I miscommunicated that, slightly. What I meant was that you still have to write defensive type checking code in Typescript, because it's only decoration, and you can't rely on the underlying JS to throw type errors either at compile time or runtime.

Javascript is a garbage layer of broken standards, a worst of breed language that shouldn't have to exist between a strict typed higher level language and lower level bytecode, but everything on the web now needs to be built on top of it, compile to it first, and run through one of a few JS engines. You can never build a full fledged language on top of something that has to go through JS... so TS, coffescript, et al are always going to be pseudo typed but not truly solid or permanent solutions.

Ultimately I think ECMA standards will finally move toward strict types in JS, and in a decade more we'll have something that for all intents and purposes is exactly what Flash was in 2009.