading: false,
};

function todoReducer(state, action) {
  switch (action.type) {
    case 'add':
      return { ...state, todos: [...state.todos, action.todo] };
    case 'toggle':
      return {
        ...state,
        todos: state.todos.map(t =>
          t.id === action.id ? { ...t, done: !t.done } : t
        ),
      };
    case 'delete':
      return { ...state, todos: state.todos.filter(t => t.id !== action.id) };
    case 'set_filter':
      return { ...state, filter: action.filter };
    case 'set_loading':
      return { ...state, loading: action.loading };
    default:
      return state;
  }
}

function TodoApp() {
  const [state, dispatch] = useReducer(todoReducer, initialState);
  // dispatch({ type: 'add', todo: { id: 1, text: 'Learn React', done: false } })
}

import { createContext, useContext, useState } from 'react';

// 1. Create context with default value
const ThemeContext = createContext('light');
const UserContext = createContext(null);

// 2. Provider component
function App() {
  const [theme, setTheme] = useState('light');
  const [user, setUser] = useState(null);

  return (
    <ThemeContext.Provider value={{ theme, setTheme }}>
      <UserContext.Provider value={{ user, setUser }}>
        <Page />
      </UserContext.Provider>
    </ThemeContext.Provider>
  );
}

// 3. Consume context
function Page() {
  const { theme } = useContext(ThemeContext);
  const { user } = useContext(UserContext);
  return <div className={'page theme-' + theme}>Hello {user?.name}</div>;
}

import { createContext, useContext, useReducer } from 'react';

// Combine Context + useReducer for global state
const StoreContext = createContext(null);

function StoreProvider({ children }) {
  const [state, dispatch] = useReducer(reducer, initialState);
  return (
    <StoreContext.Provider value={{ state, dispatch }}>
      {children}
    </StoreContext.Provider>
  );
}

// Custom hook for easy consumption
function useStore() {
  const context = useContext(StoreContext);
  if (!context) throw new Error('useStore must be used within StoreProvider');
  return context;
}

// Usage
function Component() {
  const { state, dispatch } = useStore();
  return <button onClick={() => dispatch({ type: 'action' })}>Click</button>;
}

// SPLIT contexts for performance
const ThemeContext = createContext();
const UserContext = createContext();
const CartContext = createContext();

// Each provider manages its own state
function App() {
  return (
    <ThemeProvider>
      <UserProvider>
        <CartProvider>
          <App />
        </CartProvider>
      </UserProvider>
    </ThemeProvider>
  );
}

// Component only re-renders when its context changes
function ThemedButton() {
  const { theme } = useContext(ThemeContext);
  // Won't re-render when user or cart changes
  return <button className={theme}>Button</button>;
}

// Memoize context value to prevent unnecessary re-renders
function UserProvider({ children }) {
  const [user, setUser] = useState(null);
  const value = useMemo(() => ({ user, setUser }), [user]);
  return <UserContext.Provider value={value}>{children}</UserContext.Provider>;
}

function App() {
  // Click event
  const handleClick = (e) => {
    e.preventDefault();
    console.log('Button clicked', e.target);
  };

  // With parameters (use arrow function)
  const handleDelete = (id) => {
    console.log('Delete item', id);
  };

  return (
    <div>
      <button onClick={handleClick}>Click</button>
      <button onClick={() => handleDelete(42)}>Delete</button>
      <div onMouseEnter={() => console.log('hover')}
           onMouseLeave={() => console.log('leave')}>
        Hover me
      </div>
    </div>
  );
}

function LoginForm() {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  const handleSubmit = (e) => {
    e.preventDefault();
    console.log('Email:', email, 'Password:', password);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input
        type="email"
        value={email}
        onChange={e => setEmail(e.target.value)}
        placeholder="Email"
      />
      <input
        type="password"
        value={password}
        onChange={e => setPassword(e.target.value)}
        placeholder="Password"
      />
      <button type="submit">Login</button>
    </form>
  );
}

function RegistrationForm() {
  const [formData, setFormData] = useState({
    username: '',
    email: '',
    password: '',
    country: 'us',
    agree: false,
  });

  const handleChange = (e) => {
    const { name, value, type, checked } = e.target;
    setFormData(prev => ({
      ...prev,
      [name]: type === 'checkbox' ? checked : value,
    }));
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    console.log(formData);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input name="username" value={formData.username} onChange={handleChange} />
      <input name="email" type="email" value={formData.email} onChange={handleChange} />
      <select name="country" value={formData.country} onChange={handleChange}>
        <option value="us">USA</option>
        <option value="uk">UK</option>
      </select>
      <label>
        <input type="checkbox" name="agree" checked={formData.agree} onChange={handleChange} />
        Agree to terms
      </label>
      <button type="submit">Register</button>
    </form>
  );
}

import { useRef } from 'react';

function UncontrolledForm() {
  const emailRef = useRef(null);
  const passwordRef = useRef(null);

  const handleSubmit = (e) => {
    e.preventDefault();
    console.log('Email:', emailRef.current.value);
    console.log('Password:', passwordRef.current.value);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input ref={emailRef} type="email" defaultValue="" />
      <input ref={passwordRef} type="password" defaultValue="" />
      <button type="submit">Submit</button>
    </form>
  );
}

// File input (must be uncontrolled)
function FileUpload() {
  const fileRef = useRef(null);
  return <input ref={fileRef} type="file" />;
}

function ValidatedForm() {
  const [values, setValues] = useState({ email: '', password: '' });
  const [errors, setErrors] = useState({});

  const validate = () => {
    const errs = {};
    if (!values.email) errs.email = 'Email is required';
    else if (!/\S+@\S+\.\S+/.test(values.email)) errs.email = 'Invalid email';
    if (!values.password) errs.password = 'Password is required';
    else if (values.password.length < 8) errs.password = 'Min 8 characters';
    return errs;
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    const errs = validate();
    setErrors(errs);
    if (Object.keys(errs).length === 0) {
      console.log('Form valid', values);
    }
  };

  return (
    <form onSubmit={handleSubmit}>
      <input value={values.email}
        onChange={e => setValues(v => ({ ...v, email: e.target.value }))} />
      {errors.email && <span className="error">{errors.email}</span>}
      <input type="password" value={values.password}
        onChange={e => setValues(v => ({ ...v, password: e.target.value }))} />
      {errors.password && <span className="error">{errors.password}</span>}
      <button type="submit">Submit</button>
    </form>
  );
}

function TodoList({ todos }) {
  return (
    <ul>
      {todos.map(todo => (
        <li key={todo.id}>
          <span>{todo.text}</span>
          <button onClick={() => toggle(todo.id)}>
            {todo.done ? 'Undo' : 'Done'}
          </button>
        </li>
      ))}
    </ul>
  );
}

// Filtering and sorting
function FilteredList({ items, filter }) {
  const visible = items
    .filter(item => item.category === filter)
    .sort((a, b) => a.name.localeCompare(b.name));

  return (
    <ul>
      {visible.map(item => <li key={item.id}>{item.name}</li>)}
    </ul>
  );
}

// GOOD: stable, unique keys
{todos.map(todo => (
  <TodoItem key={todo.id} todo={todo} />
))}

// BAD: index as key (causes bugs with reordering)
{todos.map((todo, index) => (
  <TodoItem key={index} todo={todo} />
))}

// When index keys are OK:
// - Static list (never reordered/filtered)
// - List items have no state
// - List is never prepended to

// Key must be unique among siblings
function List() {
  return (
    <div>
      {users.map(u => <User key={u.id} user={u} />)}
      {posts.map(p => <Post key={p.id} post={p} />)}
      {/* IDs can repeat across different lists */}
    </div>
  );
}

function UserDashboard({ user, loading, error }) {
  // 1. Early returns for loading/error states
  if (loading) return <Spinner />;
  if (error) return <ErrorMessage error={error} />;
  if (!user) return <Login />;

