Exploring Facebook Flow: First Impressions, Gotchas, and Tips

The stated goal of Flow is to "find errors in JavaScript code with little programmer effort". To paraphrase project lead Avik Chaudhuri: Flow aims to enforce the benefits of a type system while maintaing the "feel" of JavaScript. Avik promised that Flow would be open-sourced by the end of the year and, true to his word, the project appeared on GitHub a few months later.