The property prototype of an abstract constructor type is unavoidably of type any. #56801

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1. fatcerberus commented on Dec 15, 2023

   

   Bruce Pascoe (fatcerberus)

   on Dec 15, 2023

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   Possible (ugly) workaround: Omit<new () => A, "prototype"> & { prototype: A }
2. jcalz commented on Dec 15, 2023

   

   Joe Calzaretta (jcalz)

   on Dec 15, 2023

   Contributor

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   (Bruce Pascoe (@fatcerberus) Omit loses the construct signature)

   Instead of trying to use abstract new inside an object type, you can do this:

   type AAbstractConstructor = abstract new () => A;
   interface AAbstractConstructorRemedied extends AAbstractConstructor {
       prototype: A;
   }

   This will have the prototype property you care about while maintaining the abstractness of the constructor

   Playground link
3. miguel-leon commented on Dec 15, 2023

   

   Miguel Leon (miguel-leon)

   on Dec 15, 2023

   Last edited by miguel-leon

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   Thanks for the workaround Joe Calzaretta (@jcalz), silly me I didn't think of this while coming up with a minimal reproduction.

   The thing is, the type that returns the construct signature is a parametric type.
   I just tried

   // error
   interface WithPrototype<Class extends abstract new (...args: any) => any> extends Class {
       prototype: InstanceType<Class>
   }
   
   type AbstractConstructorWithPrototype<Class extends abstract new (...args: any) => any> =
       WithPrototype<abstract new (...args: ConstructorParameters<Class>) => InstanceType<Class>>;

   playground

   But the interface declaration errors.
   Do you think that as a parametric type then it becomes unavoidable? 😓
   Or maybe is there another workaround?

   Thanks anyway for the suggestion.
4. miguel-leon commented on Dec 16, 2023

   

   Miguel Leon (miguel-leon)

   on Dec 16, 2023

   Last edited by miguel-leon

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   I messed around with it for a bit and managed to make it work like this:

   type AbstractConstructor<Class extends abstract new (...args: any) => any> =
       abstract new (...args: ConstructorParameters<Class>) => InstanceType<Class>;
   
   interface AbstractConstructorWithPrototype<Class extends abstract new (...args: any) => any> extends AbstractConstructor<Class> {
       prototype: InstanceType<Class>
   }

   playground

   The fact that the "hidden" prototype is of type any instead of (at least) unknown, it's a little bit odd, but since there's a work around, I'll close this issue.
5. Miguel Leon (miguel-leon)

   closed this as completedon Dec 16, 2023
6. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025