Promises are another option in addition to callbacks, events for asynchronous programming in JavaScript. Before ES6, libraries such as bluebird (http://bluebirdjs.com) provided promises compatible with the Promises/A+ spec.
A promise represents the eventual result of an asynchronous operation, as described in the Promises/A+ spec. The result would be a successful completion or a failure. And it provides methods such as .then(), and .catch() for chaining multiple asynchronous operations together that would make the code similar to synchronous code that is easy to follow.
The features of ES6 promises are a subset of those provided by libraries such as bluebird. In this book, the promises we use are those defined in the ES6 language ...
