Hi Dianne,
Great question. There is so much more we could have included, so yes definitely!
One thing would be common idioms and
patterns for JavaScript. For instance, even though we did cover everything you'd need to know to understand the Module Pattern, it's a big leap from understanding all the pieces to actually seeing the pattern and understanding it in detail. It would take another big chapter to really explain the Module Pattern, why it's a pattern, and how it works, building on the concepts learned earlier in the book (objects, anonymous functions, function expressions, closures, etc.). I'd love to do a "Head First Extra" on this topic because it's pretty important: that pattern is ubiquitous among libraries and frameworks, so if you want to understand those, the Module Pattern is key concept.
We didn't really get a chance to cover Date much. Again, while the book teaches everything you need to know to be able to look up Date and "get it", there are many nuances to how to work with dates and times that are tricky to figure out. But we ended up leaving most of that out because, while it's important, it's not really a programming fundamental in the same sense that objects and functions are (at a more general level).
Exception handling is covered (briefly) in the appendix, but that's another topic that could have been expanded a lot.
More about how JavaScript is used to work with the browser, simply because that's still mostly how JavaScript gets used. However, because this book is focused on the language fundamentals, we are thinking about revising Head First HTML5 Programming and putting a lot of that content there, replacing the (now) slightly repetitive first four chapters in HTML5. That way there will be a better bridge between Head First JavaScript Programming and Head First HTML5 Programming. We really designed these three books to be a progression: HF HTML and CSS to HF JavaScript Programming to HF HTML5 Programming, and revising those first four chapters in HTML5 will make that progression better!
I'm sure there are many more!
Elisabeth