Matter AI | Code Reviewer Documentation

React Anti-Patterns Overview

Prop Drilling

// Anti-pattern: Prop drilling through multiple components
function App() {
  const [user, setUser] = useState({ name: 'John' });
  return <MainContent user={user} setUser={setUser} />;
}

function MainContent({ user, setUser }) {
  return <Sidebar user={user} setUser={setUser} />;
}

function Sidebar({ user, setUser }) {
  return <UserProfile user={user} setUser={setUser} />;
}

function UserProfile({ user, setUser }) {
  return <div>{user.name}</div>;
}

// Better approach: Use Context API
const UserContext = createContext();

function App() {
  const [user, setUser] = useState({ name: 'John' });
  return (
    <UserContext.Provider value={{ user, setUser }}>
      <MainContent />
    </UserContext.Provider>
  );
}

function MainContent() {
  return <Sidebar />;
}

function Sidebar() {
  return <UserProfile />;
}

function UserProfile() {
  const { user } = useContext(UserContext);
  return <div>{user.name}</div>;
}

Prop drilling makes components tightly coupled and harder to maintain. Use Context API or state management libraries like Redux for sharing state across components.

Huge Component Files

// Anti-pattern: Huge component with multiple responsibilities
function Dashboard() {
  const [users, setUsers] = useState([]);
  const [posts, setPosts] = useState([]);
  const [comments, setComments] = useState([]);
  const [activeTab, setActiveTab] = useState('users');
  
  useEffect(() => {
    fetchUsers().then(data => setUsers(data));
    fetchPosts().then(data => setPosts(data));
    fetchComments().then(data => setComments(data));
  }, []);
  
  const handleUserClick = (userId) => { /* ... */ };
  const handlePostClick = (postId) => { /* ... */ };
  const handleCommentDelete = (commentId) => { /* ... */ };
  
  return (
    <div>
      <Tabs activeTab={activeTab} onChange={setActiveTab} />
      {activeTab === 'users' && (
        <UserList users={users} onUserClick={handleUserClick} />
      )}
      {activeTab === 'posts' && (
        <PostList posts={posts} onPostClick={handlePostClick} />
      )}
      {activeTab === 'comments' && (
        <CommentList 
          comments={comments} 
          onCommentDelete={handleCommentDelete} 
        />
      )}
    </div>
  );
}

// Better approach: Split into smaller components
function Dashboard() {
  const [activeTab, setActiveTab] = useState('users');
  
  return (
    <div>
      <Tabs activeTab={activeTab} onChange={setActiveTab} />
      {activeTab === 'users' && <UserTab />}
      {activeTab === 'posts' && <PostTab />}
      {activeTab === 'comments' && <CommentTab />}
    </div>
  );
}

function UserTab() {
  const [users, setUsers] = useState([]);
  
  useEffect(() => {
    fetchUsers().then(data => setUsers(data));
  }, []);
  
  const handleUserClick = (userId) => { /* ... */ };
  
  return <UserList users={users} onUserClick={handleUserClick} />;
}

// Similar implementations for PostTab and CommentTab

Large components are difficult to understand, test, and maintain. Split them into smaller, focused components with single responsibilities.

Inline Function Definitions

// Anti-pattern: Inline function definitions
function UserList({ users }) {
  return (
    <ul>
      {users.map(user => (
        <li key={user.id} onClick={() => console.log(user.id)}>
          {user.name}
        </li>
      ))}
    </ul>
  );
}

// Better approach: Define functions outside render
function UserList({ users }) {
  const handleClick = useCallback((userId) => {
    console.log(userId);
  }, []);
  
  return (
    <ul>
      {users.map(user => (
        <li key={user.id} onClick={() => handleClick(user.id)}>
          {user.name}
        </li>
      ))}
    </ul>
  );
}

Inline function definitions create new function instances on every render, which can lead to unnecessary re-renders of child components. Use useCallback to memoize functions.

