Avoiding flash of unwanted animation on first render in React

While developing my personal website, I noticed that my mobile menu appeared for a split second and then moved out of the screen.
My mobile menu is placed on the right side of the screen by using the following CSS:

.navigation { transform: translateX(100%) } 

To make the menu come into the screen smoothly, I used the following CSS:

.navigation { transition-property: transform; transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1); transition-duration: 150ms; } 

By adding this transition in my initial CSS, the menu will be transformed to the right side of the screen but with a duration of 150ms, so this will be clearly visible when the page loads.
To avoid this behaviour, I had to find a way to only apply the transition once the React app is already mounted.
For this use case, I found a custom useIsMounted hook useful.

import { useRef, useEffect } from 'react'; const useIsMounted = () => { const isMounted = useRef(false); useEffect(() => { isMounted.current = true; return () => isMounted.current = false; }, []); return isMounted; }; export default useIsMounted; 

Then in my component, I can easily import this hook and use it:

const Navigation = () => { const isMounted = useIsMounted(); return (<div className={`navigation ${isMounted ? 'mounted' : ''}`}></div>) } 

My CSS then becomes this:

.navigation.mounted { transition-property: transform; transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1); transition-duration: 150ms; } 

And that's it! No more unwanted animations on page load.
This will also help to increase your Cumulative Layout Shift (https://web.dev/cls) if you were encountering the same thing as I was, because your layout will not shift anymore, so it's a win-win!
You can see the end result here: https://thomasledoux.be (I use Tailwind on my site, so the example code is not exactly the same).