Flow

A static typechecker for JavaScript.

The only weird part about using flow (it seems) is the need to mark each file before using it. Like:

// @flow
// Now you're flowin'

Or:

/* @flow */
/* Flowing, thriving, in my lane */

Otherwise, usage in code is similar to TypeScript, but with a few minor syntax quirks:

function foo(x: ?number): string {
  if (x) {
    return x;
  }
  return "default string";
}

To check for errors:

shell

# aliased to just flow
flow status

Flow ​