const HiMixin = Sup => class extends Sup {
  hi() {
    console.log("Hi!");
  }
}

const ByMixin = Sup => class extends Sup {
  by() {
    console.log("By!");
  }
}

class Person extends HiMixin(ByMixin(Object)) {}

const p = new Person();

p.hi();
p.by();