From what I understand, it should work with any JavaScript you feed it. Though, for badly-written source, it may even produce a faulty output when used with ADVANCED_OPTIMIZATIONS. Yikes! (Example from the docs - "Inconsistent property names": https://developers.google.com/closure/compiler/docs/api-tuto...). So it's more about the Closure Library being "certified" to work with it. Someone with more experience feel free to shed some light on this.