Repository navigation
Use typeof with generic #20719
Description
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());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
superamadeus commentedon Dec 15, 2017
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.
A solution to this is to define a generic type to describe constructors like Angular does:
from (angular/packages/core/src/type.ts)
Usage example:
RyanCavanaugh commentedon Dec 18, 2017
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
but the real type is like this
donaldpipowitch commentedon Dec 19, 2017
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.