Ah, I see what you mean now with the type annotations. It looks like they require the jsx transpiler if you use them. Still, flow isn't doing the transcoding, just the static analysis part. I wish the type annotations could have been in the form of jsdoc-style comments though.

EDIT: looks like it supports type annotations similar to closure compiler's (in docs) https://github.com/facebook/flow/blob/master/tests/docblock/...

Try running an annotated file in the browser - it's simply not valid ECMAScript syntax and no JavaScript runtime will run it 'as is'.

If the code requires transformation by a compiler aware of the language semantics and syntax - it's transpiling in my book. Just like TypeScript is a superset of ECMAScript - so is Flow (in a much lighter, closer to source sense it seems).