state setter and updater in react useState - Leebo explained

about_state_setter

To be specific, currentValue is the value the state had last time the component rendered. It isn't under any obligation to keep "up to date," because you want to start a new render when that happens. It's sort of like taking a blurry photograph vs a crisp clear one. Trying to render your component with a "always up to date" state would cause a blurry photo, because the state would be different by the time you got to the bottom of the render than it was at the top. Instead, the entire render shows you the same state from top to bottom, just like a clear photo
This is essentially one of the problems with class components vs functional components; the class architecture encouraged devs to "take blurry photos" all the time