Matter AI | Code Reviewer Documentation

React Native Anti-Patterns Overview

Inefficient List Rendering

// Anti-pattern: Inefficient list rendering
function ProductList({ products }) {
  return (
    <ScrollView>
      {products.map(product => (
        <ProductItem key={product.id} product={product} />
      ))}
    </ScrollView>
  );
}

// Better approach: Use FlatList
function ProductList({ products }) {
  return (
    <FlatList
      data={products}
      keyExtractor={item => item.id.toString()}
      renderItem={({ item }) => <ProductItem product={item} />}
      initialNumToRender={10}
      maxToRenderPerBatch={10}
      windowSize={5}
    />
  );
}

Using ScrollView with map for long lists leads to performance issues as it renders all items at once. Use FlatList or SectionList for efficient list rendering with windowing and recycling.

Not Using React Native's PureComponent or memo

// Anti-pattern: Regular component for static content
function ProfileHeader({ user }) {
  return (
    <View style={styles.header}>
      <Image source={{ uri: user.avatar }} style={styles.avatar} />
      <Text style={styles.name}>{user.name}</Text>
      <Text style={styles.bio}>{user.bio}</Text>
    </View>
  );
}

// Better approach: Use memo for functional components
const ProfileHeader = React.memo(function ProfileHeader({ user }) {
  return (
    <View style={styles.header}>
      <Image source={{ uri: user.avatar }} style={styles.avatar} />
      <Text style={styles.name}>{user.name}</Text>
      <Text style={styles.bio}>{user.bio}</Text>
    </View>
  );
});

// Or use PureComponent for class components
class ProfileHeader extends React.PureComponent {
  render() {
    const { user } = this.props;
    return (
      <View style={styles.header}>
        <Image source={{ uri: user.avatar }} style={styles.avatar} />
        <Text style={styles.name}>{user.name}</Text>
        <Text style={styles.bio}>{user.bio}</Text>
      </View>
    );
  }
}

Regular components re-render even when props haven’t changed. Use React.memo for functional components or PureComponent for class components to prevent unnecessary re-renders.

Layout Thrashing

// Anti-pattern: Layout thrashing
function AnimatedComponent() {
  const [width, setWidth] = useState(100);
  
  useEffect(() => {
    // This causes multiple layout recalculations
    setWidth(100);
    setWidth(150);
    setWidth(200);
  }, []);
  
  return (
    <Animated.View style={{ width }}>
      <Text>Content</Text>
    </Animated.View>
  );
}

// Better approach: Batch layout changes
function AnimatedComponent() {
  const width = useRef(new Animated.Value(100)).current;
  
  useEffect(() => {
    // This batches the animation
    Animated.timing(width, {
      toValue: 200,
      duration: 300,
      useNativeDriver: false,
    }).start();
  }, []);
  
  return (
    <Animated.View style={{ width }}>
      <Text>Content</Text>
    </Animated.View>
  );
}

Multiple state updates that affect layout cause performance issues. Batch layout changes and use the Animated API with useNativeDriver when possible.

Not Using useCallback for Event Handlers

// Anti-pattern: Recreating functions on each render
function ProductItem({ product, onAddToCart }) {
  // This function is recreated on every render
  const handlePress = () => {
    onAddToCart(product.id);
  };
  
  return (
    <TouchableOpacity onPress={handlePress}>
      <Text>{product.name}</Text>
    </TouchableOpacity>
  );
}

// Better approach: Use useCallback
function ProductItem({ product, onAddToCart }) {
  // This function is memoized
  const handlePress = useCallback(() => {
    onAddToCart(product.id);
  }, [product.id, onAddToCart]);
  
  return (
    <TouchableOpacity onPress={handlePress}>
      <Text>{product.name}</Text>
    </TouchableOpacity>
  );
}

Creating new function instances on every render can cause unnecessary re-renders in child components. Use useCallback to memoize functions.

Inline Styles

// Anti-pattern: Inline styles
function UserCard({ user }) {
  return (
    <View style={{ 
      padding: 10, 
      margin: 5, 
      backgroundColor: '#fff',
      borderRadius: 5,
      shadowColor: '#000',
      shadowOffset: { width: 0, height: 2 },
      shadowOpacity: 0.1,
      elevation: 2
    }}>
      <Text style={{ fontSize: 16, fontWeight: 'bold' }}>{user.name}</Text>
      <Text style={{ color: '#666' }}>{user.email}</Text>
    </View>
  );
}

// Better approach: Use StyleSheet
const styles = StyleSheet.create({
  card: {
    padding: 10,
    margin: 5,
    backgroundColor: '#fff',
    borderRadius: 5,
    shadowColor: '#000',
    shadowOffset: { width: 0, height: 2 },
    shadowOpacity: 0.1,
    elevation: 2
  },
  name: {
    fontSize: 16,
    fontWeight: 'bold'
  },
  email: {
    color: '#666'
  }
});

function UserCard({ user }) {
  return (
    <View style={styles.card}>
      <Text style={styles.name}>{user.name}</Text>
      <Text style={styles.email}>{user.email}</Text>
    </View>
  );
}

Inline styles are recreated on every render and can’t be optimized by React Native. Use StyleSheet.create() to define styles outside the component.

