Typescript Storing reference to a Generic class inside interface then creating an object from the class

Im trying to learn and understand Typescript Generics but I’m stuck with the following situation:

class A {
    helloworld() {
        console.log('hello world a')
    }
}

interface IConfig<T extends typeof A> {
    class: T
}

function testing<T extends typeof A>(config?: IConfig<T>) {
    if (config === undefined) {
        config = {class: A};
    }

    const thisClass = new config.class();
    thisClass.helloworld();
}

testing();

class B extends A {
    helloworld() {
        console.log('hello world b')
    }
}

testing({class: B})

Im getting this from WebStorm Type 'typeof A' is not assignable to type 'T'.   'typeof A' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'typeof A'.

>Solution :

You just need a minor tweak to your function:

function testing<T extends typeof A>(config?: IConfig<T | typeof A>) {
    const conf = config ?? { class: A }
    const thisClass = new conf.class();
    thisClass.helloworld();
}

Playground

You can’t in general have a default assignment with a generic like that, unless you explicitly include the default type along with the generic like I’ve included with IConfig<T | typeof A> otherwise the compiler will correctly complain that you haven’t covered all the possible T’s. Note this holds true even if your T extends your default type: T could still be a more complex superset of A.