In earlier versions of JavaScript, the callback pattern was the most common way to organize asynchronous code. It got the job done, but it didn't scale well. With callbacks, as more asynchronous functions are added, the code becomes more deeply nested, and it becomes more difficult to add to, refactor, and understand the code. This situation is commonly known as callback hell.
Promises were introduced to improve on this situation. Promises allow the relationships of asynchronous operations to be rearranged and organized with more freedom and flexibility.
The recipes in this chapter demonstrate how to use promises to create and organize asynchronous functions, as well as how to handle error conditions.
