TypeScript: initial impressions

There are several things under the TypeScript umbrella: a type system, language spec, JavaScript-targeting compiler and tooling that provides interactive code completion. I have tried TypeScript on a few small projects, the latest one being TypedPhoneGap. I also worked extensively with the language spec, wrote a parser and analyzer for the `.d.ts` contract specification fragment, pondered the semantics, and engaged in little flamewars on the language forum. My conclusion, in short, is this: the type system and language design are terrible, the compiler is OK, and tooling is excellent. Overall, it is an improvement over writing bare JavaScript, and the definition fragment is helpful (though not ideal) for communicating JavaScript API formally.