“Callback hell” is when you need to do many asynchronous actions / call several asynchronous APIs with inputs to each call depending on the delayed results of some other call(s), and you are using callbacks to organize your control flow. Often you end up with either a large set of named functions being passed around as callbacks, or a deeply nested hierarchy of anonymous callbacks. Either way, control flow can be difficult to follow or reason about, both when reading the code and when debugging. Even simple tasks take large amounts of boilerplate code.

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.