Precisely, TypeScript only exists at compile time. Half their goals relate to this fact, which leads to constraints regarding what the language can achieve.

> 3. Impose no runtime overhead on emitted programs. > 4. Emit clean, idiomatic, recognizable JavaScript code. > 7. Preserve runtime behavior of all JavaScript code. > 8. Use a consistent, fully erasable, structural type system.

https://github.com/Microsoft/TypeScript/wiki/TypeScript-Desi...