typescript factory pattern with generics

Question

I have a number of models that will be acted upon by their related controller. However there is a bit of boilerplate I would like to factor out of each controller and I thought I might be able to use a "base controller" with generics.

Hopefully example below shows what I am trying to accomplish - instantiate each controller with a factory method which will result in an instance of the controller being returned containing its specific model instantiated with its defaults.

The challenge specifically is that I can't seem to get past the

'X' only refers to a type, but is being used as a value here

error in the factory. The abstract class model seems to be the right approach, but open to all suggestions. Playground

abstract class ModelA {
    propA: string;
    propB?: number;
    propC?: number;
    constructor() {
        this.propA = "here"
    }
}

interface ModelB {
    propA: string,
    propB: number,
    propC: number,
}

class BaseController<X> {

    private _model: X

    constructor(payload: X) {
        this._model = payload
    }

    public factory():BaseController<X>
    {
        let model:X;
        model = new X();
        return new BaseController<X>(model);
    }
}

class ModelAController extends BaseController<ModelA> {
    constructor(payload: ModelA) {
        super(payload);
    }
}

let x = ModelAController.factory();

captain-yossarian from Ukraine · Accepted Answer · 2022-06-16 07:04:11Z

You want to call factory as a static method, whereas this is not static.

In order to make it work, you need:

to make factory static.

class BaseController<X> {

    private _model: X

    constructor(payload: X) {
        this._model = payload
    }

    static factory() // <------- SHOULD BE STATIC
    {
        let model:X;
        model = new X();
        return new BaseController<X>(model);
    }
}

Change this logic:

        let model:X;
        model = new X();

X is a generic, it is not a runtime value. It is not allowed to use it as a runtime value;

Consider this example:

abstract class ModelA {
    propA: string;
    propB?: number;
    propC?: number;
    constructor() {
        this.propA = "here"
    }
}

interface ModelB {
    propA: string,
    propB: number,
    propC: number,
}

class BaseController<X> {

    private _model: X

    constructor(payload: X) {
        this._model = payload
    }

    static factory<Model extends new () => any>(Model: Model) {
        return new BaseController(new Model());
    }
}

class ModelAController extends BaseController<ModelA> {
    constructor(payload: ModelA) {
        super(payload);
    }
}

class AnyClass {
    hello = 'Hello'
}

let x = ModelAController.factory(AnyClass);

I'm not sure if this example works for you, but at least this compiles.

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