ClojureScript is maintained by Cognitect, the same company that maintains Clojure.
Since version 1.7, many libraries work out of the box in both Clojure and ClojureScript.
ClojureScript uses the Google Closure Compiler to minify the code. It includes dead code elimination and other advanced optimization techniques to include only code you need before it minifies it. As a result you can include entire libraries and have Google Closure remove the parts you don't need. This greatly reduces the final bundle size.