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.
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.
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.
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
You Probably Don't Need
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.
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.