To better understand these tools, and get a real-world perspective on Closure, I reached out to Mihai Parparita, an engineer on the Google Reader team, to hear of his experience. He was gracious enough to extend a very thorough overview, explaining the tools' origin and use case, by e-mail, much of which is summarized below.
As Google Reader development started in early 2005, with Mihai, Jason Shellen, Chris Wetherell (the latter pair now are at Thing Labs working on Brizzly, which also uses Closure) and others working to make a top-notch Web-based RSS reader, the team leveraged Closure immediately after the initial prototypes. At the time, the team was less focused on download size than they are today, but the compiler's aggressive function checking improved error detection.
Closure's role at Reader, initially utilized in low level code, has "moved up the UI stack" to to the point where it is leveraged for UI widgets. Mihai says "this means that it's not a lot of work to do auto-complete widgets, menus, buttons, dialogs, drag-and-drop, etc. in Reader."
The excitement around Closure's release was palpable from developers through Silicon Valley and beyond as you could see from blog posts by Erik Arvidsson, a co-creator along with Dan Pupius, and a series of posts at bolinfest.com. Other excited Tweets came from Mike Knapp, the aforementioned Chris Wetherell and Kushal Dave.
As Mihai says, "You can tell that there's something special about this when you look at the ex-Googlers cheering about its release. If it had been some proprietary antiquated system that they had all been forced to use, they wouldn't have been so excited that it was out in the open now."
Like many other projects at Google, Closure's compiler, library and templates were derived solely as 20% projects and are largely still dependent on work done in so-called 20% time at Google. Mihai says that if one project needs a feature from the compiler or the library, they are encouraged to contribute to it as well.