  // 2. Element variables
  let greeting;
  if (user.isAdmin) {
    greeting = <h1>Welcome Admin {user.name}</h1>;
  } else {
    greeting = <h1>Welcome {user.name}</h1>;
  }

  return (
    <div>
      {greeting}

      {/* 3. Ternary for either/or */}
      {user.hasNotifications ? <NotificationBadge /> : null}

      {/* 4. && for show/hide */}
      {user.isAdmin && <AdminPanel />}

      {/* 5. Switch via object lookup */}
      {{ free: <FreePlan />, pro: <ProPlan />, enterprise: <EnterprisePlan /> }
        [user.plan]}
    </div>
  );
}

function SearchableList({ items }) {
  const [query, setQuery] = useState('');
  const [category, setCategory] = useState('all');

  // Memoize filtered results
  const filtered = useMemo(() => {
    return items.filter(item => {
      const matchesQuery = item.name.toLowerCase().includes(query.toLowerCase());
      const matchesCategory = category === 'all' || item.category === category;
      return matchesQuery && matchesCategory;
    });
  }, [items, query, category]);

  return (
    <div>
      <input
        value={query}
        onChange={e => setQuery(e.target.value)}
        placeholder="Search..."
      />
      <select value={category} onChange={e => setCategory(e.target.value)}>
        <option value="all">All</option>
        <option value="food">Food</option>
        <option value="tech">Tech</option>
      </select>

      {filtered.length === 0 ? (
        <p>No results found</p>
      ) : (
        <ul>
          {filtered.map(item => <li key={item.id}>{item.name}</li>)}
        </ul>
      )}
    </div>
  );
}

// Render different components based on type
const componentMap = {
  text: TextBlock,
  image: ImageBlock,
  video: VideoBlock,
  quote: QuoteBlock,
};

function ContentRenderer({ blocks }) {
  return (
    <div>
      {blocks.map(block => {
        const Component = componentMap[block.type];
        if (!Component) return <UnknownBlock key={block.id} type={block.type} />;

        return <Component key={block.id} {...block.props} />;
      })}
    </div>
  );
}

// Usage
const blocks = [
  { id: 1, type: 'text', props: { content: 'Hello' } },
  { id: 2, type: 'image', props: { src: 'pic.jpg', alt: 'Picture' } },
];

import { memo } from 'react';

// Memoized component: only re-renders if props change
const ExpensiveCard = memo(function ExpensiveCard({ title, content }) {
  console.log('Card rendered');
  return (
    <div className="card">
      <h2>{title}</h2>
      <p>{content}</p>
    </div>
  );
});

// Custom comparison function
const MyComponent = memo(function MyComponent(props) {
  return <div>{props.value}</div>;
}, (prevProps, nextProps) => {
  // Return true if props are equal (skip re-render)
  return prevProps.value === nextProps.value;
});

import { lazy, Suspense } from 'react';

// Lazy load component (code-split)
const AdminPanel = lazy(() => import('./AdminPanel'));
const Dashboard = lazy(() => import('./Dashboard'));

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <Router>
        <Route path="/admin" element={<AdminPanel />} />
        <Route path="/dashboard" element={<Dashboard />} />
      </Router>
    </Suspense>
  );
}

// Route-based splitting (most common)
// Component-based splitting
const HeavyChart = lazy(() => import('./HeavyChart'));

function Page({ showChart }) {
  return (
    <div>
      {showChart && (
        <Suspense fallback={<Spinner />}>
          <HeavyChart />
        </Suspense>
      )}
    </div>
  );
}

import { FixedSizeList } from 'react-window';

// Virtualized list: only renders visible items
function BigList({ items }) {
  const Row = ({ index, style }) => (
    <div style={style}>
      {items[index].name}
    </div>
  );

  return (
    <FixedSizeList
      height={600}
      width="100%"
      itemCount={items.length}
      itemSize={50}
    >
      {Row}
    </FixedSizeList>
  );
}

// Without virtualization: rendering 10,000 items is slow
// With virtualization: only ~12 visible items are rendered

import { useState, useEffect } from 'react';

// Debounce hook: delays execution until user stops typing
function useDebounce(value, delay) {
  const [debounced, setDebounced] = useState(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);

  return debounced;
}

function SearchInput() {
  const [query, setQuery] = useState('');
  const debouncedQuery = useDebounce(query, 300);

  // API call only fires when user stops typing for 300ms
  useEffect(() => {
    if (debouncedQuery) {
      fetch('/api/search?q=' + debouncedQuery);
    }
  }, [debouncedQuery]);

  return <input value={query} onChange={e => setQuery(e.target.value)} />;
}

import { Profiler } from 'react';

function App() {
  const onRender = (id, phase, actualDuration) => {
    console.log(id + ' ' + phase + ': ' + actualDuration + 'ms');
  };

  return (
    <Profiler id="App" onRender={onRender}>
      <ExpensiveComponent />
    </Profiler>
  );
}

/* Optimization checklist:
1. React.memo for expensive components
2. useMemo for expensive calculations
3. useCallback for props passed to memoized children
4. Code splitting (lazy) for routes
5. Virtualization for long lists
6. Debounce rapid events (search, resize)
7. Avoid inline objects/functions as props
8. Use keys correctly in lists
9. Profile with React DevTools Profiler
10. Check unnecessary re-renders with why-did-you-render
*/

import { useState, useEffect } from 'react';

// Reusable data fetching hook
function useFetch(url) {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    let cancelled = false;
    setLoading(true);
    fetch(url)
      .then(res => {
        if (!res.ok) throw new Error('HTTP ' + res.status);
        return res.json();
      })
      .then(data => { if (!cancelled) { setData(data); setError(null); }})
      .catch(err => { if (!cancelled) setError(err.message); })
      .finally(() => { if (!cancelled) setLoading(false); });
    return () => { cancelled = true; };
  }, [url]);

  return { data, loading, error };
}

// Usage
function UserProfile({ id }) {
  const { data: user, loading, error } = useFetch('/api/users/' + id);
  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;
  return <h1>{user.name}</h1>;
}

import { useState, useEffect } from 'react';

function useLocalStorage(key, initialValue) {
  const [value, setValue] = useState(() => {
    try {
      const saved = localStorage.getItem(key);
      return saved ? JSON.parse(saved) : initialValue;
    } catch {
      return initialValue;
    }
  });

  useEffect(() => {
    try {
      localStorage.setItem(key, JSON.stringify(value));
    } catch (e) {
      console.error('LocalStorage error:', e);
    }
  }, [key, value]);

  return [value, setValue];
}

// Usage
function ThemeToggle() {
  const [theme, setTheme] = useLocalStorage('theme', 'light');
  return (
    <button onClick={() => setTheme(t => t === 'light' ? 'dark' : 'light')}>
      Current: {theme}
    </button>
  );
}

import { Component } from 'react';

class ErrorBoundary extends Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false, error: null };
  }

  static getDerivedStateFromError(error) {
    return { hasError: true, error };
  }

  componentDidCatch(error, errorInfo) {
    console.error('Caught error:', error, errorInfo);
    // Send to error reporting service
    // logErrorToService(error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      return (
        this.props.fallback || (
          <div>
            <h1>Something went wrong.</h1>
            <p>{this.state.error?.message}</p>
            <button onClick={() => this.setState({ hasError: false })}>
              Try again
            </button>
          </div>
        )
      );
    }
    return this.props.children;
  }
}

// Usage: wrap components
<ErrorBoundary fallback={<ErrorPage />}>
  <App />
</ErrorBoundary>

