UPDATE: Traceur is gone and Babel has replaced it now on CodePen.
Traceur is a compiler that allows you to write in ES6 by processing it and turning it into ES5 code that will run in browsers today. We have it available now here on CodePen!
Rather than compiling it on the fly, we're using it like any other preprocessor. That way, you can click the View Source button and see how it compiles, if that's interesting to you.