[wukkuan]

This isn't used internally by Facebook at this point. The author mentioned this after your post. https://news.ycombinator.com/item?id=8108741

[vineet]

Thanks!

But even if it is not being used yet - do we know if Facebook uses Typescript (or some extension of it)?

[bsimpson]

To clarify your point, immutable isn't in production at FB yet; we don't know about TypeScript.