async-javascript | Asynchronous Programming in JavaScript | Reactive Programming library

Promises are usually vaguely defined as “a proxy for a value that will eventually become available”. They can be used for both synchronous and asynchronous code flows, although they make asynchronous flows easier to reason about. Promises can be chained “arbitrarily”, that is to say - you can save a reference to any point in the promise chain and then tack more promises on top of it. This is one of the fundamental points to understanding promises. Promises can be created from scratch by using new Promise(resolver). The resolver parameter is a method that will be used to resolve the promise. It takes two arguments, a resolve method and a reject method. These promises are fulfilled and rejected, respectively, on the next tick. Usually promises will resolve to some result, like the response from an AJAX call. Similarly, you’ll probably want to state the reason for your rejections – typically using an Error object.