TypeScript array of generic classes

Question

I'm having trouble getting the TypeScript compiler to not give me grief. I have a controller which has an array of classes, NOT the instance of the class, but the class itself. All of these will extend off of the same base, UnitView. For some reason I get an error if my TitleView does not also accept generics, but I get no errors if it does.

I'm not understanding why TitleView would need to accept generics because it passes the Model type explicitly to the UnitView. Can anyone explain or see anything that I'm doing wrong?

Code:

class Model { }
class View<TModel extends Model> {
    private model: TModel;
}

class UnitView<TModel extends Model> extends View<TModel> { }

class TitleView extends UnitView<Model> { }


class Controller {
    private ViewClass: typeof UnitView;

    private ViewClasses: typeof UnitView[] = [
        TitleView
    ]
}

And here is a direct link to TypeScript playground if you want to test it there

basarat · Accepted Answer · 2017-09-21 00:35:08Z

2

The error has nothing to do with arrays. Here is a minimal reproduction of your bug:

class View<TModel> {
    model: TModel;
}

class UnitView<TModel> extends View<TModel> { }
class TitleView extends UnitView<{}> { }

const example1: typeof UnitView = TitleView; // ERROR

TileView is not assignable to typeof UnitView. The key reason being that the type of T (a generic) are not compatible with {}.

This is similar to the further simplified example shown below:

class Foo<T> { 
    model: T
}
class Bar{
    model: {}
}

const example2: typeof Foo = Bar; // ERROR

TLDR

Generics T and instances (even {}) are not compatible for assignment. This is by design.

More

The only way to stay compatible is the preserve the generic as well e.g.

class Foo<T> { 
    model: T
}
class Bar<T>{
    model: T
}

const example3: typeof Foo = Bar; // Okay

edited Sep 21, 2017 at 0:35

answered Sep 21, 2017 at 0:15

basarat

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3 Comments

jas7457 Over a year ago

So what would be the correct way to write this if I want to express the array will be an array of classes that extend off of UnitView?

basarat Over a year ago

You will have to preserve the generic. Added to answer as more 🌹

jas7457 Over a year ago

I was hoping there'd be another way, as the TitleView really shouldn't be accepting generics, as that is the end of the line and the Model is well-defined at that point. I may just change the typings to an array of any instead. This design seems flawed to me, or maybe I just don't fully understand it. Thanks for the help.

Ryan Cavanaugh · Accepted Answer · 2017-09-21 00:34:57Z

0

To complement basarat's answer, here's a working declaration:

private ViewClasses: (new (...args: any[]) => UnitView<{}>)[] = [
    TitleView
]

answered Sep 21, 2017 at 0:34

Ryan Cavanaugh

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