medium-3.js

class Base {
    foo() {return 'foo in Base';}
    bar() {return 'bar in Base';}
}
class Child extends Base {
    foo() {return 'foo in Child';}
    whiz() {return 'whiz in Child';}
}

const b = new Base;
const c = new Child;

console.log(b.foo()); // foo in Base
console.log(b.bar()); // bar in Base
console.log(c.foo()); // foo in Child
console.log(c.bar()); // bar in Base
console.log(c.whiz()); // whiz in Child