Promises

We already mentioned at the beginning of the chapter that CPS is not the only way to write asynchronous code. In fact, the JavaScript ecosystem provides alternatives to the traditional callback pattern. One of these in particular is receiving a lot of momentum, especially now that it is going to be part of the ECMAScript 6 specification (also known as ES6 or Harmony), the upcoming version of the JavaScript language. We are talking, of course, about promises, and in particular about those implementations that follow the Promises/A+ specification (https://promisesaplus.com).

Note

There are other promises implementations that are not compliant to the Promises/A+ specification, and among those, the most popular is the one provided by JQuery. Most of the topics discussed in this section do not apply to those noncompliant implementations.

What is a promise?

In very simple terms, promises are an abstraction that allow an asynchronous function to return an object called a promise, which represents...