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.
Option 1 applies to mobile frontends, too. In Swift you can always call JavaScriptCore to run a script. Android can use system-wide V8 or even bundle a smaller interpreter for JS.
(In fact, JS is the only "universal" scripting language in mobile due to Apple's fun restrictions that JavaScriptCore is the only JIT engine allowed on iOS. It really is the strongest option today for language to write things in if they absolutely need to be shared across all possible operating environments.)
