Web Dev Simplified Blog

Why Do We Need useLayoutEffect?

July 20, 2020

If you have worked at all with React hooks before then you have used the useEffect hook extensively. You may not know, though, that there is a second type of useEffect hook called useLayoutEffect. In this article I will be explaining the useLayoutEffect hook and comparing it to useEffect. If you are not already familiar with useEffect check out my full article on it here.

The Biggest Difference

Everything about these two hooks is nearly identical. The syntax for them is exactly the same and they are both used to run side effects when things change in a component. The only real difference is when the code inside the hook is actually run.

In useEffect the code in the hook is run asynchronously after React renders the component. This means the code for this hook can run after the DOM is painted to the screen.

The useLayoutEffect hook runs synchronously directly after React calculates the DOM changes but before it paints those changes to the screen. This means that useLayoutEffect code will delay the painting of a component since it runs synchronously before painting, while useEffect is asynchronous and will not delay the paint.

Why Use useLayoutEffect?

So if useLayoutEffect will delay the painting of a component why would we want to use it. The biggest reason for using useLayoutEffect is when the code being run directly modifies the DOM in a way that is observable to the user.

For example, if I needed to change the background color of a DOM element as a side effect it would be best to use useLayoutEffect since we are directly modifying the DOM and the changes are observable to the user. If we were to use useEffect we could run into an issue where the DOM is painted before the useEffect code is run. This would cause the DOM element to be the wrong color at first and then change to the right color due to the useEffect code.

You Probably Don’t Need useLayoutEffect

As you can see from the previous example, use cases for useLayoutEffect are pretty niche. In general it is best to always use useEffect and only switch to useLayoutEffect when you actually run into an issue with useEffect causing flickers in your DOM or incorrect results.

Conclusion

useLayoutEffect is a very useful hook for specific situations, but in most cases you will be perfectly fine using useEffect. Also, since useEffect does not block painting it is the better option to use if it works properly.


Kyle Cook

The official Web Dev Simplified blog by Kyle Cook.

Short and simple articles on web development.