What is the reason I am unable to asign a reference to a subclass to a variable when the reference type is a superclass with Generics?

What is the reason behind not being able to assign a reference to a derived class to a variable if the reference type is a parent class with Generics?

interface IModel {
    id?: number;
}

class Model<T extends IModel> {
    public data: T;
    constructor(data: T) {
        this.data = data;
    }
}

interface IConcretModel extends IModel {
    name: string;
}

class ConcretModel extends Model<IConcretModel> {
    static sm1() { }
}

let a: typeof Model;

a = ConcretModel;

An error occurs on the last line:

Type 'typeof ConcretModel' is not assignable to type 'typeof Model'.
  Types of parameters 'data' and 'data' are incompatible.
    Type 'T' is not assignable to type 'IConcretModel'.
      Type 'IModel' is not assignable to type 'IConcretModel'.
        Property 'name' is missing in type 'IModel'.

Answer №1

When you specify a variable as typeof Model, the class will remain generic. For instance, you could use it in this way:

let a: typeof Model;
new a<IConcretModel>(null)

You cannot assign a non-generic class to the variable using the typeof syntax because it does not support specifying the generic parameter for the class. Therefore, writing typeof Model<IModel> is invalid.

To work around this limitation, you can use a constructor signature instead:

let a: new (data: IModel) => Model<IModel>;

a = ConcretModel;

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