What is the Closure Library?

The Closure Library is an industrial-strength JavaScript library. It provides user interface widgets, an event framework, a packaging and dependency resolution system, tools for DOM manipulation, tools for creating animation effects (including drag and drop), communication utilities, a unit testing framework, and a wide variety of other packages.

Google engineers developed the Closure Library in the process of creating large-scale web applications, and it is now the foundation of Gmail, Google Docs, and many other Google products. If you are developing a large or growing application, you may benefit from the Closure Library's breadth. A well-tested library can insulate you from cross-browser compatibility issues and the minutiae of client-side programming, letting you focus on the fun stuff.