// HOC: function that takes a component and returns a new one
function withLoading(Component) {
  return function WithLoading({ isLoading, ...props }) {
    if (isLoading) return <div>Loading...</div>;
    return <Component {...props} />;
  };
}

// HOC for authentication
function withAuth(Component) {
  return function WithAuth(props) {
    const { user } = useContext(AuthContext);
    if (!user) return <Redirect to="/login" />;
    return <Component {...props} user={user} />;
  };
}

// Usage
const UserList = withLoading(withAuth(BaseUserList));

// Note: Prefer hooks over HOCs for new code
// HOCs are mainly for class components or library compatibility

// Compound components: components that work together
function Select({ children, value, onChange }) {
  const [isOpen, setIsOpen] = useState(false);
  const context = { value, onChange, isOpen, setIsOpen };

  return (
    <SelectContext.Provider value={context}>
      <div className="select">{children}</div>
    </SelectContext.Provider>
  );
}

Select.Trigger = function Trigger({ children }) {
  const { isOpen, setIsOpen } = useContext(SelectContext);
  return <button onClick={() => setIsOpen(!isOpen)}>{children}</button>;
};

Select.Options = function Options({ children }) {
  const { isOpen } = useContext(SelectContext);
  return isOpen ? <div className="options">{children}</div> : null;
};

Select.Option = function Option({ value, children }) {
  const { onChange, setIsOpen } = useContext(SelectContext);
  return (
    <div onClick={() => { onChange(value); setIsOpen(false); }}>
      {children}
    </div>
  );
}

// Usage: clean, declarative API
<Select value={val} onChange={setVal}>
  <Select.Trigger>Choose...</Select.Trigger>
  <Select.Options>
    <Select.Option value="a">Option A</Select.Option>
    <Select.Option value="b">Option B</Select.Option>
  </Select.Options>
</Select>

function useFetch(url, options = {}) {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    let abortController = new AbortController();
    setLoading(true);
    fetch(url, { ...options, signal: abortController.signal })
      .then((res) => {
        if (!res.ok) throw new Error(res.statusText);
        return res.json();
      })
      .then((data) => { setData(data); setError(null); })
      .catch((err) => {
        if (err.name !== "AbortError") setError(err.message);
      })
      .finally(() => setLoading(false));
    return () => abortController.abort();
  }, [url]);

  return { data, loading, error };
}

// Usage
function Profile({ userId }) {
  const { data, loading, error } = useFetch(`/api/users/${userId}`);
  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;
  return <div>{data.name}</div>;
}

function useLocalStorage(key, initialValue) {
  const [value, setValue] = useState(() => {
    try {
      const stored = window.localStorage.getItem(key);
      return stored ? JSON.parse(stored) : initialValue;
    } catch {
      return initialValue;
    }
  });

  useEffect(() => {
    try {
      window.localStorage.setItem(key, JSON.stringify(value));
    } catch (e) {
      console.error("LocalStorage write failed:", e);
    }
  }, [key, value]);

  return [value, setValue];
}

// Usage: persists state across page reloads
function Settings() {
  const [theme, setTheme] = useLocalStorage("theme", "light");
  const [fontSize, setFontSize] = useLocalStorage("fontSize", 14);
  return (
    <div>
      <button onClick={() => setTheme("dark")}>Dark</button>
      <button onClick={() => setTheme("light")}>Light</button>
    </div>
  );
}

function useDebounce(value, delay = 500) {
  const [debounced, setDebounced] = useState(value);
  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);
  return debounced;
}

// Usage: debounce search input
function Search() {
  const [query, setQuery] = useState("");
  const debouncedQuery = useDebounce(query, 300);

  useEffect(() => {
    if (debouncedQuery) {
      fetch(`/api/search?q=${debouncedQuery}`)
        .then((res) => res.json())
        .then(setResults);
    }
  }, [debouncedQuery]);

  return <input value={query} onChange={(e) => setQuery(e.target.value)} />;
}

function usePrevious(value) {
  const ref = useRef(null);
  useEffect(() => {
    ref.current = value; // update AFTER render
  }, [value]);
  return ref.current; // returns previous value during render
}

// Usage: compare current vs previous
function Counter() {
  const [count, setCount] = useState(0);
  const prevCount = usePrevious(count);

  return (
    <div>
      <p>Now: {count}, before: {prevCount}</p>
      {count > prevCount && <p>Increased!</p>}
      <button onClick={() => setCount(count + 1)}>+</button>
    </div>
  );
}

// useWindowSize hook
function useWindowSize() {
  const [size, setSize] = useState({
    width: window.innerWidth,
    height: window.innerHeight,
  });
  useEffect(() => {
    const handler = () =>
      setSize({ width: innerWidth, height: innerHeight });
    window.addEventListener("resize", handler);
    return () => removeEventListener("resize", handler);
  }, []);
  return size;
}

function useToggle(initial = false) {
  const [on, setOn] = useState(initial);
  const toggle = useCallback(() => setOn((p) => !p), []);
  return [on, toggle, setOn];
}

function useClipboard(timeout = 2000) {
  const [copied, setCopied] = useState(false);
  const copy = useCallback((text) => {
    navigator.clipboard.writeText(text).then(() => {
      setCopied(true);
      setTimeout(() => setCopied(false), timeout);
    });
  }, [timeout]);
  return [copied, copy];
}

// Usage
function CopyButton({ text }) {
  const [copied, copy] = useClipboard();
  return (
    <button onClick={() => copy(text)}>
      {copied ? "Copied!" : "Copy"}
    </button>
  );
}

function Modal({ children }) {
  const [isOpen, toggle] = useToggle(false);
  return (
    <>
      <button onClick={toggle}>Open</button>
      {isOpen && <div className="modal">{children}</div>}
    </>
  );
}

import { createPortal } from "react-dom";

function Modal({ children, onClose }) {
  return createPortal(
    <div className="modal-overlay" onClick={onClose}>
      <div className="modal-content" onClick={(e) => e.stopPropagation()}>
        <button onClick={onClose}>✕</button>
        {children}
      </div>
    </div>,
    document.body  // render target outside the DOM hierarchy
  );
}

// Usage: the modal renders at body level,
// escaping any overflow:hidden or z-index stacking contexts
function App() {
  const [show, setShow] = useState(false);
  return (
    <div style={{ overflow: "hidden", position: "relative" }}>
      <button onClick={() => setShow(true)}>Open Modal</button>
      {show && <Modal onClose={() => setShow(false)}>Hello!</Modal>}
    </div>
  );
}

function Modal({ isOpen, onClose, children }) {
  const modalRef = useRef(null);

  useEffect(() => {
    if (!isOpen) return;
    const modal = modalRef.current;
    const previouslyFocused = document.activeElement;
    modal.focus();

    const handleKey = (e) => {
      if (e.key === "Escape") onClose();
    };
    document.addEventListener("keydown", handleKey);

    // Prevent body scroll
    document.body.style.overflow = "hidden";

    return () => {
      document.removeEventListener("keydown", handleKey);
      document.body.style.overflow = "";
      previouslyFocused.focus(); // restore focus
    };
  }, [isOpen, onClose]);

  if (!isOpen) return null;

  return createPortal(
    <div className="overlay" onClick={onClose}>
      <div ref={modalRef} tabIndex={-1} className="modal">
        {children}
      </div>
    </div>,
    document.body
  );
}

function Tooltip({ children, text }) {
  const [visible, setVisible] = useState(false);
  const [coords, setCoords] = useState({ x: 0, y: 0 });
  const targetRef = useRef(null);

  const show = () => {
    const rect = targetRef.current.getBoundingClientRect();
    setCoords({ x: rect.left, y: rect.top - 40 });
    setVisible(true);
  };

  return (
    <>
      <span
        ref={targetRef}
        onMouseEnter={show}
        onMouseLeave={() => setVisible(false)}
      >
        {children}
      </span>
      {visible && createPortal(
        <div style={{ position: "fixed", left: coords.x, top: coords.y }}
             className="tooltip">
          {text}
        </div>,
        document.body
      )}
    </>
  );
}

