Flow is the JavaScript type checker I have been waiting for

I'd love to a side-by-side feature comparison of Flow and TypeScript, since they seem very similar. I honestly don't know how to pick one over the other now.

Flow doesn't actually transpile; that's done by the JSX transformer tool, which is an interesting separation.

But both implement a subset of ES6, which in itself is very useful. However, they both add non-standard extensions, meaning that TS code might not work with Flow in the future, and vice versa.

A well thought out and timely piece on Flow from the facebook team.