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

[denis-migdal]

🔍 Search Terms

generic methods

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Use prefix <T> to access methods with generics:

class Klass {
    method<T>() {}
}

type Method = Klass[<number>"method"] // <number>() => void

Could also be extended for all <T>(...args: any[]) => any types :

type Fct = <T>() => void;
type FctT<U> = <U>Fct; // FctT<U> = <U>() => void;

I think there are no ambiguity with this syntax:

typeA<T>typeB  // syntax error
<T>typeA.typeB // = (<T>typeA).typeB
typeA.<T>typeB // = <T>(typeA.typeB)

📃 Motivating Example

Currently, there are no ways to access a method with generics from a class.

This would solve numerous issues :

• Assume type X = <T>() => Z<T>; as having an optionnal generic type parameter to enable deducing the return type on generic methods #57102
• Introduce mechanism for manipulating function types without losing generic type information #50481
• Instantiation expressions #47607 (comment)
• Trying to get the type of a generic method of a class results in a parse error #53410

💻 Use Cases

1. What do you want to use this for?

Access methods with generics

3. What shortcomings exist with current approaches?

No current approaches.

4. What workarounds are you using in the meantime?

• Assume type X = <T>() => Z<T>; as having an optionnal generic type parameter to enable deducing the return type on generic methods #57102 (comment) : won't work if the class isn't known
• Introduce mechanism for manipulating function types without losing generic type information #50481 (comment) : requires to register in an interface all classes that will be used.

[whzx5byb]

Workaround: access method from prototype.

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

[denis-migdal]

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...

[RyanCavanaugh]

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.

[denis-migdal]

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).
     }
}

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.