Professional JavaScript for Web Developers, 5th Edition

11Promises and Async/Await

WHAT'S IN THIS CHAPTER?

• Introduction to Asynchronous Programming
• Promises
• Async Functions

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.wiley.com/go/projavascript5e.

Promises and async/await are two of the most important features in JavaScript, offering developers elegant and efficient ways to handle asynchronous operations. Promises offer a powerful abstraction over callbacks, allowing developers to write cleaner and more maintainable code. Async/await enhances the simplicity of promises by providing a more natural syntax that looks synchronous but runs asynchronously, making it easier to reason about asynchronous code.

NOTE Throughout this chapter, examples make extensive use of the asynchronous logging function console.asyncLog() to log the state of promise instances. This function is not native to the browser, but it can easily be defined as follows:

console.asyncLog = (...args) => setTimeout(console.log, 0, ...args)

The log output will be shown in the chapter 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. Head to the “Promises and the Microtask Queue” section to understand why this is necessary.

Additionally, a browser's console output will often print information about objects that is ...