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typescriptclassobjecttypechecking

Check that object is not instance of any class in typescript

I have a function which takes objects and proxies them.

I do not want it to proxy classes, and I'd like to assert that with the argument type.

How can I do this?

type NonClassObject = ???;
export function createObjectProxy(obj: NonClassObject) {
  return new Proxy(obj, {});
}

const foo: NonClassObject = {};
const bar: NonClassObject = {} as HTMLElement; // this should be a type error

There doesn't seem to be much to distinguish the two except for the typeof the constructor. I'm not sure if there's a way to say Exact<tyepof OBJ.constructor, Object>... but I'm looking into Utility types now.

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On to something: enter image description here

Dead end: https://github.com/Microsoft/TypeScript/issues/3841

Solution

  • There's an open issue about making the constructor type more defined (https://github.com/Microsoft/TypeScript/issues/3841) but it's 4 years old and not seeming to get anywhere.

    As a stopgap solution, I've just started adding { _noMyProjectProxy: true } to all my classes, and listing out anything I don't have access to. It looks like this:

    type NonClassObject<TYPE> = 
  TYPE extends HTMLElement ? never :
  TYPE extends Event ? never :
  TYPE extends OtherClass ? never :
  TYPE extends { _noMyProjectProxy: any } ? never : 
  TYPE;

export function createObjectProxy<TYPE extends object>(obj: NonClassObject<TYPE>) {
  return new Proxy(obj, {});
}

    It's not pretty or elegant, but it works when nothing else did.