🧬 React Lifecycle & Behavior – Understand How Components Work (2025 Guide)

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🧲 Introduction – Why Learn React Lifecycle?

React.js components mount, update, and unmount during their lifetime. Understanding the lifecycle behavior of components is essential to manage:

• 🧠 State updates
• 🔄 Side effects
• 🧹 Cleanup
• ⚠️ Performance optimization

While class components had distinct lifecycle methods, React Hooks now provide similar control in functional components using useEffect, useLayoutEffect, useRef, and more.

🎯 In this guide, you’ll learn:

• React’s component lifecycle stages
• How lifecycle differs between class and functional components
• How to replicate each stage using Hooks
• Best practices and debugging tips

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🏗️ 1. What Is a Component Lifecycle?

The component lifecycle describes the phases a React component goes through from creation to removal:

Phase	Description
Mounting	Component is created and inserted into the DOM
Updating	Component re-renders due to prop/state changes
Unmounting	Component is removed from the DOM

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🏛️ 2. Lifecycle in Class Components

Method	Triggered When
constructor()	Before mounting
componentDidMount()	After first render
shouldComponentUpdate()	Before re-render on props/state change
componentDidUpdate()	After re-render
componentWillUnmount()	Before component removal

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⚙️ 3. Replicating Lifecycle with React Hooks

In functional components, the useEffect hook handles most lifecycle behavior:

✅ Mounting

useEffect(() => {
  console.log('Mounted');
}, []); // empty array → runs once

✅ Updating

useEffect(() => {
  console.log('Count changed');
}, [count]); // dependency array → runs when `count` changes

✅ Unmounting (Cleanup)

useEffect(() => {
  const timer = setInterval(() => { ... }, 1000);
  return () => clearInterval(timer); // cleanup before unmount
}, []);

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📦 4. Lifecycle Phases Explained with Hooks

Phase	Hook Strategy
Mount	useEffect(() => { ... }, [])
Update	useEffect(() => { ... }, [deps])
Unmount	useEffect(() => { return () => {...} }, [])
DOM Updates	useLayoutEffect() for post-DOM adjustments

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🔍 5. useLayoutEffect vs useEffect

Hook	Runs When
useEffect	After the DOM is painted
useLayoutEffect	Before the DOM paint (sync)

📘 Use useLayoutEffect when you need to measure DOM size or apply layout changes before render

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🧠 6. Tracking Lifecycle with useRef

You can track lifecycle phases using refs:

const isFirstRender = useRef(true);

useEffect(() => {
  if (isFirstRender.current) {
    console.log('First render');
    isFirstRender.current = false;
  } else {
    console.log('Updated');
  }
}, [value]);

✅ Detect mount vs update in a useEffect

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⚠️ 7. Common Lifecycle Gotchas

Issue	Fix
State updates after unmount	Use cleanup with useEffect
Stale values in effects	Add correct dependencies in [dep] array
useEffect runs multiple times	React Strict Mode double-invokes effects (dev only)
DOM changes not visible in useEffect	Use useLayoutEffect instead

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📘 Best Practices

✅ Use separate useEffect() calls for unrelated effects
✅ Use cleanup functions for subscriptions, timers, or listeners
✅ Avoid direct DOM manipulation (use refs if necessary)
✅ Keep effects pure – avoid async logic without cleanup
✅ Profile and debug long-running effects

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📌 Summary – Recap & Next Steps

React’s component lifecycle is predictable and powerful. Whether using class components or functional ones with Hooks, understanding the lifecycle phases helps you build efficient, maintainable, and bug-free components.

🔍 Key Takeaways:

• Mounting, updating, and unmounting define the lifecycle
• Hooks like useEffect, useLayoutEffect, and useRef simulate class lifecycle methods
• Cleanup effects to prevent memory leaks
• Be cautious of infinite loops caused by bad dependencies
• Use useLayoutEffect only when layout measurement is critical

⚙️ Real-World Relevance:
Lifecycle logic powers real-time apps, form validation, animations, analytics tracking, and component teardown in apps like Trello, Slack, and Notion.

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❓ FAQ Section

❓ Does useEffect run on every render?
❌ Not if you provide a dependency array. It only runs when dependencies change.

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❓ What’s the difference between useEffect and useLayoutEffect?
✅ useEffect runs after the paint. useLayoutEffect runs before the paint and is synchronous.

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❓ How do I run code only once when a component mounts?
✅ Use:

useEffect(() => {
  // mount-only code
}, []);

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❓ How do I clean up an effect like a timer or event?
✅ Return a cleanup function inside useEffect:

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

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❓ Can I track previous state values?
✅ Yes. Use useRef to persist previous values across renders:

const prev = useRef();
useEffect(() => {
  prev.current = value;
}, [value]);

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