// Usage
<Tooltip text="Click to save">💾</Tooltip>

function Dropdown({ trigger, children }) {
  const [open, setOpen] = useState(false);
  const [pos, setPos] = useState({ top: 0, left: 0 });
  const ref = useRef(null);

  const handleOpen = () => {
    const rect = ref.current.getBoundingClientRect();
    setPos({ top: rect.bottom + 4, left: rect.left });
    setOpen(true);
  };

  useEffect(() => {
    if (!open) return;
    const handleClick = (e) => {
      if (!ref.current?.contains(e.target)) setOpen(false);
    };
    const handleScroll = () => setOpen(false); // close on scroll
    document.addEventListener("mousedown", handleClick);
    window.addEventListener("scroll", handleScroll, true);
    return () => {
      document.removeEventListener("mousedown", handleClick);
      window.removeEventListener("scroll", handleScroll, true);
    };
  }, [open]);

  return (
    <>
      <div ref={ref} onClick={handleOpen}>{trigger}</div>
      {open && createPortal(
        <div style={{ position: "fixed", ...pos }} className="dropdown">
          {children}
        </div>,
        document.body
      )}
    </>
  );
}

function PortalExample() {
  // Despite rendering in document.body, events bubble
  // through the React tree, not the DOM tree
  return (
    <div onClick={() => console.log("Parent clicked!")}>
      <p>Click the button — parent handler fires!</p>
      {createPortal(
        <button onClick={() => console.log("Button clicked!")}>
          I'm in a portal
        </button>,
        document.body
      )}
    </div>
  );
}
// Clicking logs: "Button clicked!" then "Parent clicked!"

// This means context, state, and event delegation
// all work as if the portal were inline

// But CSS inheritance does NOT cross the portal boundary:
// document.body styles won't inherit into the portal content
// unless you explicitly apply them

import { lazy, Suspense } from "react";

// Lazy-load component (code-split)
const Dashboard = lazy(() => import("./Dashboard"));
const Settings = lazy(() => import("./Settings"));

function App() {
  return (
    <Suspense fallback={<div>Loading page...</div>}>
      <nav>
        <button onClick={() => setPage("dash")}>Dashboard</button>
        <button onClick={() => setPage("settings")}>Settings</button>
      </nav>
      {page === "dash" && <Dashboard />}
      {page === "settings" && <Settings />}
    </Suspense>
  );
}

<Suspense fallback={<PageSkeleton />}>
  <Header />
  <Suspense fallback={<MainSkeleton />}>
    <MainContent />  {/* loads first */}
    <Suspense fallback={<CommentsSkeleton />}>
      <Comments />  {/* loads independently, doesn't block MainContent */}
    </Suspense>
  </Suspense>
  <Sidebar />
</Suspense>

// Suspense "peeling" effect:
// 1. PageSkeleton shows
// 2. Header + Sidebar load → PageSkeleton peels away
// 3. MainSkeleton shows until MainContent loads
// 4. MainContent shows, CommentsSkeleton shows
// 5. Comments load → everything visible

class ErrorBoundary extends React.Component {
  state = { hasError: false, error: null };
  static getDerivedStateFromError(error) {
    return { hasError: true, error };
  }
  componentDidCatch(error, info) {
    console.error("Chunk load failed:", error, info);
  }
  render() {
    if (this.state.hasError) {
      return (
        <div>
          <p>Failed to load. {this.state.error.message}</p>
          <button onClick={() => window.location.reload()}>
            Retry
          </button>
        </div>
      );
    }
    return this.props.children;
  }
}

// Wrap lazy components: network failures need handling
<ErrorBoundary>
  <Suspense fallback={<Loader />}>
    <LazyComponent />
  </Suspense>
</ErrorBoundary>

// React 18+ Suspense for data fetching (experimental)
import { use } from "react"; // React 19+

// Wrap a promise with use()
function UserProfile({ userId }) {
  // 'use' suspends until the promise resolves
  const user = use(fetchUser(userId));
  return <div>{user.name}</div>;
}

function fetchUser(id) {
  return fetch(`/api/users/${id}`).then((r) => r.json());
}

// Parent provides Suspense boundary
function App() {
  return (
    <Suspense fallback={<Spinner />}>
      <UserProfile userId={1} />
    </Suspense>
  );
}

// Concurrent: multiple suspends resolve together
function Dashboard() {
  const user = use(fetchUser(1));
  const posts = use(fetchPosts(user.id));
  // Both fetch in parallel, Suspense shows until all resolve
  return <div>{user.name}: {posts.length} posts</div>;
}

// SuspenseList controls reveal order of multiple Suspense
import { SuspenseList, Suspense } from "react";

function Article({ id }) {
  const data = use(fetchArticle(id));
  return <article>{data.title}</article>;
}

function Feed() {
  return (
    <SuspenseList revealOrder="forwards" tail="collapsed">
      {/* "forwards": reveals top-to-bottom as they load */}
      {/* "together": waits for all, reveals together */}
      {/* "backwards": reveals bottom-to-top */}

      <Suspense fallback={<Skeleton />}>
        <Article id={1} />
      </Suspense>
      <Suspense fallback={<Skeleton />}>
        <Article id={2} />
      </Suspense>
      <Suspense fallback={<Skeleton />}>
        <Article id={3} />
      </Suspense>
    </SuspenseList>
  );
}

import { BrowserRouter, Routes, Route, Link } from "react-router-dom";

function App() {
  return (
    <BrowserRouter>
      <nav>
        <Link to="/">Home</Link>
        <Link to="/about">About</Link>
        <Link to="/users/123">User 123</Link>
      </nav>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/about" element={<About />} />
        <Route path="/users/:id" element={<UserPage />} />
        <Route path="*" element={<NotFound />} />
      </Routes>
    </BrowserRouter>
  );
}

function UserPage() {
  const { id } = useParams();
  return <h1>User ID: {id}</h1>;
}

function App() {
  return (
    <BrowserRouter>
      <Routes>
        <Route path="/" element={<Layout />}>
          <Route index element={<Home />} />
          <Route path="about" element={<About />} />
          <Route path="users" element={<Users />}>
            <Route path=":id" element={<UserDetail />} />
          </Route>
        </Route>
      </Routes>
    </BrowserRouter>
  );
}

function Layout() {
  return (
    <div>
      <nav>Navigation here</nav>
      <Outlet />  {/* Child routes render here */}
    </div>
  );
}

function Users() {
  return (
    <div>
      <h2>Users</h2>
      <Outlet />  {/* Nested :id route renders here */}
    </div>
  );
}

import { useNavigate, Navigate, NavLink, useLocation } from "react-router-dom";

function Login() {
  const navigate = useNavigate();
  const location = useLocation();

  const handleLogin = async () => {
    await auth.login();
    // Redirect to intended page or home
    const from = location.state?.from || "/";
    navigate(from, { replace: true });
  };
  return <button onClick={handleLogin}>Login</button>;
}

// Declarative redirect
function ProtectedRoute({ user, children }) {
  if (!user) {
    return <Navigate to="/login" state={{ from: location }} replace />;
  }
  return children;
}

