I stumbled across a gem of an online service called Codewars when reading an article about coding as often as possible to keep those coding skills up to date. Codewars works very much in a Martial Arts/Kata system, whereby you start as a Kyu and work your way up to being a Dan. I recommend the JavaScript developers give it a try, as my experience with the site has been great. There is also CoffeeScript and Ruby language support too.