medium-6.js

class A {
    foo() {return 'foo in A';}
}
class B {
    foo() {return 'foo in B';}
}

class C extends A {
    foo() {
        console.log(super.foo()); // foo in A
        Object.setPrototypeOf(C.prototype, B.prototype);
        console.log(super.foo()); // foo in B
    }
}

c = new C;
c.foo();