Google just open sourced Closure, their robust JavaScript library. Closure is used in a variety of Google products, notably GMail and Google Docs. I had a chance to play with it for a few minutes, and I’m posting my first impressions.

Google Closure is a fairly complete JavaScript framework. In that sense, it duplicates a lot of the functionality of existing JavaScript libraries, such as jQuery and YUI. It contains behaviors, AJAX, event handling, selectors, UI components, and more. The syntax is fairly straightforward, but it’s not the same as other libraries, so there’s a learning curve in getting acquainted with it.

Closure has excellent dependency management through a robust loading mechanism. Basically, if you don’t reference a certain piece of the library, rest assured that it’s not going to load (and slow down your page as a result). In addition, it comes with the Closure Compiler, which will walk your JavaScript, determine what libraries you need, then aggregate and compress all the required files. Being able to deploy one file instead of many is a great way of speeding up your site.

One thing that irks me is that there only way to get Closure is through Subversion access to the trunk. If Google wants Closure to be adopted widely, they’re going to need to start offering discrete, packaged versions of it. Many developers (and novices) will be put off by the current distribution method otherwise.

The API is well documented, and has links to the actual code for each method (something that I’ve not really seen before in API documentation). The API itself is very robust, with a ton of methods and accessors on each object. I’m not sure I’m a big fan of the way things are laid out in the API, but I don’t have enough experience with it to say anything definitively here.

One doesn’t have to stretch the imagination to believe that Closure has amazing potential. This is, after all, what GMail is built from. I’m excited that it’s been open-sourced and I can’t wait to use it a bit more. Google has given a grand gift to the developer community with this release.