[kinjba11]

> A JS engine can do this trivially

I'm not one to argue, computers are fast. However, this would be a huge paradigm shift for the JavaScript ecosystem, where compile-to-JavaScript-tools like TypeScript are dominant. These tools have explicit goals to do type checking at compile time only and avoid generating anything at runtime. Runtime checking is redundant in this paradigm.

Now if you're arguing that the JS engines like V8 should be able to use type information for performance gains, I understand that wish. WebAssembly seems to be taking over the performance angle these days though, as it was designed from the start for that purpose, unlike JavaScript.

> You're kidding right? .. will be stripped before a JS engine ever sees it?

Yes. We're talking JavaScript here - billions of lines of which are compiled every year to ES5 ;-; with polyfills. Throwing away hundreds of useful features of ES2016+. This is par for the course, part of the JavaScript paradigm which this proposal understands.

> sounds like a feature for your console

That is fair, though it seems unlikely Chrome would adopt such a feature for their console without motivation. Perhaps.

> Which enforced the types

Python does not enforce types at runtime? You might be referring to something I'm not familiar with. Fired up Python 3.9. This works fine even though I'm passing strings for int.

  def add(a: int, b: int):
      return a + b
  add("not ", "used") # 'not used'

> the cost of a mode switch is massive

But earlier you said a JS engine can do this trivially? I don't understand how this is now difficult. Ultimately, if the types are enforced, you may as well go the full monty and support TypeScript in the browser, with Deno leading the way. Stop beating the JavaScript horse and all. That's the dream. This proposal is a compromise without a doubt, with the understanding that TypeScript in the browser is politically untenable at this point.