Ideally you want to have all your state in one place, the "single point of truth", in tightly modeled data structures, keeping it as lean and immutable as possi

Ideally you want to have all your state in one place, the "single point of truth", in tightly modeled data structures, keeping it as lean and immutable as possible. The program is then just a function of the state, consisting mainly of self-contained functions, that take arguments and produce a result, without causing side effects. Realistically, that doesn't mean there are no classes or objects with internal vars, but these are more of ephemeral nature, and can be created predicatively and deterministically from your centralized global state. In the best case you can have complex applications with tens of thousands LoCs that feed from just a few dozen state variables.