You mention CoffeeScript being one of the worst, and I don't necessarily disagree, but how has it become so popular despite its poor documentation? Is it because CS is just /that/ good? Or is it that it has developed a great community early on that compensates for the lack of documentation? Either way, writing good documentation seems like an either path than doing those two things.
There's also a third reason: it's very easy to see what CoffeeScript compiles to - the JavaScript source is easy enough to read and comprehend - and that behaviour is well documented.
Most of the times I'm looking at CoffeeScript to just figure out what it compiles to!
