You're assuming a web client. The example I had in mind was a mobile client, where we actually used a cross compiler to generate java byte code from swift to allow code sharing between iOS/Android clients, but that wasn't usable for the backends (which were C# mostly). Which is why I went with a server-precomputed lookup table in the cases the number of possible inputs made that feasible. If not, reg-exes for validation (with tooling to enable sharing between codebases) are a decent alternative in many cases. But I was curious what other options HN readers might have tried.