typeguards.ts

export abstract class A {

    static make(input: boolean):A {
        return  input ? new B() : new C();
    }

    abstract isB(): this is B;
    abstract isC(): this is C;
    abstract get(): number;
}

export class B extends A {
readonly className: "B"  // add this line

    isB(): this is B {
        return true;
    }

    isC(): this is C {
        return false;
    }

    get() {
        return 5;
    }
}

export class C extends A {
readonly className: "C"  // add this line

    isB(): this is B {
        return false;
    }

    isC(): this is C {
        return true;
    }

    get() {
        return 6;
    }
}

const x = A.make(true); // new C();
if (x.isB()) {
    console.log("B!")
} else {
    console.log(x.get());
}