This isn’t available yet, but was discussed at @Scale, https://code.facebook.com/atscale:

At Facebook, we’re writing more and more JavaScript every day. The code we write not only exploits the full flexibility and expressiveness offered by the language, but it also demands strong assurances of robustness, performance, and hackability. In such an environment, the lack of static typing becomes extremely costly, because all the tooling that is needed to support development at this level of scale and complexity requires code intelligence, and the foundations of code intelligence are static types. Flow is a static type system for JavaScript that we’ve built at Facebook, with the overall goal of making JavaScript development an awesome experience. Flow supports several flexible and expressive JavaScript idioms that require advanced type system features that go beyond traditional languages.