Head First JavaScript Programming

Chapter 9. Handling events

Asynchronous Coding

After this chapter you’re going to realize you aren’t in Kansas anymore. Up until now, you’ve been writing code that typically executes from top to bottom—sure, your code might be a little more complex than that, and make use of a few functions, objects and methods, but at some point the code just runs its course. Now, we’re awfully sorry to break this to you this late in the book, but that’s not how you typically write JavaScript code. Rather, most JavaScript is written to react to events. What kind of events? Well, how about a user clicking on your page, data arriving from the network, timers expiring in the browser, changes happening in the DOM and that’s just a few examples. In fact, all kinds of events are happening all the time, behind the scenes, in your browser. In this chapter we’re going rethink our approach to JavaScript coding, and learn how and why we should write code that reacts to events.

Brain Power

You know what a browser does, right? It retrieves a page and all that page’s contents and then renders the page. But the browser’s doing a lot more than just that. What else is it doing? Choose any of the tasks below you suspect the browser is doing behind the scenes. If you aren’t sure just make your best guess.

Knows when the page is fully loaded and displayed.

Keeps track of all the clicks you make to ...