Honestly, I didn't see much that doesn't still apply to writing JavaScript today. However, I think overall code organization and re-usability is something that today's JS devs need to think more about. It's too easy to whip together a bunch of "cowboy code" with so many plug-ins and libraries available.
Is the "cowboy code" any less reliable than non cowboy code? I feel like the extra speed of writing in this case may be more valuable than using strict javascript.
Just as notes you take for yourself are not the same as an edited manuscript, code you write in the cowboy way is not code that other developers should be expected to easily understand and maintain.
If you're a solo developer writing small applications, this might not be a problem. If you're growing an application and a business this is a very serious concern. As you grow, you need to bring in more developers, but with a cowboy codebase it will be harder and harder to keep everyone productive. Instead of writing new features, they'll spend more and more time just trying to undersand the code, and to fix the inevitable bugs.
Quick fixes are a fact of life though, but use them only until a solid patch can be crafted.
