Yup, Opa is pretty nice. It just highlights the differences between "compilers" and "transpilers" which came up in another thread recently. There's a number of

Yup, Opa is pretty nice. It just highlights the differences between "compilers" and "transpilers" which came up in another thread recently. There's a number of transpilers that are fairly simplistic transforms of source languages, or JS->JS translation (lowering ES7/6 to ES5, etc) which do limited whole world optimization, and mostly consist of peephole passes. Closure, Opa, GWT, Dart, and a few others (ScalaJS?) run fully optimizing compiler passes like a traditional old school compiler. For example, IIRC, Dart transforms (or used to) to Hydrogen IR/SSA internally, runs lots of optimizations, and then transforms back before JS output. This is not to bash transpilers, but I don't consider Webpack/Babel/Uglify in the same category.