> If they were using useState() correctly and provided a button or link to call the update function, yeah, React would call into their code as many times as the user clicked the button. As it is though, it's just once (useEffect with that second argument is "only run the passed-in function once on mount").

As I understand it both the useState and useEffect create points where their code will be suspended and resumed as a coroutine, even if everything only gets run once? But yeah, the more important point is that in the general case React will run it multiple times in ways that are not really under the user's control (at least not without understanding the internal details of React quite deeply).

Nope:

The first run through, useState() returns the default value and your code runs to completion. When the update function is called, that entire React function is run again, but this time useState() returns the new value instead of the default value.

Likewise useEffect(), your React function runs all the way through and the callback passed to it is run afterwards at various times depending on the dependencies array. First run is immediately after, on component mount, then every time the dependencies change (if dependencies are specified), or every rerender (if null/undefined instead of an empty array).

> The first run through, useState() returns the default value and your code runs to completion.

Your function was still sliced into the part before and the continuation (Dispatch) after though, right? The React runtime calls that continuation immediately with the default value, so you don't notice the suspension, but control flow wise it's still React calling your code twice rather than once.

Nope, it's a normal function that updates some variables in the React module then returns two values. There's a little bit of magic going on, with how it knows which component it's in, but that's happening outside of the individual hooks.