Flow tries to integrate with the existing ecosystem as much as possible. By taking advantage of Babel, ESLint, Atom, etc.

With Flow you don't even have to opt-in to a new syntax. You can just use comments:

    function foo(val /* : boolean */) /* : string */ {}

Using really powerful inference you can also write much fewer types. If you have well types libraries [you sometimes don't need types in your code at all](https://medium.com/@thejameskyle/flow-mapping-an-object-373d...).