Mastering React useEffect: A Comprehensive Guide to Side Effect Management Patterns

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

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

  useEffect(() => {
    // This is the side effect function
    console.log('Component rendered or count changed:', count);

    // Optional cleanup function
    return () => {
      console.log('Cleanup for count:', count);
    };
  }, [count]); // Dependency array

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

            

              
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            useEffect(() => {
  const subscription = subscribeToChat(props.chatId);

  return () => {
    // Cleanup: Unsubscribe when chatId changes or component unmounts
    unsubscribeFromChat(subscription);
  };
}, [props.chatId]);

            

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

function UserProfile() {
  const [userData, setUserData] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    const fetchUserData = async () => {
      try {
        const response = await fetch('https://api.example.com/users/1');
        if (!response.ok) {
          throw new Error(`HTTP error! status: ${response.status}`);
        }
        const data = await response.json();
        setUserData(data);
      } catch (err) {
        setError(err);
      } finally {
        setLoading(false);
      }
    };

    fetchUserData();
  }, []); // Empty array ensures this runs only once on mount

  if (loading) return <p>Loading user data...</p>;
  if (error) return <p>Error: {error.message}</p>;
  if (!userData) return <p>No user data found.</p>;

  return (
    <div>
      <h2>{userData.name}</h2>
      <p>Email: {userData.email}</p>
      <p>Location: {userData.location}</p>
    </div>
  );
}

            

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

function UserPosts({ userId }) {
  const [posts, setPosts] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    if (!userId) { // Handle cases where userId might be undefined initially
      setPosts([]);
      setLoading(false);
      return;
    }

    const fetchUserPosts = async () => {
      setLoading(true);
      setError(null);
      try {
        const response = await fetch(`https://api.example.com/users/${userId}/posts`);
        if (!response.ok) {
          throw new Error(`HTTP error! status: ${response.status}`);
        }
        const data = await response.json();
        setPosts(data);
      } catch (err) {
        setError(err);
      } finally {
        setLoading(false);
      }
    };

    fetchUserPosts();
  }, [userId]); // Re-fetch whenever userId changes

  if (loading) return <p>Loading posts...</p>;
  if (error) return <p>Error: {error.message}</p>;
  if (posts.length === 0) return <p>No posts found for this user.</p>;

  return (
    <div>
      <h3>Posts by User {userId}</h3>
      <ul>
        {posts.map(post => (
          <li key={post.id}>{post.title}</li>
        ))}
      </ul>
    </div>
  );
}

            

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

function ProductDetails({ productId }) {
  const [product, setProduct] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    const controller = new AbortController();
    const signal = controller.signal;

    const fetchProduct = async () => {
      setLoading(true);
      setError(null);
      setProduct(null); // Clear previous product data

      try {
        const response = await fetch(`https://api.example.com/products/${productId}`, { signal });
        if (!response.ok) {
          throw new Error(`HTTP error! status: ${response.status}`);
        }
        const data = await response.json();
        setProduct(data);
      } catch (err) {
        if (err.name === 'AbortError') {
          console.log('Fetch aborted');
        } else {
          setError(err);
        }
      } finally {
        setLoading(false);
      }
    };

    fetchProduct();

    return () => {
      // Abort ongoing fetch request if component unmounts or productId changes
      controller.abort();
    };
  }, [productId]);

  if (loading) return <p>Loading product details...</p>;
  if (error) return <p>Error: {error.message}</p>;
  if (!product) return <p>No product found.</p>;

  return (
    <div>
      <h2>{product.name}</h2>
      <p>Price: ${product.price}</p>
      <p>Description: {product.description}</p>
    </div>
  );
}

            

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

function WindowSizeLogger() {
  const [windowSize, setWindowSize] = useState({
    width: window.innerWidth,
    height: window.innerHeight,
  });

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

    window.addEventListener('resize', handleResize);

    return () => {
      // Clean up the event listener when component unmounts
      window.removeEventListener('resize', handleResize);
    };
  }, []); // Empty array: add/remove listener only once on mount/unmount

  return (
    <div>
      <p>Window Width: {windowSize.width}px</p>
      <p>Window Height: {windowSize.height}px</p>
    </div>
  );
}

            

              
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            import React, { useEffect, useState } from 'react';
// Assume chatService is an external module providing subscribe/unsubscribe methods
import { chatService } from './chatService'; 

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

  useEffect(() => {
    const handleNewMessage = (message) => {
      setMessages((prevMessages) => [...prevMessages, message]);
    };

    const subscription = chatService.subscribe(roomId, handleNewMessage);

    return () => {
      chatService.unsubscribe(subscription);
    };
  }, [roomId]); // Re-subscribe if roomId changes

  return (
    <div>
      <h3>Chat Room: {roomId}</h3>
      <div className="messages">
        {messages.length === 0 ? (
          <p>No messages yet.</p>
        ) : (
          messages.map((msg, index) => (
            <p key={index}><strong>{msg.sender}:</strong> {msg.text}</p>
          ))
        )}
      </div>
    </div>
  );
}

