Comment by chrisaycock

He's using ClojureScript. He states at the bottom of the page:

  It's no secret that I really like Clojure and as a lisp,
  it was the easiest language for me to start the prototype
  with, but there's no reason this couldn't be done for any
  language with a dynamic runtime. The rest is mostly simple
  analysis of an AST and some clever inference. So could
  Light Table have used JS instead? Certainly - and
  hopefully it will get there sooner rather than later.