We hadn't at the time because the only JS engines we could think of were 10x larger than CMake because they were built for browsers and had all kinds of cruft for a DOM that CMake doesn't care about hanging around. Maybe QuickJS would work, but (personally) I don't really think the language would be all that much of an improvement over what we have today (my issues are mostly around the oddities of which JS has quite a number of due to its roots). Then again, some of the ideas that have been tossed around for what could happen are probably interfering and I don't think JS would make those things any easier.