            

              
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            import React, { useEffect } from 'react';

function PageTitleUpdater({ title }) {
  useEffect(() => {
    document.title = `My App | ${title}`;
  }, [title]); // Update title whenever 'title' prop changes

  return (
    <h2>Welcome to the {title} Page!</h2>
  );
}

            

              
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            import React, { useEffect, useRef } from 'react';
import Chart from 'chart.js/auto'; // Assuming Chart.js is installed

function MyChartComponent({ data, labels }) {
  const chartRef = useRef(null); // Ref to hold the canvas element
  const chartInstance = useRef(null); // Ref to hold the chart instance

  useEffect(() => {
    if (chartRef.current) {
      // Destroy existing chart instance before creating a new one
      if (chartInstance.current) {
        chartInstance.current.destroy();
      }

      const ctx = chartRef.current.getContext('2d');
      chartInstance.current = new Chart(ctx, {
        type: 'bar',
        data: {
          labels: labels,
          datasets: [{
            label: 'Sales Data',
            data: data,
            backgroundColor: 'rgba(75, 192, 192, 0.6)',
            borderColor: 'rgba(75, 192, 192, 1)',
            borderWidth: 1
          }]
        },
        options: {
          responsive: true,
          maintainAspectRatio: false,
        }
      });
    }

    return () => {
      // Cleanup: Destroy the chart instance on unmount
      if (chartInstance.current) {
        chartInstance.current.destroy();
      }
    };
  }, [data, labels]); // Re-render chart if data or labels change

  return (
    <div style={{ width: '600px', height: '400px' }}>
      <canvas ref={chartRef}></canvas>
    </div>
  );
}

            

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

function CountdownTimer({ initialSeconds }) {
  const [seconds, setSeconds] = useState(initialSeconds);

  useEffect(() => {
    if (seconds <= 0) return; // Stop timer when it reaches zero

    const timerId = setInterval(() => {
      setSeconds(prevSeconds => prevSeconds - 1);
    }, 1000);

    return () => {
      // Cleanup: Clear the interval when component unmounts or seconds become 0
      clearInterval(timerId);
    };
  }, [seconds]); // Re-run effect if seconds changes to set up new interval (e.g. if initialSeconds changes)

  return (
    <div>
      <h3>Countdown: {seconds} seconds</h3>
      {seconds === 0 && <p>Time's up!</p>}
    </div>
  );
}

            

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

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

  useEffect(() => {
    // This effect wants to log the count after 2 seconds.
    // If count changes within these 2 seconds, this will log the OLD count!
    const timer = setTimeout(() => {
      console.log('Stale Count:', count); 
    }, 2000);

    return () => {
      clearTimeout(timer);
    };
  }, []); // Problem: 'count' is not in dependencies, so it's stale

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

            

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

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

  useEffect(() => {
    const timer = setTimeout(() => {
      console.log('Correct Count:', count);
    }, 2000);

    return () => {
      clearTimeout(timer);
    };
  }, [count]); // Solution: 'count' is now a dependency. Effect re-runs when count changes.

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

            

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

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

  useEffect(() => {
    const interval = setInterval(() => {
      setCount(prevCount => prevCount + 1); // Functional update
    }, 1000);

    return () => clearInterval(interval);
  }, []); // 'count' is not a dependency because we use functional update

  return <p>Count: {count}</p>;
}

            

              
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            import React, { useEffect, useRef, useState } from 'react';

function LatestValueLogger() {
  const [count, setCount] = useState(0);
  const latestCountRef = useRef(count); // Create a ref

  // Keep the ref's current value updated with the latest count
  useEffect(() => {
    latestCountRef.current = count;
  }, [count]);

  useEffect(() => {
    const interval = setInterval(() => {
      // Access the latest count via the ref, avoiding stale closure
      console.log('Latest Count:', latestCountRef.current);
    }, 2000);

    return () => clearInterval(interval);
  }, []); // Empty dependency array, as we are not directly using 'count' here

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

            

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

function UserSettings() {
  const [userId, setUserId] = useState(1);
  const [settings, setSettings] = useState({});

  const fetchSettings = async () => {
    // This function is re-created on every render
    console.log('Fetching settings for user:', userId);
    const response = await fetch(`https://api.example.com/users/${userId}/settings`);
    const data = await response.json();
    setSettings(data);
  };

  useEffect(() => {
    fetchSettings();
  }, [fetchSettings]); // Problem: fetchSettings changes on every render

  return (
    <div>
      <p>User ID: {userId}</p>
      <button onClick={() => setUserId(userId + 1)}>Next User</button>
      <pre>{JSON.stringify(settings, null, 2)}</pre>
    </div>
  );
}

            

