[bcyn]

I agree it's not intuitive at first. As someone pointed out on Twitter (can't find link at the moment):

A good mental model is that the second argument to useEffect defines which pieces of state that effect syncs with.

useEffect(() => {}, undefined) // Sync with all state

useEffect(() => {}, []) // Sync with no state

useEffect(() => {}, [a, b]) // Sync with `a` & `b`

[acemarke]

Original source was Ryan Florence:

https://twitter.com/ryanflorence/status/1125041041063665666