Let's not forget that if you are not passing immutable data-structures into your pure functions, all bets are off. Unless you code very defensively, there's no

Let's not forget that if you are not passing immutable data-structures into your pure functions, all bets are off. Unless you code very defensively, there's no way you can guarantee purity, especially in async programming. Given f(x) = y, if x is a regular JS object, you can't guarantee purity. It works many times because of the single-threaded nature of most JS engines, but the moment you do something async, you can't guarantee x hasn't changed. That's one of the many reasons I like ClojureScript. It offers interesting guarantees that let me write robust code.