"Invent Angular 2 before Google does!"
On a serious note, one of the biggest things for me learning JavaScript was lack of knowing what was really out there. So many different roles, jobs, directions you could take on the web. I've always been really passionate about design and web, so knowing what was possible back then would've helped fast forward a few years I think :)
Things I wish I knew... I think the most important one that's taken my skills to new levels is understanding and implementing design patterns. We can all write a "for" loop, but again writing it as part of a bigger program that does more powerful things is something that is tough when you're brand new to patterns and best practice ways to attempt things.
Thanks for the great question (was a tough one)!