When working with React, you’ll often need to perform side effects in your functional components. React provides two hooks for this purpose: useEffect and useLayoutEffect. While they serve a similar purpose, their timing and use cases differ significantly. Let’s explore these differences to help you choose the right hook for your needs.

useEffect

The useEffect hook is the most commonly used hook for handling side effects in React. It runs after the component renders, which means it executes after the DOM has been updated. This makes it ideal for tasks like fetching data, setting up subscriptions, or manually changing the DOM.

Key Points:

• Runs asynchronously after the DOM update.
• Suitable for tasks that do not require immediate DOM measurement or manipulation.
• And the most important one, does not block the browser’s painting process.

useLayoutEffect

The useLayoutEffect hook is similar to useEffect, but it runs synchronously after all DOM mutations but before the browser has painted. This means it fires after the DOM is updated but before the browser repaints the screen. useLayoutEffect is useful for scenarios where you need to read layout information from the DOM and re-render based on that information.

Key Points:

• Runs synchronously after the DOM mutations but before the paint.
• Ideal for reading layout and performing calculations based on the DOM.
• Ensures that the measurements and DOM changes are done before the user sees the updated content.
• blocks the browser from repainting

Summary

As you might guessed, useLayoutEffect can hurt performance, so you must always use useEffect and reserve useLayoutEffect only for cases where you need to synchronize with the DOM immediately after changes but before the browser paints.

About The Author

Emad Dehnavi is a Senior Software Engineer and Certified Auth0 Developer at Distology Studios, bringing extensive product engineering experience when working at the first Cryptobanking platform and now applying those principles and methodologies to Consumer Identity use cases.