React provides a powerful hook called useLayoutEffect that allows you to perform side-effects after the DOM has been updated but before the browser has painted the screen. This can be useful when you need to access or manipulate DOM elements in your React components.

In this blog post, we will explain what the useLayoutEffect hook is, how it differs from the useEffect hook, and provide some examples of when and how to use it effectively.

What is the useLayoutEffect Hook?

The useLayoutEffect hook is similar to the useEffect hook, but it fires synchronously after all DOM mutations have taken place, but before the browser has painted those changes on the screen. This means that any visual updates triggered by your component will be reflected before the user sees them.

Differences Between useEffect and useLayoutEffect

The main difference between useEffect and useLayoutEffect is the timing of when they are executed.

• useEffect is asynchronous and runs after the component is rendered and the DOM is updated. This is useful for most cases when you want to perform side effects like fetching data, subscribing to events, or modifying the document title.

• useLayoutEffect, on the other hand, runs synchronously after the DOM mutation, but before the browser has painted. This is useful when you need to measure layout or interact with the DOM immediately after an update.

When to Use useLayoutEffect

Here are a few scenarios where using the useLayoutEffect hook can be beneficial:

• Measuring Layout: If you need to measure the size or position of a DOM element, useLayoutEffect is a good choice, as it runs after the layout calculations have been performed, ensuring accurate measurements.

• DOM Manipulation: If you need to interact with the DOM directly after it has been updated, such as setting focus or scrolling to a specific element, you can use useLayoutEffect to ensure that the DOM is ready for manipulation.

Example: Observing Scroll Position

Let’s consider a simple example where we want to observe the current scroll position of the page and update our component’s state accordingly:

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

function ScrollPosition() {
  const [scrollY, setScrollY] = useState(window.scrollY);

  useLayoutEffect(() => {
    const handleScroll = () => {
      setScrollY(window.scrollY);
    };

    window.addEventListener('scroll', handleScroll);

    return () => {
      window.removeEventListener('scroll', handleScroll);
    };
  }, []);

  return (
    <div>
      <h1>Scroll Position: {scrollY}px</h1>
    </div>
  );
}

export default ScrollPosition;

In the above code, we use the useLayoutEffect hook to attach an event listener to the ‘scroll’ event of the window object. Each time the scroll event is triggered, we update the scrollY state variable with the current scroll position.

Conclusion

The useLayoutEffect hook provides a way to perform side-effects that need access to the updated DOM immediately without causing layout thrashing. It is especially useful when dealing with layout measurements or interacting with the DOM after an update.

Remember, while useLayoutEffect is powerful, it should be used with caution, as it may impact performance if not used properly.