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Something like this?

"Safe TypeScript: Safe and Efficient Gradual Typing for TypeScript"

http://research.microsoft.com/apps/video/default.aspx?id=226...

That was precisely the paper I was thinking of.

This is a neat idea. Even with a significant perf hit, it could be used during development, testing, and debugging. You should propose this to the Typescript team! https://github.com/Microsoft/TypeScript/issues

I recall some paper from people who had already implemented it.

Edit: It might have been this one: http://research.microsoft.com/apps/pubs/?id=224900

You can add type safe guards which uses reflection at runtime.

No, and you wouldn't want the overhead. You can turn on warnings though if you want to make sure all your code is typed on compile. After compiling all type definitions are stripped and you're left with plain javascript.

How do you enable warnings?

swax is probably referring to the compiler option "noImplicitAny", which can literally turn variables whose type is not explicitly specified and cannot be inferred from their declaration as errors, instead of assuming they have the transparent `any` type.