// NavLink: active styling
<NavLink to="/about" className={({ isActive }) =>
  isActive ? "nav-active" : "nav"
}>
  About
</NavLink>

import { createBrowserRouter, RouterProvider } from "react-router-dom";

const router = createBrowserRouter([
  {
    path: "/users/:id",
    element: <UserPage />,
    loader: async ({ params }) => {
      const res = await fetch(`/api/users/${params.id}`);
      if (!res.ok) throw new Response("Not found", { status: 404 });
      return res.json();
    },
    errorElement: <ErrorPage />,
  },
]);

function UserPage() {
  const user = useLoaderData(); // data from loader
  return <h1>{user.name}</h1>;
}

// Action for form submissions
{
  path: "/users/new",
  element: <NewUser />,
  action: async ({ request }) => {
    const formData = await request.formData();
    const res = await fetch("/api/users", {
      method: "POST",
      body: formData,
    });
    return redirect(`/users/${res.id}`);
  },
}

function App() {
  return <RouterProvider router={router} />;
}

function RequireAuth({ children }) {
  const { user } = useAuth();
  const location = useLocation();

  if (!user) {
    return <Navigate to="/login" state={{ from: location }} replace />;
  }
  return children;
}

function App() {
  return (
    <BrowserRouter>
      <Routes>
        <Route path="/login" element={<Login />} />
        <Route path="/" element={<Layout />}>
          <Route index element={<Home />} />
          <Route path="dashboard" element={
            <RequireAuth><Dashboard /></RequireAuth>
          } />
          <Route path="admin" element={
            <RequireAuth><RequireRole role="admin"><Admin /></RequireRole></RequireAuth>
          } />
        </Route>
      </Routes>
    </BrowserRouter>
  );
}

function RequireRole({ role, children }) {
  const { user } = useAuth();
  if (user?.role !== role) return <Navigate to="/forbidden" />;
  return children;
}

const AuthContext = createContext(null);

function AuthProvider({ children }) {
  const [user, setUser] = useState(null);
  const login = async (credentials) => {
    const user = await api.login(credentials);
    setUser(user);
  };
  const logout = () => setUser(null);
  return (
    <AuthContext.Provider value={{ user, login, logout }}>
      {children}
    </AuthContext.Provider>
  );
}

function useAuth() {
  const ctx = useContext(AuthContext);
  if (!ctx) throw new Error("useAuth must be inside AuthProvider");
  return ctx;
}

// Usage
function Navbar() {
  const { user, logout } = useAuth();
  return user
    ? <button onClick={logout}>Logout {user.name}</button>
    : <Link to="/login">Login</Link>;
}

// Wrap app: <AuthProvider><App /></AuthProvider>

const CartContext = createContext();

function cartReducer(state, action) {
  switch (action.type) {
    case "ADD":
      const existing = state.find((i) => i.id === action.item.id);
      if (existing) {
        return state.map((i) =>
          i.id === action.item.id ? { ...i, qty: i.qty + 1 } : i
        );
      }
      return [...state, { ...action.item, qty: 1 }];
    case "REMOVE":
      return state.filter((i) => i.id !== action.id);
    case "CLEAR":
      return [];
    default:
      return state;
  }
}

function CartProvider({ children }) {
  const [items, dispatch] = useReducer(cartReducer, []);
  const value = useMemo(() => ({
    items,
    total: items.reduce((s, i) => s + i.price * i.qty, 0),
    addItem: (item) => dispatch({ type: "ADD", item }),
    removeItem: (id) => dispatch({ type: "REMOVE", id }),
  }), [items]);
  return <CartContext.Provider value={value}>{children}</CartContext.Provider>;
}

import { configureStore, createSlice } from "@reduxjs/toolkit";
import { useSelector, useDispatch } from "react-redux";

const counterSlice = createSlice({
  name: "counter",
  initialState: { value: 0 },
  reducers: {
    increment: (state) => { state.value += 1; }, // Immer: mutate safely
    decrement: (state) => { state.value -= 1; },
    addBy: (state, action) => { state.value += action.payload; },
  },
});

const store = configureStore({
  reducer: { counter: counterSlice.reducer },
});

export const { increment, decrement, addBy } = counterSlice.actions;

// Usage in component
function Counter() {
  const count = useSelector((state) => state.counter.value);
  const dispatch = useDispatch();
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => dispatch(increment())}>+</button>
      <button onClick={() => dispatch(addBy(5))}>+5</button>
    </div>
  );
}

import { create } from "zustand";

const useStore = create((set, get) => ({
  count: 0,
  user: null,
  increment: () => set((state) => ({ count: state.count + 1 })),
  setUser: (user) => set({ user }),
  reset: () => set({ count: 0, user: null }),
  // Access other state with get()
  doubleCount: () => get().count * 2,
}));

// Usage: select only what you need (prevents re-renders)
function Counter() {
  const count = useStore((state) => state.count);
  const increment = useStore((state) => state.increment);
  return <button onClick={increment}>{count}</button>;
}

// Multiple selections
function Profile() {
  const { user, setUser } = useStore(
    (state) => ({ user: state.user, setUser: state.setUser })
  );
  // Use shallow comparison for object selectors
  // import { shallow } from "zustand/shallow";
  // useStore(selector, shallow);
}

import { useQuery, useMutation, QueryClient, QueryClientProvider } from "@tanstack/react-query";

const queryClient = new QueryClient();

function App() {
  return (
    <QueryClientProvider client={queryClient}>
      <Users />
    </QueryClientProvider>
  );
}

function Users() {
  const { data, isLoading, error, refetch } = useQuery({
    queryKey: ["users"],
    queryFn: () => fetch("/api/users").then((r) => r.json()),
    staleTime: 60000, // fresh for 60s
    refetchOnWindowFocus: true,
  });

  const mutation = useMutation({
    mutationFn: (newUser) =>
      fetch("/api/users", { method: "POST", body: JSON.stringify(newUser) }),
    onSuccess: () => queryClient.invalidateQueries(["users"]),
  });

  if (isLoading) return <p>Loading...</p>;
  if (error) return <p>Error: {error.message}</p>;
  return (
    <div>
      {data.map((u) => <div key={u.id}>{u.name}</div>)}
      <button onClick={() => mutation.mutate({ name: "New" })}>Add</button>
    </div>
  );
}

import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { Counter } from "./Counter";

test("counter increments on click", async () => {
  const user = userEvent.setup();
  render(<Counter />);

  // Find by accessible role (not test-id)
  expect(screen.getByRole("heading")).toHaveTextContent("0");

  await user.click(screen.getByRole("button", { name: /increment/i }));

  expect(screen.getByRole("heading")).toHaveTextContent("1");
});

test("displays error for invalid input", async () => {
  const user = userEvent.setup();
  render(<Form />);

  await user.type(screen.getByLabelText(/email/i), "not-an-email");
  await user.click(screen.getByRole("button", { name: /submit/i }));

  expect(screen.getByRole("alert")).toHaveTextContent(/invalid email/i);
});

import { renderHook, act } from "@testing-library/react";
import { useCounter } from "./useCounter";

test("useCounter increments and decrements", () => {
  const { result } = renderHook(() => useCounter(0));

  expect(result.current.count).toBe(0);

  // Wrap state updates in act()
  act(() => result.current.increment());
  expect(result.current.count).toBe(1);

  act(() => result.current.decrement());
  expect(result.current.count).toBe(0);

  act(() => result.current.reset());
  expect(result.current.count).toBe(0);
});

// Testing with initial props that change
test("useEffect runs on dependency change", () => {
  const { result, rerender } = renderHook(
    ({ id }) => useFetchUser(id),
    { initialProps: { id: 1 } }
  );
  rerender({ id: 2 });
  // Effect re-ran with new id
});

import { render, screen, waitFor } from "@testing-library/react";
import { rest } from "msw";
import { setupServer } from "msw/node";
import { UserProfile } from "./UserProfile";

