Definitely "Javascript Web Applications" out of those options. Apps is misleading and informal. "Javascript Applications" is ambiguous, and the only people who

Definitely "Javascript Web Applications" out of those options. Apps is misleading and informal. "Javascript Applications" is ambiguous, and the only people who will understand "Stateful" probably already know what's in the book. "Modern Javascript" is also good because that's basically what you're talking about here. If you focused on the realtime aspect of web app dev, you could even go with something like "Realtime Javascript Applications" - I bet there are a LOT of people searching and looking for books on that subject (I know I was).