> open up `.js` files and have them Just Work

Nod. A single big grammar just wasn't what I'd thoughtlessly expected. When I did something similar, there was a gaggle of small grammars slicing up javascript aspects and history, then variously composed into assorted language grammar variants. But part of the motivation for that was sharing grammar fragments across languages, and "parse exactly language version X" in support of parser validation and compiler use. So a bit different use cases.