Flow offers some interesting control over things like optional types (nullable) and intelligent understanding of type tests in conditionals or calls to librarie

Flow offers some interesting control over things like optional types (nullable) and intelligent understanding of type tests in conditionals or calls to libraries like invariant. I've had these sorts of features catch quite a few bugs in my editor before even running a single test. Flow is also based on a daemon which continuously monitors your code. As such, it incrementally checks your modules allowing it to be extremely fast once started. I ended up using flow since it fit with the language feature set I was using with Babel (JSX in react code being one example). Since then, I've grown to enjoy the flow based typing and the rather good support for optional and non-optional types. Looking at the code lately seems to show some interesting stuff in the pipeline too. Taint tracking seems like a very useful one that I'd love to implement and seems like a natural extension of the code-flow based type system. Flow could definitely use more documentation and more example code but I found the community on IRC extremely helpful and polite so perhaps it's not a bad time to start if you're willing to do a little reading on the site first: http://flowtype.org/docs/getting-started.html http://flowtype.org/docs/getting-started.html