use-is-mounted-ref.js

export function useIsMountedRef(){
    // We need something similar to the plain mutable class member.
    const isMounted = useRef( true );
    
    // And, we need something similar to componentWillUnmount.
    useEffect( () => {
        // Whatever we return is a cleanup effect.
        return () => { // <- componentWillUnmount
            isMounted.current = false
        }
    }, []); // [] never changes, so the "cleanup" function will be fired on unmount only.

    return isMounted;
}