HTML5, JavaScript, and jQuery 24-Hour Trainer

Lesson 36

Promises

Throughout this book, you have made extensive use of callback functions. You have used callback functions:

• To register event listeners that fire when specific events occur
• To listen for the completion of IndexedDB operations, such as the insertion of data or the opening of the database
• When using the JavaScript filter, map, and reduce functions to process arrays
• When listening for messages from web workers
• When waiting for AJAX responses

As you can see, callback-based programming is enormously important to many JavaScript APIs, and it is impossible to gain a solid understanding of JavaScript without understanding callback functions.

Although callback-based APIs are enormously popular, they do bring their own set of problems. These problems are often referred to as “Callback hell” and stem largely from the following issues:

• It is often necessary to nest callbacks inside other callbacks, and this nesting can extend to several levels. As this happens, code can become difficult to read because it is not always obvious where each level of nesting ends.
• It is difficult to determine the behavior of an application because it is not possible to logically follow code with your eye.
• Data scoping can become difficult with callbacks. You can see this primarily in relation to the identity of this and the corresponding necessity to use bind on each callback to ensure the correct this instance was set.

This lesson examines an alternative mechanism for implementing callback ...