Sorry about that, I'll try to elaborate based on my own experiences...

Let's take Angular for example. A junior would be expected to have experience in either Angular or some kind of front end framework (Backbone, React, etc). This means either experience building your own project in one of these technologies from scratch, or working at a company (internship or otherwise) where you worked with one of these frameworks.

I'd expect a junior, given an Angular code base, to be able to figure out what's going on and make some light commits/changes within 1 week, 2 tops. These likely would be bug fixes (IMO this is the best way to ease into a code base). After that amount of time, I'd expect them to be able to implement some enhancements or small new features with a little bit of guidance from someone senior.

To be able to function at this level, you'd need to know how JS frameworks work in general (I know Backbone/React != Angular, but they're similar enough where you should be able to pick up the others if you know one of them). I'd expect an understanding of concepts like the DOM, data-binding, dependency injection, routing, asynchronous promises, etc. I wouldn't expect you to be an expert in any of these, but I'd expect you to be able to figure them out on your own for the most part (of course questions here and there are encouraged!). I'd expect you to make a mistake here or there, but I'd also expect you to be good enough to where you're not committing total garbage/broken code to our repository. I'd also expect you to take your time to try and gain a full understanding of the scope of the changes you're making, to make sure you're following whatever style guides/design patterns are in place.

To prove you can do this in an interview, I'd want to see that you can answer general questions about the framework you've worked with before. I'd also want to see professional experience in the framework, or a completed (or close to completed) project in the tech I'm looking for that demonstrates you can make things work. This stuff can be learned, though, so above all else I'd be looking for a proven ability to learn fast and a positive, motivated attitude.