The first bits of code I come across on this codebase isn't functional: https://github.com/prettydiff/share-file-systems/blob/master.... You're modifying state. You're implementing iteration with do..while loops. That doesn't scream functional to me. Iteration is done with things like Array.map, Array.filter, Array.reduce in functional JS. Higher order functions are key. Functional code is declarative. do..while is imperative.