Callback Hell
At first when I was reading this JavaScript coding guide, I thought “Isn’t this exactly what promises address?” but that is then addressed further down:
Before looking at more advanced solutions, remember that callbacks are a fundamental part of JavaScript (since they are just functions) and you should learn how to read and write them before moving on to more advanced language features, since they all depend on an understanding of callbacks.
Fair enough. In any case, what you’ll find here is mainly good advice for writing modular code.