// Mock API with MSW (Mock Service Worker)
const server = setupServer(
  rest.get("/api/users/:id", (req, res, ctx) => {
    return res(ctx.json({ id: 1, name: "Alice" }));
  })
);

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

test("displays user after fetch", async () => {
  render(<UserProfile id={1} />);

  // findBy waits for async update
  expect(await screen.findByText("Alice")).toBeInTheDocument();
  expect(screen.queryByText("Loading")).not.toBeInTheDocument();
});

test("shows error on fetch failure", async () => {
  server.use(
    rest.get("/api/users/:id", (req, res, ctx) =>
      res(ctx.status(500))
    )
  );
  render(<UserProfile id={1} />);
  expect(await screen.findByText(/error/i)).toBeInTheDocument();
});

// Custom render that wraps with providers
import { render } from "@testing-library/react";
import { ThemeProvider } from "./ThemeProvider";

function customRender(ui, { theme = "light", ...options } = {}) {
  function Wrapper({ children }) {
    return <ThemeProvider initialTheme={theme}>{children}</ThemeProvider>;
  }
  return render(ui, { wrapper: Wrapper, ...options });
}

// Re-export everything
export * from "@testing-library/react";
export { customRender as render };

// In test files, import from your test-utils:
// import { render, screen } from "../test-utils";

test("button uses theme color", () => {
  customRender(<Button>Click</Button>, { theme: "dark" });
  expect(screen.getByRole("button")).toHaveClass("btn-dark");
});

// Testing with router
import { MemoryRouter } from "react-router-dom";
render(
  <MemoryRouter initialEntries={["/users/123"]}>
    <App />
  </MemoryRouter>
);

import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";

test("form submission flow", async () => {
  const onSubmit = jest.fn();
  const user = userEvent.setup();
  render(<LoginForm onSubmit={onSubmit} />);

  // Fill form fields
  await user.type(screen.getByLabelText(/email/i), "[email protected]");
  await user.type(screen.getByLabelText(/password/i), "password123");

  // Submit
  await user.click(screen.getByRole("button", { name: /login/i }));

  expect(onSubmit).toHaveBeenCalledWith({
    email: "[email protected]",
    password: "password123",
  });
});

test("keyboard navigation", async () => {
  const user = userEvent.setup();
  render(<Modal />);
  await user.tab(); // focus first element
  expect(screen.getByRole("button", { name: /close/i })).toHaveFocus();
  await user.keyboard("{Escape}"); // press Escape
  expect(screen.queryByRole("dialog")).not.toBeInTheDocument();
});

// Basic props
interface ButtonProps {
  text: string;
  onClick: () => void;
  variant?: "primary" | "secondary"; // union type
  disabled?: boolean;
}

function Button({ text, onClick, variant = "primary", disabled }: ButtonProps) {
  return (
    <button
      className={`btn btn-${variant}`}
      onClick={onClick}
      disabled={disabled}
    >
      {text}
    </button>
  );
}

// Children prop
interface CardProps {
  title: string;
  children: React.ReactNode; // any renderable content
}

// Extending HTML attributes
interface InputProps extends React.InputHTMLAttributes<HTMLInputElement> {
  label: string;
  error?: string;
}

function Input({ label, error, ...rest }: InputProps) {
  return (
    <label>
      {label}
      <input {...rest} />
      {error && <span className="error">{error}</span>}
    </label>
  );
}

// useState with types
const [count, setCount] = useState<number>(0);
const [user, setUser] = useState<User | null>(null);
const [items, setItems] = useState<string[]>([]);

// useRef
const inputRef = useRef<HTMLInputElement>(null);
// Access: inputRef.current?.focus()

// useContext
interface ThemeContextType {
  theme: "light" | "dark";
  toggle: () => void;
}
const ThemeContext = createContext<ThemeContextType | undefined>(undefined);

function useTheme() {
  const ctx = useContext(ThemeContext);
  if (!ctx) throw new Error("useTheme must be inside ThemeProvider");
  return ctx; // ctx is now ThemeContextType, not undefined
}

// useReducer
type Action = { type: "increment" } | { type: "set"; value: number };
const [state, dispatch] = useReducer((state: number, action: Action) => {
  switch (action.type) {
    case "increment": return state + 1;
    case "set": return action.value;
  }
}, 0);

// Generic component: works with any type
interface ListProps<T> {
  items: T[];
  renderItem: (item: T) => React.ReactNode;
  keyExtractor: (item: T) => string;
}

function List<T>({ items, renderItem, keyExtractor }: ListProps<T>) {
  return (
    <ul>
      {items.map((item) => (
        <li key={keyExtractor(item)}>{renderItem(item)}</li>
      ))}
    </ul>
  );
}

// Usage: TypeScript infers T from items
<List
  items={[{ id: "1", name: "Alice" }, { id: "2", name: "Bob" }]}
  renderItem={(user) => <span>{user.name}</span>}
  keyExtractor={(user) => user.id}
/>

// Generic hook
function useArray<T>(initial: T[]) {
  const [array, setArray] = useState(initial);
  const push = (item: T) => setArray((prev) => [...prev, item]);
  return { array, push, setArray };
}

// Event types
function handleChange(e: React.ChangeEvent<HTMLInputElement>) {
  console.log(e.target.value); // string
}

function handleSubmit(e: React.FormEvent<HTMLFormElement>) {
  e.preventDefault();
  const formData = new FormData(e.currentTarget);
}

function handleClick(e: React.MouseEvent<HTMLButtonElement>) {
  console.log(e.clientX, e.clientY);
}

// forwardRef with TypeScript
interface InputProps extends React.InputHTMLAttributes<HTMLInputElement> {
  label: string;
}

const Input = React.forwardRef<HTMLInputElement, InputProps>(
  ({ label, ...props }, ref) => (
    <label>
      {label}
      <input ref={ref} {...props} />
    </label>
  )
);
Input.displayName = "Input";

// Usage: const ref = useRef<HTMLInputElement>(null);
// <Input ref={ref} label="Email" />

// Pick: select specific props
interface ButtonProps {
  text: string;
  onClick: () => void;
  color: string;
  size: "sm" | "md" | "lg";
}
type IconButtonProps = Pick<ButtonProps, "onClick" | "size"> & {
  icon: React.ReactNode;
};

// Omit: exclude specific props
type LinkButtonProps = Omit<ButtonProps, "onClick"> & {
  href: string;
};

// Partial: all props optional (for defaults)
type DefaultProps = Partial<ButtonProps>;

// ComponentProps: extract props from existing component
type MyButtonProps = React.ComponentProps<typeof Button> & {
  variant?: "custom";
};

// ReturnType: type of a function's return
type User = ReturnType<typeof fetchUser>;

// Record for prop maps
type ButtonVariants = Record<"primary" | "danger" | "ghost", string>;

import { useTransition, useState } from "react";

function SearchResults() {
  const [isPending, startTransition] = useTransition();
  const [query, setQuery] = useState("");
  const [results, setResults] = useState([]);

  const handleSearch = (value) => {
    setQuery(value); // urgent: update input immediately
    startTransition(() => {
      // non-urgent: heavy filtering can be interrupted
      const filtered = heavyFilter(allItems, value);
      setResults(filtered);
    });
  };

  return (
    <div>
      <input value={query} onChange={(e) => handleSearch(e.target.value)} />
      {isPending && <span>Updating...</span>}
      <ul>{results.map((r) => <li key={r.id}>{r.name}</li>)}</ul>
    </div>
  );
}

function Search() {
  const [query, setQuery] = useState("");
  // query updates immediately; deferredQuery lags behind
  const deferredQuery = useDeferredValue(query);
  const isStale = query !== deferredQuery;

  return (
    <div>
      <input value={query} onChange={(e) => setQuery(e.target.value)} />
      <Results query={deferredQuery} isStale={isStale} />
    </div>
  );
}

