Welcome to the second part in my series of posts about creating a Facebook application. I am using Django as my web development framework, but this post doesn't have much to do with Django, since it deals with the front end. In particular, it talks about how to write JavaScript that can work both in and out of Facebook.

As I mentioned last time, Facebook lets developers use a subset of JavaScript, which they call FBJS. The FBJS is transformed on the fly into JavaScript as the page is loaded. All variables and functions you define or reference are prepended with a string like "a123456789_", including calls to document.getElementById and setTimer and the like. This is done in order to restrict what you can do with DOM elements, to avoid cross-site-scripting attacks and unwanted user-hostile behavior. FBJS is fairly well documented, so if you plan to do some Facebook JavaScript development, you should start there.

The biggest restriction that FBJS imposes is that you can no longer access the attributes of DOM elements directly, but must go through an abstraction API consisting of a series of setters and getters. For example, instead of saying something like imageEl.src = myImageUrl, you instead need to call imageEl.setSrc(myImageUrl).