⚛️ How to Use useLayoutEffect Effectively

useLayoutEffect is a React Hook that runs synchronously after all DOM mutations but before the browser repaints. It’s similar to useEffect, but it blocks the paint until it finishes executing. This makes it suitable for reading layout and making immediate changes to the DOM.

🧠 Basic Syntax

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import { useLayoutEffect } from 'react';

useLayoutEffect(() => {

  // Your logic here (DOM reads/writes)

  return () => {

    // Cleanup if necessary

  };

}, [dependencies]);

✅ When to Use useLayoutEffect

Use useLayoutEffect when:

You need to measure DOM elements (e.g. widths, heights) before the screen paints.

You’re making synchronous DOM mutations that must happen before the user sees the update.

You’re coordinating animations or scroll position adjustments.

🆚 useLayoutEffect vs useEffect

Feature useLayoutEffect useEffect

Timing Before paint After paint

Blocks rendering? ✅ Yes ❌ No

Use case DOM measurements, layout fixes Data fetching, logging, side effects

Performance impact Higher if overused Lower

❗ Avoid using useLayoutEffect unnecessarily, as it may lead to performance issues by delaying painting.

📦 Practical Examples

1. Measuring DOM Size Before Paint

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import { useLayoutEffect, useRef, useState } from 'react';

function Box() {

  const boxRef = useRef(null);

  const [width, setWidth] = useState(0);

  useLayoutEffect(() => {

    const box = boxRef.current;

    if (box) {

      setWidth(box.getBoundingClientRect().width);

    }

  }, []);

  return (

    <>

      <div ref={boxRef} style={{ width: '50%' }}>Resize me</div>

      <p>Width: {width}px</p>

    </>

  );

}

2. Fixing Scroll Jumps

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useLayoutEffect(() => {

  // Scroll to bottom of chat

  chatContainerRef.current.scrollTop = chatContainerRef.current.scrollHeight;

}, [messages]);

If you used useEffect here, the scroll might happen after the browser repaints, resulting in a visual "jump."

⚠️ Best Practices

✅ Use it only when needed (for DOM reads/writes that affect layout).

❌ Don’t fetch data or use async code inside useLayoutEffect.

✅ Always clean up any side effects that impact the DOM.

❌ Avoid running heavy logic inside it — it delays rendering.

🧪 Debugging Tip

If your component flickers or re-renders weirdly, try switching useLayoutEffect to useEffect and compare the behavior. Use useLayoutEffect only if that flicker is due to layout being read/modified too late.

📝 Summary

Do Use useLayoutEffect When Don’t Use It When

Reading DOM size or position Fetching data

Controlling scroll position Delayed visual effects

Triggering animations before paint Logging/debugging only

Would you like a performance comparison demo or a working example in a sandbox link?

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