Not Using React Keys Properly

// Anti-pattern: Using index as key
function TodoList({ todos }) {
  return (
    <ul>
      {todos.map((todo, index) => (
        <li key={index}>{todo.text}</li>
      ))}
    </ul>
  );
}

// Better approach: Use unique IDs
function TodoList({ todos }) {
  return (
    <ul>
      {todos.map(todo => (
        <li key={todo.id}>{todo.text}</li>
      ))}
    </ul>
  );
}

Using array indices as keys can lead to performance issues and bugs when items are added, removed, or reordered. Use stable, unique identifiers for keys.

Overusing useState

// Anti-pattern: Overusing useState
function UserForm() {
  const [firstName, setFirstName] = useState('');
  const [lastName, setLastName] = useState('');
  const [email, setEmail] = useState('');
  const [phone, setPhone] = useState('');
  const [address, setAddress] = useState('');
  const [city, setCity] = useState('');
  const [state, setState] = useState('');
  const [zip, setZip] = useState('');
  
  // Multiple handlers for each field
  const handleFirstNameChange = (e) => setFirstName(e.target.value);
  const handleLastNameChange = (e) => setLastName(e.target.value);
  // ... more handlers
  
  return (
    <form>
      <input value={firstName} onChange={handleFirstNameChange} />
      <input value={lastName} onChange={handleLastNameChange} />
      {/* More inputs */}
    </form>
  );
}

// Better approach: Use useReducer for complex state
function formReducer(state, action) {
  return { ...state, [action.field]: action.value };
}

function UserForm() {
  const [formState, dispatch] = useReducer(formReducer, {
    firstName: '',
    lastName: '',
    email: '',
    phone: '',
    address: '',
    city: '',
    state: '',
    zip: ''
  });
  
  const handleChange = (e) => {
    dispatch({ field: e.target.name, value: e.target.value });
  };
  
  return (
    <form>
      <input 
        name="firstName" 
        value={formState.firstName} 
        onChange={handleChange} 
      />
      <input 
        name="lastName" 
        value={formState.lastName} 
        onChange={handleChange} 
      />
      {/* More inputs */}
    </form>
  );
}

Using multiple useState hooks for related data makes state management complex. Use useReducer for complex state or objects with multiple fields.

Side Effects in Render

// Anti-pattern: Side effects in render
function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  
  // Wrong: API call in render
  if (!user) {
    fetchUser(userId).then(data => setUser(data));
  }
  
  return user ? <div>{user.name}</div> : <div>Loading...</div>;
}

// Better approach: Use useEffect
function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);
  
  useEffect(() => {
    setLoading(true);
    fetchUser(userId)
      .then(data => setUser(data))
      .finally(() => setLoading(false));
  }, [userId]);
  
  return loading ? <div>Loading...</div> : <div>{user.name}</div>;
}

Performing side effects during render can cause infinite loops and unexpected behavior. Use useEffect for side effects.

Not Memoizing Expensive Calculations

// Anti-pattern: Recalculating on every render
function ProductList({ products }) {
  // This will run on every render
  const sortedProducts = products
    .filter(p => p.inStock)
    .sort((a, b) => a.price - b.price);
  
  return (
    <ul>
      {sortedProducts.map(product => (
        <li key={product.id}>{product.name} - ${product.price}</li>
      ))}
    </ul>
  );
}

// Better approach: Use useMemo
function ProductList({ products }) {
  const sortedProducts = useMemo(() => {
    return products
      .filter(p => p.inStock)
      .sort((a, b) => a.price - b.price);
  }, [products]);
  
  return (
    <ul>
      {sortedProducts.map(product => (
        <li key={product.id}>{product.name} - ${product.price}</li>
      ))}
    </ul>
  );
}

Expensive calculations are recomputed on every render, impacting performance. Use useMemo to memoize expensive calculations.

