Static type checking for Javascript (TypeScript vs Flow)

I've never been a big fan of Javascript for large applications (nothing beyond proxies and simple services) and that is partially because in my experience the lack of static typing ends up making very easy to make mistakes and very difficult to refactor code

Because of that I was very excited when I discovered TypeScript some months ago (Disclaimer: I'm not a JS expert) and I was very curious about the differences between TypeScript and Flow when some colleage pointed me to it today.   So I tried to play find the seven differences, but I'm lazy and I stopped after finding one.

Apart from cosmetic differences and tools availability both TypeScript and Flow support type definition based on annotations, type inference and class/modules support based on EcmaScript 6 syntax.    The relevant difference I found after reading/playing with them (for half an hour) is that because of the way they implement type inference Flow can detect type changes of the variables after the initial declaration making it more appropriate for legacy Javascript code where adding annotations can be not possible.

This is some code I used to play with it with some inline comments:

var s = "hello";
s.length  // Both TS and Flow know that s is a string and they check they have a length method


var s: string = null;
s = "hello";
s.length  // Both TS and Flow know that s is a string and they check they have a length method

var s = null;
s = "hello";
s.length // TS doesn't know that this is a string but Flow knows and can check it has a length method