I guess since ES5 is the language spec I've used the longest, I find refactoring vanilla JS to be enjoyable since most frameworks exist to clean up ugliness. Many frameworks add boilerplate and foreign patterns that can make this difficult, though. When I run against this, however, I have to re-assess the value of the framework I'm adding and if it still exists, refactoring is not what's needed as much as a re-write. This is the primary reason for writing code that is modular on as many levels as possible.

I worry about the React/Flux patterns at this point in that they are creating codebases where modularity is specific to their pattern which could be tomorrow's anti-pattern (Prototypejs anyone?). I suspect we'll be cursing refactoring these codebases soon.