Not Using Proper Image Optimization

// Anti-pattern: Not optimizing images
function ProfileScreen({ user }) {
  return (
    <View>
      <Image 
        source={{ uri: user.highResAvatar }} 
        style={{ width: 50, height: 50 }} 
      />
    </View>
  );
}

// Better approach: Optimize images
function ProfileScreen({ user }) {
  return (
    <View>
      <Image 
        source={{ uri: `${user.avatar}?width=100&height=100` }} 
        style={{ width: 50, height: 50 }}
        resizeMode="cover"
        fadeDuration={300}
      />
    </View>
  );
}

Loading full-size images for small UI elements wastes bandwidth and memory. Request appropriately sized images and use proper resizeMode.

Not Using React Navigation Properly

// Anti-pattern: Creating navigator inside component
function App() {
  return (
    <NavigationContainer>
      <Stack.Navigator>
        <Stack.Screen name="Home" component={HomeScreen} />
        <Stack.Screen name="Details" component={DetailsScreen} />
      </Stack.Navigator>
    </NavigationContainer>
  );
}

// Better approach: Separate navigation configuration
const Stack = createStackNavigator();

function AppNavigator() {
  return (
    <Stack.Navigator 
      screenOptions={{
        headerStyle: {
          backgroundColor: '#f4511e',
        },
        headerTintColor: '#fff',
      }}
    >
      <Stack.Screen 
        name="Home" 
        component={HomeScreen} 
        options={({ route }) => ({ title: route.params?.title || 'Home' })} 
      />
      <Stack.Screen name="Details" component={DetailsScreen} />
    </Stack.Navigator>
  );
}

function App() {
  return (
    <NavigationContainer>
      <AppNavigator />
    </NavigationContainer>
  );
}

Poor navigation structure leads to maintainability issues. Separate navigation configuration from components and use proper screen options.

Not Handling Platform Differences Properly

// Anti-pattern: Platform-specific code scattered throughout
function Button({ title, onPress }) {
  return (
    <TouchableOpacity 
      onPress={onPress}
      style={{
        padding: 10,
        backgroundColor: Platform.OS === 'ios' ? '#007AFF' : '#2196F3',
        borderRadius: Platform.OS === 'ios' ? 10 : 4,
      }}
    >
      <Text style={{ 
        color: 'white',
        fontWeight: Platform.OS === 'ios' ? '600' : 'bold',
      }}>
        {title}
      </Text>
    </TouchableOpacity>
  );
}

// Better approach: Use Platform.select or separate files
const styles = StyleSheet.create({
  button: {
    padding: 10,
    ...Platform.select({
      ios: {
        backgroundColor: '#007AFF',
        borderRadius: 10,
      },
      android: {
        backgroundColor: '#2196F3',
        borderRadius: 4,
      },
    }),
  },
  buttonText: {
    color: 'white',
    ...Platform.select({
      ios: { fontWeight: '600' },
      android: { fontWeight: 'bold' },
    }),
  },
});

function Button({ title, onPress }) {
  return (
    <TouchableOpacity onPress={onPress} style={styles.button}>
      <Text style={styles.buttonText}>{title}</Text>
    </TouchableOpacity>
  );
}

// Or use platform-specific files:
// Button.ios.js and Button.android.js

Scattering platform-specific code makes components hard to maintain. Use Platform.select() or platform-specific files (e.g., Component.ios.js and Component.android.js).

Not Using Proper State Management

// Anti-pattern: Prop drilling for global state
function App() {
  const [user, setUser] = useState(null);
  const [cart, setCart] = useState([]);
  
  return (
    <View>
      <Header user={user} cart={cart} />
      <Main user={user} cart={cart} setCart={setCart} />
      <Footer user={user} />
    </View>
  );
}

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

function App() {
  const [user, setUser] = useState(null);
  const [cart, setCart] = useState([]);
  
  return (
    <UserContext.Provider value={{ user, setUser }}>
      <CartContext.Provider value={{ cart, setCart }}>
        <View>
          <Header />
          <Main />
          <Footer />
        </View>
      </CartContext.Provider>
    </UserContext.Provider>
  );
}

function Header() {
  const { user } = useContext(UserContext);
  const { cart } = useContext(CartContext);
  return <View>{/* Use user and cart */}</View>;
}

Prop drilling makes components tightly coupled and hard to maintain. Use Context API for simpler apps or Redux/MobX for complex state management.

