Something that has been kind of on my mind is the relationship between JavaScript's undefined and Ren-C's TRASH state.
I've set it up so that if you have a JS-NATIVE that doesn't have a return statement, that returns TRASH. That's consistent with plain functions that don't have a return statement.
But should reb.Value("print {Hello}"); return undefined (the way that reb.Value("select [a 10 b 20] 'c"); returns null?)
It seems there's no value in JavaScript that can cause an error on conditional testing, so I guess maybe mapping it to undefined is about the best we can do.