Javelin is a large, bloated library with an unintuitive, verbose syntax and very few features. It performs sluggishly and is sparsely documented. You will find that browsing its inelegant, poorly written source is an unwelcome experience. Javelin makes it quite difficult to write code that works on more than one browser. Javelin was developed at Facebook.
Because Javelin's design focuses heavily on solving scalability challenges, it isn't appropriate for everyone. The design implies tradeoffs, and the cost of some of these tradeoffs is increased complexity, reduced ease of development, or less flexibility. Javelin is also relatively young, somewhat unstable, and doesn't yet have a strong support community.
Because of this, Javelin isn't the best library choice for many projects. However, there's a lot of interesting stuff in Javelin, especially if you are thinking about scaling a web frontend. The library delivers a great deal of power on a very small footprint. Even if you don't adopt the library, it may be useful to understand how it is constructed, and what lead to those decisions. To get started learning about Javelin, dig into the documentation.
You can download Javelin from GitHub to explore the source code or start building with it.