Could someone please explain what's Callback hell and why Toffee-script is doing to resolve it? The compiled JS code looks harder to read than using callbacks to me.
The generated code is using callbacks. The point is that instead of writing something like (a cleaner, but still ugly, version of) the generated code on the right, you’d instead write the (quite easy to read) toffeescript code on the left.
That said, I think coffeescript’s “it’s just javascript” slogan (and goal of generating readable code) is one of its most important features. It has to be quite a bit harder to debug than the coffeescript version, since the generated code has control flow that’s so different looking from the input.
As an analogy, consider what would happen if Outlook was written in VBA and you opened OUTLOOK.DOC in Word to read your email. Well that's GMail.
That explains the many ways to structure your async code in pure JavaScript (no helper libraries or transpilers) so you don't create callback hell code.
With good tools and a sufficiently correct compiler, that should be as irrelevant as worrying that compiled x86 assembly is harder to read than the C source.