Not Handling Permissions Properly

// Anti-pattern: Not handling permissions properly
function CameraScreen() {
  const takePicture = async () => {
    try {
      const photo = await camera.takePictureAsync();
      // Process photo
    } catch (error) {
      console.error('Failed to take picture:', error);
    }
  };
  
  return (
    <View>
      <Camera ref={ref => (camera = ref)} />
      <Button title="Take Picture" onPress={takePicture} />
    </View>
  );
}

// Better approach: Check and request permissions
function CameraScreen() {
  const [hasPermission, setHasPermission] = useState(null);
  const [camera, setCamera] = useState(null);
  
  useEffect(() => {
    (async () => {
      const { status } = await Camera.requestPermissionsAsync();
      setHasPermission(status === 'granted');
    })();
  }, []);
  
  const takePicture = async () => {
    if (camera) {
      try {
        const photo = await camera.takePictureAsync();
        // Process photo
      } catch (error) {
        console.error('Failed to take picture:', error);
      }
    }
  };
  
  if (hasPermission === null) {
    return <View><Text>Requesting camera permission...</Text></View>;
  }
  
  if (hasPermission === false) {
    return <View><Text>No access to camera</Text></View>;
  }
  
  return (
    <View>
      <Camera ref={ref => setCamera(ref)} />
      <Button title="Take Picture" onPress={takePicture} />
    </View>
  );
}

Not handling permissions properly leads to crashes and poor user experience. Always check and request permissions before using device features.

Not Using Proper Error Boundaries

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

// Better approach: Use error boundaries
class ErrorBoundary extends React.Component {
  state = { hasError: false };
  
  static getDerivedStateFromError() {
    return { hasError: true };
  }
  
  componentDidCatch(error, errorInfo) {
    // Log error to error reporting service
    logErrorToService(error, errorInfo);
  }
  
  render() {
    if (this.state.hasError) {
      return (
        <View style={styles.errorContainer}>
          <Text style={styles.errorText}>Something went wrong</Text>
          <Button 
            title="Try Again" 
            onPress={() => this.setState({ hasError: false })} 
          />
        </View>
      );
    }
    
    return this.props.children;
  }
}

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

Without error boundaries, a single component crash can bring down the entire app. Use error boundaries to gracefully handle component errors.

Not Using Proper Accessibility Features

// Anti-pattern: Not considering accessibility
function LoginButton({ onPress }) {
  return (
    <TouchableOpacity onPress={onPress}>
      <Text>Log In</Text>
    </TouchableOpacity>
  );
}

// Better approach: Include accessibility props
function LoginButton({ onPress }) {
  return (
    <TouchableOpacity 
      onPress={onPress}
      accessible={true}
      accessibilityLabel="Log in to your account"
      accessibilityHint="Navigates to the login screen"
      accessibilityRole="button"
    >
      <Text>Log In</Text>
    </TouchableOpacity>
  );
}

Ignoring accessibility makes your app unusable for many users. Include accessibility props like accessible, accessibilityLabel, and accessibilityHint.

Not Optimizing App Launch Time

// Anti-pattern: Heavy operations in App component
function App() {
  // Heavy initialization during app launch
  const data = processLargeData();
  initializeAllServices();
  
  return (
    <NavigationContainer>
      <AppNavigator initialData={data} />
    </NavigationContainer>
  );
}

// Better approach: Defer non-critical operations
function App() {
  const [isReady, setIsReady] = useState(false);
  const [initialData, setInitialData] = useState(null);
  
  useEffect(() => {
    async function prepare() {
      try {
        // Only initialize critical services synchronously
        initializeCriticalServices();
        
        // Defer non-critical operations
        setTimeout(() => {
          initializeAnalytics();
          initializeNonCriticalServices();
        }, 1000);
        
        // Load initial data asynchronously
        const data = await loadInitialDataAsync();
        setInitialData(data);
      } catch (e) {
        console.warn(e);
      } finally {
        setIsReady(true);
      }
    }
    
    prepare();
  }, []);
  
  if (!isReady) {
    return <AppLoading />;
  }
  
  return (
    <NavigationContainer>
      <AppNavigator initialData={initialData} />
    </NavigationContainer>
  );
}

Heavy operations during app launch increase startup time. Defer non-critical operations and use splash screens with AppLoading component.

Not Using Hermes Engine

// In android/app/build.gradle

// Anti-pattern: Not enabling Hermes
project.ext.react = [
    enableHermes: false // Not using Hermes engine
]

// Better approach: Enable Hermes
project.ext.react = [
    enableHermes: true // Using Hermes engine for better performance
]

Not using Hermes (JavaScript engine optimized for React Native) can result in slower startup times and higher memory usage. Enable Hermes for better performance, especially on Android.

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