Given function is pure, for any given input state, it will return state + 1, the state is an accumulated result of all invocations, there are no arguments nor external factors that would cause it to behave differently.
There are no side-effects either, it solely relies on it's input state to produce the output, it doesn't mutate any variables outside it's scope, doesn't interact with the outside world, nor modify any global state.
Therefore it is perfectly pure by the classic definition of pure, react did not redefine anything in that matter, it is a pure functional renderer by design.
