# React useLayoutEffect Hook

The `useLayoutEffect` hook in React is similar to the `useEffect` hook, but it runs synchronously after all DOM mutations are applied. It allows you to perform imperative operations that rely on the updated DOM layout before the browser paints the screen.

Here's a concise explanation of the `useLayoutEffect` hook:

* `useLayoutEffect` is a React hook that runs after the DOM has been updated but before the browser paints the screen.
    
* It has the same API as `useEffect`, which means you can provide a callback function as the first argument.
    
* The difference is that `useLayoutEffect` runs synchronously and blocks the painting process until it's finished.
    
* This makes it suitable for tasks that require accurate measurements or immediate DOM updates.
    
* However, you should use `useEffect` in most cases since it doesn't block painting and is more performant.
    
* Only use `useLayoutEffect` when you need to interact with the updated layout before it's rendered.
    

Here is an example:

```javascript
import React, { useState, useLayoutEffect } from 'react';

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

  useLayoutEffect(() => {
    function updateWidth() {
      setWidth(window.innerWidth);
    }

    // Add event listener to update width when the window is resized
    window.addEventListener('resize', updateWidth);

    // Call the update function once to get the initial width
    updateWidth();

    // Clean up the event listener when the component unmounts
    return () => {
      window.removeEventListener('resize', updateWidth);
    };
  }, []); // Empty dependency array ensures the effect runs only once

  return (
    <div>
      Window width: {width}px
    </div>
  );
}
```

In this example, the `useLayoutEffect` hook is used to update the `width` state variable whenever the window is resized. It first defines an `updateWidth` function that retrieves the current window width and updates the state.

Inside `useLayoutEffect`, an event listener is added to the `resize` event, which calls the `updateWidth` function. This ensures that the width is updated whenever the window is resized.

The effect also includes a cleanup function that removes the event listener when the component unmounts. This is important to prevent memory leaks.

Finally, the component renders the `width` value, which reflects the current window width.