Turns out I missed most of ES5, too: https://github.com/es-shims/es5-shim
Browser-based repl: http://babeljs.io/repl/
https://github.com/lukehoban/es6features https://babeljs.io/docs/learn-es6/ https://robots.thoughtbot.com/replace-coffeescript-with-es6 https://hacks.mozilla.org/category/es6-in-depth/