              
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            import React, { useEffect, useState, useCallback } from 'react';

function UserSettingsOptimized() {
  const [userId, setUserId] = useState(1);
  const [settings, setSettings] = useState({});

  const fetchSettings = useCallback(async () => {
    console.log('Fetching settings for user:', userId);
    const response = await fetch(`https://api.example.com/users/${userId}/settings`);
    const data = await response.json();
    setSettings(data);
  }, [userId]); // fetchSettings only changes when userId changes

  useEffect(() => {
    fetchSettings();
  }, [fetchSettings]); // Now fetchSettings is a stable dependency

  return (
    <div>
      <p>User ID: {userId}</p>
      <button onClick={() => setUserId(userId + 1)}>Next User</button>
      <pre>{JSON.stringify(settings, null, 2)}</pre>
    </div>
  );
}

            

              
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            import React, { useEffect, useMemo, useState } from 'react';

function ProductList({ categoryId, sortBy }) {
  const [products, setProducts] = useState([]);

  // Memoize the filter/sort criteria object
  const fetchCriteria = useMemo(() => ({
    category: categoryId,
    sort: sortBy,
  }), [categoryId, sortBy]);

  useEffect(() => {
    // fetch products based on fetchCriteria
    console.log('Fetching products with criteria:', fetchCriteria);
    // ... API call logic ...
  }, [fetchCriteria]); // Effect runs only when categoryId or sortBy changes

  return (
    <div>
      <h3>Products in Category {categoryId} (Sorted by {sortBy})</h3>
      <!-- Render product list -->
    </div>
  );
}

            

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

function InfiniteLoopExample() {
  const [data, setData] = useState([]);

  useEffect(() => {
    // This will cause an infinite loop!
    // setData causes a re-render, which re-runs the effect, which calls setData again.
    setData([1, 2, 3]); 
  }, [data]); // 'data' is a dependency, and we're always setting a new array reference

  return <p>Data length: {data.length}</p>;
}

            

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

function CorrectDataSetup() {
  const [data, setData] = useState([]);

  useEffect(() => {
    // This runs only once on mount
    setData([1, 2, 3]); 
  }, []); // Empty array prevents re-runs

  return <p>Data length: {data.length}</p>;
}

            

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

function UserDashboard({ userId }) {
  const [profile, setProfile] = useState(null);
  const [activityLog, setActivityLog] = useState([]);

  // Effect for fetching user profile (depends only on userId)
  useEffect(() => {
    const fetchProfile = async () => {
      // ... fetch profile data ...
      console.log('Fetching profile for', userId);
      const response = await fetch(`https://api.example.com/users/${userId}/profile`);
      const data = await response.json();
      setProfile(data);
    };
    fetchProfile();
  }, [userId]);

  // Effect for fetching activity log (also depends on userId, but separate concern)
  useEffect(() => {
    const fetchActivity = async () => {
      // ... fetch activity data ...
      console.log('Fetching activity for', userId);
      const response = await fetch(`https://api.example.com/users/${userId}/activity`);
      const data = await response.json();
      setActivityLog(data);
    };
    fetchActivity();
  }, [userId]);

  return (
    <div>
      <h2>User Dashboard: {userId}</h2>
      <h3>Profile:</h3>
      <pre>{JSON.stringify(profile, null, 2)}</pre>
      <h3>Activity Log:</h3>
      <pre>{JSON.stringify(activityLog, null, 2)}</pre>
    </div>
  );
}

            

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

// Custom Hook: useFetch.js
function useFetch(url, dependencies = []) {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    const abortController = new AbortController();
    const signal = abortController.signal;

    const fetchData = async () => {
      setLoading(true);
      setError(null);
      try {
        const response = await fetch(url, { signal });
        if (!response.ok) {
          throw new Error(`HTTP error! status: ${response.status}`);
        }
        const result = await response.json();
        setData(result);
      } catch (err) {
        if (err.name === 'AbortError') {
          console.log('Fetch aborted');
        } else {
          setError(err);
        }
      } finally {
        setLoading(false);
      }
    };

    fetchData();

    return () => {
      abortController.abort();
    };
  }, [url, ...dependencies]); // Re-run if URL or any extra dependency changes

  return { data, loading, error };
}

// Component using the custom hook: UserDataDisplay.js
function UserDataDisplay({ userId }) {
  const { data: userData, loading, error } = useFetch(
    `https://api.example.com/users/${userId}`,
    [userId] // Pass userId as a dependency to the custom hook
  );

  if (loading) return <p>Loading user data...</p>;
  if (error) return <p>Error: {error.message}</p>;
  if (!userData) return <p>No user data.</p>;

  return (
    <div>
      <h2>{userData.name}</h2>
      <p>Email: {userData.email}</p>
    </div>
  );
}
```

function ProductCalculator({ price, quantity }) {
  const [total, setTotal] = useState(0);

  useEffect(() => {
    setTotal(price * quantity); // Unnecessary effect
  }, [price, quantity]);

  return <p>Total: ${total.toFixed(2)}</p>;
}

            

              
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