I'm not terribly convinced with memoization though. You're using extra memory, so it's not free optimization. We have Redux memoized selectors everywhere. I can't help but wonder how much of that is actually a memory leak (i.e. it's never used more than once). Granted, components are a bit different.

I always do cringe when I see a lint rule forcing you to use a spread operator in an array reduce(). It's such a stupid self-inflicted way to turn an O(N) into an O(N^2) while adding GC memory pressure. All to serve some misguided dogma of immutability. I feel there is a need for a corollary to the "premature optimization is the root of all evil" rule.