useEffect is treating undefined as meaning undefined.

It makes perfect sense from inside of useEffect.

If there are no dependencies ([]), the dependencies can never change, so React can always reuse the old effect.

If the dependencies aren’t defined, there’s no way to tell if the old effect is OK, so React must always rebuild it.

**

But `useEffect(()=>{})` doesn’t explicitly show that React will get `dependencies = undefined`, and in this case that’s unusually important.

You could have ESlint force you to change that to `useEffect(()=>{}, undefined)`, I suppose…