Typing function factory taking dynamic class types varying constructor arguments.

[prasannavl]

Simply put, I'm trying to achieve a mixin like functionality.
The pattern I'm using is this:

Breaking it down into javascript:

const Factory = function factory(X) {
    return class extends X implements IService {
        private _service: IService;
        constructor(service: IService) {
            super(service);
            this._service= service;
        }
    }
}

Typing it in typescript I ended up with this:

const Factory = function factory<T>(X: (FunctionConstructor & (new(service: IService<T>) => any))) {
    return class extends X<T> implements IService<T> {
        private _service: IService<string>;
        constructor(service: IService<string>) {
            super(service);
            this._service= service;
        }
    }
}

But I'm still unable to get it to work. FunctionConstructor seems to do the trick the if constructor has no arguments. Even using any doesn't help. Is there a way to achieve this? Or is this a bug?

[mhegazy]

the issue here is that you can not extend a generic type parameter. this is tracked by #2225.

if i am not wrong, you want to define factory as:

const Factory = function factory<T>(X: { new (): T }) {
    return class extends X {

    }
}

[mhegazy]

having said that, you can work around this by casting to any at the extend point, and casting to the intersection type at the output:

interface IService<T> {
    service: T;
}
class MyClass { o: any }

const Factory = function factory<T>(X: { new (): T }): { new (): (T & IService<T>) } {
    var base: { new (): Object } = X;
    var C = class extends base implements IService<T> {
        service: T
    };
    return <any>C;
}


var x = new (Factory(MyClass))();
x.o;
x.service; // IService<MyClass>

[mhegazy]

closing in favor of #2225