Not Using React.memo for Pure Components

// Anti-pattern: Not memoizing pure components
function ProductItem({ product, onAddToCart }) {
  return (
    <div>
      <h3>{product.name}</h3>
      <p>${product.price}</p>
      <button onClick={() => onAddToCart(product.id)}>
        Add to Cart
      </button>
    </div>
  );
}

// Better approach: Use React.memo
const ProductItem = React.memo(function ProductItem({ product, onAddToCart }) {
  return (
    <div>
      <h3>{product.name}</h3>
      <p>${product.price}</p>
      <button onClick={() => onAddToCart(product.id)}>
        Add to Cart
      </button>
    </div>
  );
});

Components re-render even when their props haven’t changed, causing unnecessary renders. Use React.memo to skip re-rendering when props are unchanged.

Improper useEffect Dependencies

// Anti-pattern: Missing or incorrect dependencies
function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  
  // Missing dependency
  useEffect(() => {
    fetchUser(userId).then(data => setUser(data));
  }, []); // userId should be in the dependency array
  
  return user ? <div>{user.name}</div> : <div>Loading...</div>;
}

// Better approach: Proper dependencies
function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  
  useEffect(() => {
    fetchUser(userId).then(data => setUser(data));
  }, [userId]); // Correctly includes userId
  
  return user ? <div>{user.name}</div> : <div>Loading...</div>;
}

Missing or incorrect dependencies in useEffect can lead to stale closures and bugs. Always include all values from the component scope that the effect uses.

Not Using Fragment for Multiple Elements

// Anti-pattern: Unnecessary div wrapper
function UserInfo({ user }) {
  return (
    <div>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
    </div>
  );
}

// Better approach: Use Fragment
function UserInfo({ user }) {
  return (
    <>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
    </>
  );
}

Unnecessary wrapper divs add to the DOM depth and can break layouts. Use React Fragments (<>...</> or <React.Fragment>) to group elements without adding extra nodes to the DOM.

Not Using Error Boundaries

// Anti-pattern: No error handling
function App() {
  return (
    <div>
      <Header />
      <UserProfile /> {/* If this crashes, the whole app crashes */}
      <Footer />
    </div>
  );
}

// Better approach: Use Error Boundaries
class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }
  
  static getDerivedStateFromError(error) {
    return { hasError: true };
  }
  
  componentDidCatch(error, errorInfo) {
    logErrorToService(error, errorInfo);
  }
  
  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    
    return this.props.children;
  }
}

function App() {
  return (
    <div>
      <Header />
      <ErrorBoundary>
        <UserProfile />
      </ErrorBoundary>
      <Footer />
    </div>
  );
}

Without error boundaries, a runtime error in a component can break the entire application. Use error boundaries to gracefully handle errors and display fallback UIs.

Not Using Controlled Components for Forms

// Anti-pattern: Uncontrolled component
function LoginForm() {
  const handleSubmit = (e) => {
    e.preventDefault();
    const username = e.target.elements.username.value;
    const password = e.target.elements.password.value;
    // Submit logic
  };
  
  return (
    <form onSubmit={handleSubmit}>
      <input name="username" />
      <input name="password" type="password" />
      <button type="submit">Login</button>
    </form>
  );
}

// Better approach: Controlled component
function LoginForm() {
  const [username, setUsername] = useState('');
  const [password, setPassword] = useState('');
  
  const handleSubmit = (e) => {
    e.preventDefault();
    // Submit logic with username and password
  };
  
  return (
    <form onSubmit={handleSubmit}>
      <input 
        value={username} 
        onChange={(e) => setUsername(e.target.value)} 
      />
      <input 
        type="password"
        value={password} 
        onChange={(e) => setPassword(e.target.value)} 
      />
      <button type="submit">Login</button>
    </form>
  );
}

Uncontrolled components make it harder to validate and manipulate form data. Use controlled components for better control over form inputs and validation.

Not Using React DevTools

// Anti-pattern: Debugging with console.log
function ComplexComponent({ data }) {
  console.log('Component rendered', data);
  // Component logic
  return <div>{/* JSX */}</div>;
}

// Better approach: Use React DevTools and named components
function ComplexComponent({ data }) {
  // No console.log, use React DevTools instead
  return <div>{/* JSX */}</div>;
}

Using console.log for debugging is inefficient. Use React DevTools for inspecting component hierarchies, props, state, and performance.

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