React Hooks, introduced in React 16.8, have completely changed the way we write components in React. They allow us to use state and other React features without writing a class. Two of these hooks, useEffect and useLayoutEffect, are used to perform side effects in function components. But when should we use each one? Let's explore both hooks and find out.

What is useEffect?

The useEffect hook is used to perform side effects in function components. A side effect could be anything that affects something outside of the scope of the current function being executed. Examples include data fetching, setting up a subscription, manually changing the DOM, and so on.

Here is an example of useEffect:

useEffect(() => {
  document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes

The function passed to useEffect will run after the render is committed to the screen. Think of effects as an escape hatch from React’s purely functional world into the imperative world.

What is useLayoutEffect?

The useLayoutEffect hook has the same signature as useEffect. However, it fires synchronously after all DOM mutations. This can be useful when you need to make new updates and measurements after the DOM updates, but before the browser has a chance to "paint" those changes, such as reading layout from the DOM or synchronously re-rendering.

Here is an example of useLayoutEffect:

useLayoutEffect(() => {
  ref.current.style.color = 'blue';
}, []); // Only run once

When to use useEffect vs useLayoutEffect?

The primary difference between useEffect and useLayoutEffect lies in the timing of execution. useEffect runs asynchronously and after a render is painted to the screen. useLayoutEffect, on the other hand, runs synchronously after a render but before the screen is updated.

If you’re migrating code from a class component, note useLayoutEffect fires in the same phase as componentDidMount and componentDidUpdate.

Here are some general rules of thumb:

• If you need to mutate the DOM and/or do measurements and then make further updates, you'd want to use useLayoutEffect to ensure these updates occur before the browser has a chance to paint. This can help prevent flickering on the screen.
• For other cases, including data fetching and subscriptions, useEffect is the way to go. It won't block the painting process, leading to better perceived performance.

Always remember, each tool has its place. Understanding the difference between useEffect and useLayoutEffect allows us to make better decisions about which to use when, for the best possible user experience.

Remember, while useLayoutEffect can prevent flickering, using it excessively can lead to performance problems, as it blocks visual updates. So, unless you need to make updates and measurements before the browser "paints", stick with useEffect.

In conclusion, understanding the difference between useEffect and useLayoutEffect is crucial to ensure your React app's performance. Use the right hook at the right time, and you're on your way to creating a smooth and efficient React application.