[kagia]

I feel that typescript being only 2.0, should be in a position to break things once in a while.

If you pay too much attention to keeping legacy code 'alive' you end up complicating matters with either compiler-flags all over the place or redundant API's i.e win32 api (old+new+newer versions of the same function).

[cakoose]

The fact that the flag is global could actually slow down adoption of the new type system.

Let's say I want to use the new type system, but some library I depend on hasn't been updated. If I enable the new type system, I'll get both false positives and negatives when type checking.

If a library writer updated to the new type system, they'd break compatibility with callers that haven't updated. The safest option would be to maintain two almost-identical copies of the library -- one for the old type system and one for the new one.

A better option would be to allow specifying the type system mode at the top of each file (like "use strict"). This allows me to use the new type system even if not all my libraries have updated yet. This also allows libraries writers to use the new type system features without breaking existing users.