Flow: Open Source JavaScript Static Type Checker from Facebook

http://flowtype.org/

---

Example 1:

/* @flow */
function foo(x) {
  return x * 10;
}
foo('Hello, world!');

$> flow

hello.js:5:5,19: string
This type is incompatible with
  hello.js:3:10,15: number

Example 2:

/* @flow */
function foo(x: string, y: number): string {
  return x.length * y;
}
foo('Hello', 42);

$> flow

hello.js:3:10,21: number
This type is incompatible with
  hello.js:2:37,42: string