如何在TypeScript中键入类的函数的返回值作为其类的属性的值？

Question

Provided i have a Class Factory who takes in another class Product (or its subclass) as an argument and has methods that return instantiated Products will a lot of extra done to them, like so:

abstract class Product {}
class Wheel extends Product {}
class Roof extends Product {
    is_asbestos(){ return false } }
type ProductClass = {new (): Product}

class Factory {
    public product_cls: ProductClass;
    constructor(product_cls: ProductClass, private regulation_code?: number) {
        this.product_cls = product_cls;
    }
    build_for_france(){
        return new this.product_cls();
    }
    build_for_britain(){
        return new this.product_cls();
    }
}

let wheel_factory = new Factory(Wheel);
let roof_factory = new Factory(Roof, 123);
let new_roof = roof_factory.build_for_france();

new_roof.is_asbestos();  // error

Calling the is_asbestos method gives me this error.

Property 'is_asbestos' does not exist on type 'Product'.

So, how do i tell TypeScript that the return value of that function is an instance of the value of product_cls , which is a class.

Now, i can do the below way and it works, but doing that at every function call is silly.

let new_roof = <Roof>roof_factory.build_for_france();
new_roof.is_asbestos();

Doing the following gives me a syntax error.

build_for_france(){
    let french_product: this.product_cls = new this.product_cls();
    return french_product;
}

Finally, I'm using Typescript 2.0.10. I do not mind upgrading the TypeScript version.

Answer 1

You must use generics to specify which type your factory constructs.

abstract class Product {}
class Wheel extends Product {}
class Roof extends Product {
    is_asbestos(){ return false } }
type ProductClass<T> = {new (): T}

class Factory<T extends Product> {
    public product_cls: ProductClass<T>;
    constructor(product_cls: ProductClass<T>, private regulation_code?: number) {
        this.product_cls = product_cls;
    }
    build_for_france(): T {
        return new this.product_cls();
    }
    build_for_britain(): T {
        return new this.product_cls();
    }
}

let wheel_factory = new Factory<Wheel>(Wheel);
let roof_factory = new Factory<Roof>(Roof, 123);
let new_roof = roof_factory.build_for_france();

new_roof.is_asbestos();  // ok