During development it would be helpful to have type checking enabled at runtime behind some flag or configuration. I want code to fail fast and early in my development environment. In production the code will be stripped and minified.

I read the FAQ in the proposal but don't understand the challenges in adding type introspection at runtime. It seems weird to add all this syntax and not have any way to leverage it during runtime. Although maybe it wouldn't be that useful? Still making up my mind about this.

Glad to see that these questions are at least being considered and explored as part of the proposal.

Great job from everyone involved, this proposal shows a lot of promise and it makes me excited for the future of JS.