JavaScript Cookbook, 3rd Edition

Chapter 9. Asynchronous Programming

JavaScript was built as a single-threaded programming language, with one call stack, one memory heap, and able to execute just one code routine at a time. But over the years, JavaScript has grown. It’s acquired the ability to send network messages, read files, and wait for user confirmation—all operations that might take time and could lock up the user interface. To handle these operations safely, JavaScript has introduced its own asynchronous programming patterns.

In the early days, JavaScript’s asynchronous support revolved around callbacks. With a callback, you request an operation (say, fetching an image from the web) and the browser does the work on another thread, outside of your application code. When the image has finished downloading and your application is idle, JavaScript triggers your callback and passes the data back to your code. The end result is that your application code is still single-threaded, but you have the ability to launch asynchronous work through a set of standardized web APIs.

Callbacks are still found all over JavaScript, but in recent years they’ve been wrapped with more polished language features, like promises and the async and await keywords. The underlying plumbing is the same, but now it’s possible to create sophisticated applications that manage concurrent asynchronous tasks, handle sequences of asynchronous calls, and deal gracefully with unexpected errors.

In this chapter, you’ll use callback and promises ...