I've been doing some serious JavaScript development for around a year now. It started out with some innocent Harmony hacking at Spring. After Summer I received an offer I just could not pass.
I started developing CanvasDraw for Rate My Drawings somewhere around September. The application is currently at beta and close to a public release. Quite a few people have tried it out already.
During a recent seminar I wrote a paper about the application and its development. I hope it gives you some idea of what can be achieved with some effort. JavaScript isn't definitely a toy language.
The paper contains some very basic information about JavaScript and surrounding concepts. It also discusses RequireJS and RightJS briefly. If you have any questions, I'm willing to answer them at the comments section.
There's also a brief presentation available. The slides are in Finnish so it's likely you won't be able to understand them without some considerable effort. After all there are like six million people proficient in it. :)