🧠 useEffect vs useLayoutEffect in React: When and Why

React gives us powerful hooks to manage side effects, but one of the most confusing pairs is: 👉 useEffect vs useLayoutEffect

They sound similar—but their timing and impact on rendering are very different. Let’s break them down clearly 👇

🧩 What is useEffect?

useEffect runs after the browser paints the UI.

✅ Great for:

• Fetching data
• Subscribing to events
• Logging
• Setting timers

useEffect(() => {

  console.log('Runs after paint');

}, []);

🟢 Pros:

• Doesn’t block UI rendering.
• Ideal for non-visual side effects.

🧩 What is useLayoutEffect?

useLayoutEffect runs before the browser paints the screen.

✅ Great for:

• Measuring DOM elements
• Updating layout synchronously
• Avoiding flickers

useLayoutEffect(() => {

  const width = elementRef.current.offsetWidth;

  console.log('Runs before paint with width:', width);

}, []);

🔴 Caution:

It blocks rendering until the code inside runs—can lead to performance issues if misused.

⚖️ Key Differences (in simple terms)

🚀 Real-World Example

Imagine a tooltip that adjusts position based on element width:

useLayoutEffect(() => {

  const rect = ref.current.getBoundingClientRect();

  setPosition({ left: rect.left + 10 });

}, []);

🔁 If you used useEffect here, the tooltip might flicker before adjusting—because it runs after paint. ✅ useLayoutEffect prevents that by updating before the user sees it.

✅ Final Advice

🔹 Use useEffect by default—it’s faster and non-blocking. 🔹 Use useLayoutEffect only when you need precise layout control. 🔹 Avoid useLayoutEffect for async tasks like API calls or logging.

💬 Have you faced bugs due to incorrect use of these hooks? Share your experience—let’s untangle this together!

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