You have to restrict your code to a certain style (and there are warnings for whenever you're defeating the DCE, I believe). You'll particularly see this when you're accessing properties via array-index-like lookup (your myObj['f'+'oo']) example) instead of e.g. myObj.foo under advanced compilation, where the property name will have been munged and doesn't exist.

Like anything else, there are tradeoffs, and Closure asks that you restrict yourself to a more predictable set of patterns in return for the benefits it offers.