Prefix <T> to access methods with generics : Klass[<T>"method"] #57297

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1. whzx5byb commented on Feb 5, 2024

   

   whzx5byb

   on Feb 5, 2024

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   Workaround: access method from prototype.

   type Method = typeof Klass.prototype.method<number>;

   👍React with 👍1Reacted by Ryan Cavanaugh
2. denis-migdal commented on Feb 5, 2024

   

   Denis Migdal (denis-migdal)

   on Feb 5, 2024

   Last edited by denis-migdal

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   Workaround: access method from prototype.

   type Method = typeof Klass.prototype.method<number>;

   Nope.
   You didn't "access a method with generics from a class" in TS, but asked the type of a value that happens to be a method.

   class Klass {
   	method<T>() {
   
   	}
   }
   
   // from the class
   type T = Klass;
   type m1 = T["method"]
   
   // from the type of the class
   type TK = typeof Klass;
   type m2 = TK["prototype"]["method"];

   Your solution could work using functions :

   function _getMethod<T extends BaseClass, U>(t: typeof T) {
         return t.prototype.method<U>;
   }
   
   type getMethod<T extends BaseClass, U> = ReturnType<typeof _getMethod<T, U>>

   But due to Typescript widening, that will return BaseClass.prototype.method<U> instead of T.prototype.method<U>

   cf #57286

   Also, we could help TS by precising the return type :

   function _getMethod<T extends BaseClass, U>(t: typeof T): T["prototype"]["method"] {
         return t.prototype.method<U>;
   }

   But we have currently no ways to precise the generic parameters to the type...
3. Ryan Cavanaugh (RyanCavanaugh)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Feb 5, 2024
4. RyanCavanaugh commented on Feb 5, 2024

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Feb 5, 2024

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   Nope. You didn't "access a method with generics from a class" in TS, but asked the type of a value that happens to be a method.

   I don't understand what you're getting at here. whzx5byb's answer isn't even a workaround; it's a solution.

   I also don't see what this being in a class has to do with anything. There are existing issues around generic instantiation of a type as opposed to a value.
5. denis-migdal commented on Feb 5, 2024

   

   Denis Migdal (denis-migdal)

   on Feb 5, 2024

   Last edited by denis-migdal

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   I also don't see what this being in a class has to do with anything. There are existing issues around generic instantiation of a type as opposed to a value.

   My suggestion wasn't about generic instantiation of a type, but to be able to fetch a generic version of a method.
   It is not (2 operations) :

   1. operator []: we fetch Klass["method"]
   2. operator <>: we make a generic instantiation of the returned type.

   But (1 operation):

   1. operator [<> ]: we fetch Klass[<T>"method"] directly as a type.

   Sorry if I'm not clear in my expression, I am not quite familiar with TS nomenclature and internal stuff.

   I then suggested an extension to the syntax for a more generic usage, that is indeed a generic instantiation of a type.
   Which would give (in the same case as above): <T>(Klass["method"]). So it is slightly different.

   I made another issue as other issues doesn't seem active / followed.
   I think I suggested a way that seems to not have the drawback of other cited suggestions:

   • no syntax ambiguity (?)
   • simple, clear and concise syntax.
   • I guess not too much trouble to implement (?).

   Nope. You didn't "access a method with generics from a class" in TS, but asked the type of a value that happens to be a method.

   I don't understand what you're getting at here. whzx5byb's answer isn't even a workaround; it's a solution.

   It isn't, as he gets the type from a value that happens to be a method, not from the class in TS.

   When manipulating types, there are lot of cases where you don't have access to the values :

   function foo<T extends Record<string, X>>(...) {
   
        type T2<U> = {
            [K in keyof T]: T[K][<U>"method"] // can't have access to the class value as we are in TS (type manipulation, not value manipulation).
        }
   }
6. typescript-bot commented on Feb 8, 2024

   

   TypeScript Bot (typescript-bot)

   on Feb 8, 2024

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   This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.
7. TypeScript Bot (typescript-bot)

   closed this as not plannedon Feb 8, 2024
8. microsoft

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