Understanding the Differences: useEffect vs. useLayoutEffect in React

Introduction

In the world of React, Hooks have been a game-changer, allowing developers to use state and other React features in functional components. Among these, useEffect and useLayoutEffect are crucial for managing side effects, but they often cause confusion. Understanding the nuances between them is essential for optimal and efficient React development.

What is useEffect?

useEffect is a React Hook that lets you perform side effects in your functional components. It is a workhorse of React Hooks, handling operations that don't directly impact the DOM or require synchronisation with it. This includes fetching data, setting up subscriptions, and manually changing the DOM in React components after rendering.

For instance:

useEffect(() => {
  // Fetch data or subscribe to a service
}, [dependencies]);        

This hook runs after the render is committed to the screen, making it asynchronous and non-blocking for UI updates.

What is useLayoutEffect?

useLayoutEffect is similar to useEffect in its syntax and purpose, but with a critical difference in its execution timing. This Hook is used for tasks that need synchronisation with DOM updates, like reading layout from the DOM or making DOM mutations that need to be visually synchronous. React guarantees that the code inside useLayoutEffect and any state updates scheduled inside it will be processed before the browser repaints the screen. This lets you render the tooltip, measure it, and re-render the tooltip again without the user noticing the first extra render. In other words, useLayoutEffect blocks the browser from painting

For instance:

useLayoutEffect(() => {
  // Read layout from the DOM or make synchronous DOM mutations
}, [dependencies]);        

It runs synchronously after all DOM mutations, just before the browser paints, ensuring that any updates made within this hook are reflected immediately on the next screen paint.

Key Differences

The primary difference lies in their timing. useEffect runs after the DOM has been updated and the browser has painted the screen, making it suitable for most side effects that don't need to interact with the DOM. Conversely, useLayoutEffect runs right after DOM updates but before the browser has a chance to paint, which is crucial for tasks like measurements or synchronous re-rendering.

When to Use Each

Generally, useEffect should be your default choice for side effects. It's less performance-intensive and handles most use cases effectively. useLayoutEffect, on the other hand, should be used sparingly and only for tasks that need to measure or mutate the DOM before the browser paints, like adjusting the scroll position of an item.

Using useLayoutEffect unnecessarily can lead to performance bottlenecks since it blocks visual updates until its tasks are complete.

Practical Examples

Consider a scenario where you need to fetch user data from an API. This is a perfect case for useEffect:

useEffect(() => {
  fetchData().then(data => setUser(data));
}, []);        

For useLayoutEffect, imagine you need to adjust the scroll position of a list when it loads:

useLayoutEffect(() => {
  if (listRef.current) {
    listRef.current.scrollTop = desiredPosition;
  }
}, [desiredPosition]);        

Conclusion

Understanding the difference between useEffect and useLayoutEffect is critical in React development. While they may seem similar, their differences in execution timing can have significant impacts on performance and behaviour. Choose the right hook based on the nature of your side effects for a smooth, efficient application.

References:-

https://react.dev/reference/react/useEffect

https://react.dev/reference/react/useLayoutEffect