medium-5.js

class A {
    foo() {return 'foo in A';}
}
class B extends A {
    foo() {return 'foo in B';}
}
class C {
    foo() {return 'foo in C';}
}
class D extends C {
    foo() {return super.foo();}
}

b = new B;
console.log(b.foo()); // foo in B

B.prototype.foo = D.prototype.foo
console.log(b.foo()); // foo in C
console.log(b instanceof C); // false