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Comment by jakelazaroff | Hacker News Reader

That is outside of the scope of TypeScript, which is to be a statically typed superset of JavaScript with no runtime cost.

It's easy enough to write a runtime type checking library; I wrote a tiny one that I use in basically all my projects: https://npmjs.com/package/narrows

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> That is outside of the scope of TypeScript, which is to be a statically typed superset of JavaScript with no runtime cost.

Yes, that is the thing being criticized here.

With your library one has to write types twice.

I understand that it’s not TypeScript’s job at the moment, but it’d be great to enable runtime checks with the flick of a switch without typing extra code.

    function upper(s: string) {
      return s.toUpperCase();
    }

This function could have a nice implicit TypeError instead of “undefined is not a function”

I’m not sure what you mean. My library has a `TypeOf` type that lets you extract static type information from validation functions. And calling `toUpperCase` on undefined already results in a TypeError.

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