This document discusses how to prevent "callback hell" in JavaScript code. It defines callback hell as hard to understand code from nested callbacks. It recommends naming functions clearly, defining new functions, and using Promise objects and generators to avoid callback nesting. Specifically, it suggests using the co library to wrap Promise objects in generator functions so code can use yield instead of nested callbacks. In conclusion, callback nesting is part of JavaScript but there are options like Promises or naming callbacks well to make code more readable.