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Comment by jfengel | Hacker News Reader

I'm disappointed that it still doesn't incorporate a tool for one common case, runtime type checking. Node.js REST servers receive a lot of JSON objects and it would be nice to automatically check that they actually follow type.

There are several aftermarket add-ons (io-ts, runtypes, class-validator) but they all seem to impose extra steps or rewriting code -- and different projects will use different ones. It seems like it should be something they can build into the language. I'd rather have one good standard than a dozen alternatives. (Can you guess I come from a Java background?)

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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.

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

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.

> 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.

> I'm disappointed that it still doesn't incorporate a tool for one common case, runtime type checking. Node.js REST servers receive a lot of JSON objects and it would be nice to automatically check that they actually follow type.

You are using the wrong tool for the job then. There are json-schema libraries made for that purpose. It has absolutely nothing to do with Typescript which is just basically a static type analyser, not a language with a runtime, since it does absolutely nothing at runtime.

json-schema is hideous compared to TS type definitions, though.

json-schema works at runtime, that's what you want to validate your serialized messages. None of that stuff is related to typescript.

Typescript is a static code analyser, nothing more. It doesn't do anything at runtime.

I see the utility of json-schema, I just wish the schemas were beautiful and succinct like TS type definitions are.

I know one is runtime and one is compile-time, but they both allow you to specify the "shape" of a particular JSON object and verify an example against it.

I agree, it should be built in. But have you tried Zod? It’s amazing.

> I agree, it should be built in. But have you tried Zod? It’s amazing.

Not it shouldn't, since it would affect runtime performances.

Or other alternatives

https://github.com/moltar/typescript-runtime-type-benchmarks

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