// Memoize the expensive component so it only re-renders
// when deferredQuery changes (not on every keystroke)
const Results = React.memo(function Results({ query, isStale }) {
  const items = expensiveSearch(query); // heavy computation
  return (
    <div style={{ opacity: isStale ? 0.5 : 1 }}>
      {items.map((i) => <div key={i.id}>{i.name}</div>)}
    </div>
  );
});

import { useOptimistic } from "react";

function ThumbsUp({ likes, addLike }) {
  // Optimistic state: updates immediately, reverts on error
  const [optimisticLikes, addOptimisticLike] = useOptimistic(
    likes,
    (state, newLike) => [...state, newLike]
  );

  const handleClick = async () => {
    const newLike = { id: Date.now(), pending: true };
    addOptimisticLike(newLike); // instant UI update
    try {
      await addLike(newLike); // actual API call
    } catch {
      // Reverts automatically on re-render with real data
    }
  };

  return (
    <div>
      <button onClick={handleClick}>👍 {optimisticLikes.length}</button>
      {optimisticLikes.some((l) => l.pending) && <span>Saving...</span>}
    </div>
  );
}

import { use } from "react";

// Read context with use (works in conditions!)
function Theme() {
  // Unlike useContext, use() can be inside conditions
  if (showTheme) {
    const theme = use(ThemeContext); // conditional context!
    return <div style={{ background: theme.color }} />;
  }
  return null;
}

// Read promises with use (Suspense integration)
function UserProfile({ userPromise }) {
  // Suspends until promise resolves
  const user = use(userPromise);
  return <h1>{user.name}</h1>;
}

// Parent passes promise (starts fetching during render)
function App() {
  const userPromise = fetchUser(); // starts immediately
  return (
    <Suspense fallback={<Loading />}>
      <UserProfile userPromise={userPromise} />
    </Suspense>
  );
}

// Pattern 1: Deferred search with transition
function App() {
  const [tab, setTab] = useState("home");
  const [isPending, startTransition] = useTransition();

  return (
    <>
      <nav>
        <button
          onClick={() => startTransition(() => setTab("analytics"))}
          disabled={isPending}
        >
          {isPending ? "Loading..." : "Analytics"}
        </button>
      </nav>
      {tab === "home" && <Home />}
      {tab === "analytics" && <HeavyAnalytics />}
    </>
  );
}

// Pattern 2: useSyncExternalStore for external state
function useOnlineStatus() {
  return useSyncExternalStore(
    (callback) => {
      window.addEventListener("online", callback);
      window.addEventListener("offline", callback);
      return () => {
        window.removeEventListener("online", callback);
        window.removeEventListener("offline", callback);
      };
    },
    () => navigator.onLine, // client snapshot
    () => true // server snapshot (SSR)
  );
}

const ThemeContext = React.createContext({ theme: 'light', toggle: () => {} });

function ThemeProvider({ children }) {
  const [theme, setTheme] = useState('light');
  const toggle = useCallback(() => setTheme((t) => (t === 'light' ? 'dark' : 'light')), []);
  return (
    <ThemeContext.Provider value={{ theme, toggle }}>
      {children}
    </ThemeContext.Provider>
  );
}

function ThemedButton() {
  const { theme, toggle } = useContext(ThemeContext);
  return (
    <button onClick={toggle} style={{ background: theme === 'dark' ? '#333' : '#eee' }}>
      Toggle Theme
    </button>
  );
}

const StoreContext = React.createContext(null);

function storeReducer(state, action) {
  switch (action.type) {
    case 'add': return { items: [...state.items, action.item] };
    case 'remove': return { items: state.items.filter((_, i) => i !== action.index) };
    default: return state;
  }
}

function StoreProvider({ children }) {
  const [state, dispatch] = useReducer(storeReducer, { items: [] });
  return <StoreContext.Provider value={{ state, dispatch }}>{children}</StoreContext.Provider>;
}

const StateContext = React.createContext(null);
const DispatchContext = React.createContext(null);

function Provider({ children }) {
  const [state, dispatch] = useReducer(reducer, initial);
  return (
    <StateContext.Provider value={state}>
      <DispatchContext.Provider value={dispatch}>
        {children}
      </DispatchContext.Provider>
    </StateContext.Provider>
  );
}

export function useAuth() {
  const ctx = useContext(AuthContext);
  if (!ctx) throw new Error('useAuth must be used within AuthProvider');
  return ctx;
}

function reducer(state, action) {
  switch (action.type) {
    case 'increment': return { count: state.count + 1 };
    case 'decrement': return { count: state.count - 1 };
    case 'reset':     return { count: 0 };
    default: throw new Error('Unknown action: ' + action.type);
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, { count: 0 });
  return <button onClick={() => dispatch({ type: 'increment' })}>{state.count}</button>;
}

function init(initialCount) {
  return { count: initialCount, history: [] };
}

function reducer(state, action) {
  switch (action.type) {
    case 'increment': return { ...state, count: state.count + 1 };
    case 'reset':     return init(action.payload);
    default: return state;
  }
}

const [state, dispatch] = useReducer(reducer, initialCount, init);

const initialState = { users: [], loading: false, error: null, filter: 'all' };

function reducer(state, action) {
  switch (action.type) {
    case 'fetch-start': return { ...state, loading: true, error: null };
    case 'fetch-success': return { ...state, loading: false, users: action.users };
    case 'fetch-error': return { ...state, loading: false, error: action.error };
    case 'set-filter': return { ...state, filter: action.filter };
    default: return state;
  }
}

function todoReducer(state, action) {
  switch (action.type) {
    case 'add': return [...state, { id: Date.now(), text: action.text, done: false }];
    case 'toggle': return state.map((t) => t.id === action.id ? { ...t, done: !t.done } : t);
    case 'delete': return state.filter((t) => t.id !== action.id);
    default: return state;
  }
}

export function TodoProvider({ children }) {
  const [todos, dispatch] = useReducer(todoReducer, []);
  return <TodoContext.Provider value={{ todos, dispatch }}>{children}</TodoContext.Provider>;
}

const ACTIONS = { ADD: 'add', UPDATE: 'update', DELETE: 'delete' };

function reducer(state, action) {
  switch (action.type) {
    case ACTIONS.ADD:
      return [...state, { id: action.id, ...action.payload }];
    case ACTIONS.UPDATE:
      return state.map((item) => item.id === action.id ? { ...item, ...action.payload } : item);
    case ACTIONS.DELETE:
      return state.filter((item) => item.id !== action.id);
    default: return state;
  }
}

function ProductList({ products, filter }) {
  const filtered = useMemo(() => {
    return products.filter((p) => p.category === filter);
  }, [products, filter]);

  const sorted = useMemo(() => [...filtered].sort((a, b) => a.price - b.price), [filtered]);
  return <ul>{sorted.map((p) => <li key={p.id}>{p.name}</li>)}</ul>;
}

function Parent() {
  const [count, setCount] = useState(0);
  const handleClick = useCallback(() => setCount((c) => c + 1), []);
  return <MemoizedChild onClick={handleClick} />;
}

const ExpensiveItem = React.memo(function ExpensiveItem({ value, onClick }) {
  return <li onClick={onClick}>{value}</li>;
});

// With custom comparison
const DeepChild = React.memo(
  ({ user }) => <div>{user.name}</div>,
  (prev, next) => prev.user.id === next.user.id
);

// GOOD: expensive computation
const sorted = useMemo(() => heavySort(data), [data]);

// GOOD: callback passed to memoized child
const onSelect = useCallback((id) => setSelected(id), []);

