TypeScript is a typed superset of JavaScript that compiles to plain JavaScript.
It is very active and it is gaining popularity at a rapid rate
You can understand more about TypeScript directly from the official website and the TypeScript handbook
- Compatible syntax and semantics as JavaScript.
- Gradual Type system
- Better IDE support
- Support latest ESMCScript syntax
TypeScript allows you to write ES2015+ syntax today by transpiling the code back to ES3/ES5 JavaScript.
There are other transpilers such as babel and traceur that allows you to do similar things.
babel is pretty much winning the transpiler race against traceur and support more new syntax compare to traceur and TypeScript.
In this front, TypeScript is trailing behind because on the added complexity of the type system and does not support plugins as babel does.
However, TypeScript has the benefit of type system and the ability to transpile to ES3 (babel and traceur only transpile to ES5).
flow is a static type checker for JavaScript.