Professional JavaScript for Web Developers, 4th Edition

11Promises and Async Functions

WHAT'S IN THIS CHAPTER?

• Introduction to Asynchronous Programming
• Promises
• Async Functions

WROX.COM DOWNLOADS FOR THIS CHAPTER

Please note that all the code examples for this chapter are available as a part of this chapter's code download on the book's website at www.wrox.com/go/projavascript4e on the Download Code tab.

In ECMAScript editions beginning with ES6, support and tooling for asynchronous behavior has undergone a renaissance. ECMAScript 6 introduces a formal Promise reference type, allowing for elegant definition and organization of asynchronous behavior. Later editions also extended the language to support asynchronous functions with the async and await keywords.

NOTE   Throughout this chapter, examples make extensive use of asynchronous logging setTimeout(console.log, 0, …params) to demonstrate order of operation and other asynchronous behavior characteristics. The log output will be shown as though it is printed synchronously when, in fact, it is printed asynchronously. This is done to allow for values such as promises to assume their final state.

Additionally, a browser's console output will often print information about objects that is not otherwise available to the JavaScript runtime (such as the state of a promise). This feature is used extensively in examples throughout the chapter to aid in broadening the reader's understanding.

INTRODUCTION TO ASYNCHRONOUS PROGRAMMING

The duality between synchronous and asynchronous ...