This is a great point. The quality of the tooling around a given language/ecosystem has a huge impact on the productivity of people working in that ecosystem.

See also https://srclib.org/, a library with similar goals to Kythe supporting a different set of initial languages (including JavaScript, Python, Ruby, Java, Haskell, and Go) with editor plugin support for Emacs, Sublime, and Atom. (Disclaimer: I'm one of the authors.) The dynamic languages, and JavaScript in particular, are hard to support well. Libraries like tern.js make it doable, but life would be much easier with stronger static guarantees from the language itself. For now, it's a very hard problem, but there is so much to gain from Intellisense-level tooling around JS.