Description

Donald Pipowitch (donaldpipowitch)

opened on Dec 15, 2017

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TypeScript Version: Current playground version (?)

Code

class GenericGroup<T> {
  items: Array<T> = [];
  constructor(name: string) {}
}

type CheckboxValues = string;
type CheckboxesGroup = new (name: string) => GenericGroup<CheckboxValues>;
const Checkboxes: CheckboxesGroup = GenericGroup;
const checkboxes = new Checkboxes('checkboxes');
checkboxes.items.map(item => item.toUpperCase());


type RadioValues = string;
type RadiosGroup = typeof GenericGroup<RadioValues>;
const Radios: RadiosGroup = GenericGroup;
const radios = new Radios('radios');
radios.items.map(item => item.toUpperCase());

link

Expected behavior:

typeof GenericGroup<RadioValues>; is equivalent to new (name: string) => GenericGroup<RadioValues>;

Actual behavior:

Syntax error, because typeof GenericGroup<RadioValues>; is not supported.

Motivation:

In my use case I have a class with a generic (like GenericGroup) which extends a class from a 3rd party lib. The class from the 3rd party lib uses multiple params in its constructor. When I alias my class with the filled generic I don't want to write the parameters for the 3rd party lib every time (as done with new (name: string) => GenericGroup<CheckboxValues>;) as I don't really maintain them.

(There seem to be multiple related issues, but I couldn't found an issue with exactly this problem.)

Activity

1. superamadeus commented on Dec 15, 2017

   

   superamadeus

   on Dec 15, 2017

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   The 'typeof' operator is not meant to describe the constructor of a type, but rather the type of a value. SomeGeneric<T> is not a value, it is a type in and of itself. Any other interface/type (like the one defined below) will not work in this way either.

   interface MyInterface {
       name: string;
   }
   
   let m: typeof MyInterface; // error "MyInterface only refers to a type, but is being used as a value here."

   A solution to this is to define a generic type to describe constructors like Angular does:

   // angular/packages/core/src/type.ts
   
   export interface Type<T> extends Function { 
       new (...args: any[]): T; 
   }

   from (angular/packages/core/src/type.ts)

   Usage example:

   type RadioValues = string;
   type RadiosGroup = Type<GenericGroup<RadioValues>>;
   const Radios: RadiosGroup = GenericGroup;
   const radios = new Radios('radios');
   radios.items.map(item => item.toUpperCase());

   Reacted by Ryan Cavanaugh, Apperside and XGHeaven
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   QuestionAn issue which isn't directly actionable in codeAn issue which isn't directly actionable in code

   on Dec 18, 2017
3. RyanCavanaugh commented on Dec 18, 2017

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Dec 18, 2017

   Member

   More actions

   A subtle thing is that generic class constructor functions are not contained in a generic type. You can tell that because the type parameter is not in scope for static members! It "feels" like you have a type like this

   interface GenericCtor<T> {
     new(x: string): InstanceType<T>;
   }

   but the real type is like this

   interface GenericCtor {
     new<T>(x: string): InstanceType<T>;
   }

   Reacted by superamadeus and kuzhelov
4. donaldpipowitch commented on Dec 19, 2017

   

   Donald Pipowitch (donaldpipowitch)

   on Dec 19, 2017

   ContributorAuthor

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   Thank you both. I need to wrap my head around this. In my case I extend from a 3rd party constructor (React.Component) so I need to figure out how to apply your feedback there. Thanks.
5. Donald Pipowitch (donaldpipowitch)

   closed this as completedon Dec 19, 2017
6. microsoft

   locked and limited conversation to collaborators on Jun 21, 2018

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