Babel started out as a transpiler to let you write the latest version of the ECMAScript specification but ship to environments that don't implement those features yet. But it has become much more than that. "Compilers are the New Frameworks" says Tom Dale and I could not agree more. We're seeing more and more compile-time optimizations for libraries and frameworks. I'm not talking about syntax extensions to the language, but simple code transformations that enable patterns that would be difficult to accomplish otherwise.