// BAD: cheap operation, no perf issue
const label = useMemo(() => first + ' ' + last, [first, last]);

function Search({ query }) {
  const params = useMemo(() => ({ q: query, limit: 10 }), [query]);
  useEffect(() => {
    api.search(params).then(setData);
  }, [params]); // Without useMemo, this fires every render
}

import { createPortal } from 'react-dom';

function Modal({ open, onClose, children }) {
  if (!open) return null;
  return createPortal(
    <div className="modal-overlay" onClick={onClose}>
      <div className="modal" onClick={(e) => e.stopPropagation()}>{children}</div>
    </div>,
    document.body
  );
}

function FocusInput() {
  const inputRef = useRef(null);
  const focus = () => inputRef.current?.focus();
  return (
    <>
      <input ref={inputRef} type="text" />
      <button onClick={focus}>Focus</button>
    </>
  );
}

const FancyInput = React.forwardRef(function FancyInput({ label, ...props }, ref) {
  return (
    <label>{label}<input ref={ref} {...props} className="fancy-input" /></label>
  );
});

const VideoPlayer = React.forwardRef(function VideoPlayer(props, ref) {
  const videoRef = useRef(null);
  useImperativeHandle(ref, () => ({
    play: () => videoRef.current?.play(),
    pause: () => videoRef.current?.pause(),
    seek: (time) => { if (videoRef.current) videoRef.current.currentTime = time; },
  }));
  return <video ref={videoRef} src={props.src} />;
});

function Stopwatch() {
  const [seconds, setSeconds] = useState(0);
  const intervalRef = useRef(null);
  const start = () => {
    if (intervalRef.current) return;
    intervalRef.current = setInterval(() => setSeconds((s) => s + 1), 1000);
  };
  const stop = () => { clearInterval(intervalRef.current); intervalRef.current = null; };
  useEffect(() => () => clearInterval(intervalRef.current), []);
  return <>{seconds}<button onClick={start}>Start</button><button onClick={stop}>Stop</button></>;
}

class ErrorBoundary extends React.Component {
  constructor(props) { super(props); this.state = { hasError: false, error: null }; }
  static getDerivedStateFromError(error) { return { hasError: true, error }; }
  componentDidCatch(error, info) { console.error('Caught:', error, info); }
  render() {
    if (this.state.hasError) return this.props.fallback || <h1>Something went wrong.</h1>;
    return this.props.children;
  }
}

function App() {
  return (
    <ErrorBoundary fallback={<ErrorPage />}>
      <Header />
      <ErrorBoundary fallback={<SidebarCrash />}><Sidebar /></ErrorBoundary>
      <ErrorBoundary fallback={<ContentCrash />}><MainContent /></ErrorBoundary>
    </ErrorBoundary>
  );
}

class ErrorBoundary extends React.Component {
  state = { hasError: false, error: null };
  static getDerivedStateFromError(error) { return { hasError: true, error }; }
  reset = () => this.setState({ hasError: false, error: null });
  render() {
    if (this.state.hasError) return <div><p>Failed.</p><button onClick={this.reset}>Try again</button></div>;
    return this.props.children;
  }
}

import { ErrorBoundary } from 'react-error-boundary';

<ErrorBoundary
  FallbackComponent={ErrorFallback}
  onError={(error, info) => logError(error, info)}
  onReset={() => window.location.reload()}
  resetKeys={[location.pathname]}
>
  <Routes />
</ErrorBoundary>

function AsyncComponent() {
  const [state, setState] = useState({ data: null, error: null });
  useEffect(() => {
    let active = true;
    fetchData()
      .then((data) => { if (active) setState({ data, error: null }); })
      .catch((error) => { if (active) setState({ data: null, error }); });
    return () => { active = false; };
  }, []);
  if (state.error) throw state.error;
  if (!state.data) return <Loading />;
  return <div>{state.data}</div>;
}

import { useVirtualizer } from '@tanstack/react-virtual';

function BigList({ items }) {
  const parentRef = useRef(null);
  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 50,
  });
  return (
    <div ref={parentRef} style={{ height: 600, overflow: 'auto' }}>
      <div style={{ height: virtualizer.getTotalSize(), position: 'relative' }}>
        {virtualizer.getVirtualItems().map((vi) => (
          <div key={vi.key} style={{ position: 'absolute', top: vi.start, height: vi.size }}>
            {items[vi.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}

import { lazy, Suspense } from 'react';
const Admin = lazy(() => import('./Admin'));
const Chart = lazy(() => import('./Chart'));

function Page({ showChart }) {
  return (
    <Suspense fallback={<Skeleton />}>
      {showChart && <Chart data={data} />}
    </Suspense>
  );
}

// Use React DevTools Profiler to record renders
// Look for:
// - Components rendering too often
// - Long commit phases
// - Wasted renders (props didn't change)

// Wrap expensive renders to find bottlenecks
function MyComponent({ data }) {
  console.time('render');
  const result = heavyCompute(data);
  console.timeEnd('render');
  return <div>{result}</div>;
}

function Search({ query }) {
  const deferredQuery = useDeferredValue(query);
  const isStale = query !== deferredQuery;
  const results = useMemo(() => expensiveSearch(deferredQuery), [deferredQuery]);
  return <div style={{ opacity: isStale ? 0.7 : 1 }}>{results.map((r) => <div key={r.id}>{r.name}</div>)}</div>;
}

import { useTransition, useDeferredValue } from 'react';

// useTransition: mark updates as non-urgent
const [isPending, startTransition] = useTransition();
const filterResults = (q) => startTransition(() => setResults(search(q)));

// Automatic batching (React 18): state updates in promises batch automatically
fetch('/api').then(() => {
  setLoading(false);   //  \
  setData(data);       //   > single re-render
  setError(null);      //  /
});

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';

test('increments on click', async () => {
  const user = userEvent.setup();
  render(<Counter />);
  expect(screen.getByText('Count: 0')).toBeInTheDocument();
  const button = screen.getByRole('button', { name: /increment/i });
  await user.click(button);
  expect(screen.getByText('Count: 1')).toBeInTheDocument();
});

// getBy: throws if 0 or >1 matches (strict)
screen.getByRole('button', { name: 'Submit' });

// queryBy: returns null if 0 matches (for assertions of absence)
expect(screen.queryByText('Error')).not.toBeInTheDocument();

// findBy: returns a Promise, waits for match (async)
const element = await screen.findByText('Loaded');

// getAllBy: returns array (multiple matches)
const items = screen.getAllByRole('listitem');

import userEvent from '@testing-library/user-event';

test('form submission', async () => {
  const user = userEvent.setup();
  const onSubmit = vi.fn();
  render(<Form onSubmit={onSubmit} />);
  await user.type(screen.getByLabelText(/email/i), '[email protected]');
  await user.click(screen.getByRole('button', { name: /submit/i }));
  expect(onSubmit).toHaveBeenCalled();
});

import { waitFor, within } from '@testing-library/react';

test('shows data after fetch', async () => {
  render(<UserList />);
  await waitFor(() => {
    const list = screen.getByRole('list');
    expect(within(list).getAllByRole('listitem')).toHaveLength(3);
  });
});

// findBy is often cleaner than waitFor + getBy
test('shows data (cleaner)', async () => {
  render(<UserList />);
  expect(await screen.findAllByRole('listitem')).toHaveLength(3);
});

import { rest } from 'msw';
import { setupServer } from 'msw/node';

const server = setupServer(
  rest.get('/api/user', (req, res, ctx) => res(ctx.json({ name: 'Alice' })))
);

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

test('shows user name', async () => {
  render(<UserProfile />);
  expect(await screen.findByText('Alice')).toBeInTheDocument();
});