const Flyable = {
  fly() { return 'flying'; }
};

const Swimmable = {
  swim() { return 'swimming'; }
};

function mixin(Base, ...mixins) {
  mixins.forEach(mixin => Object.assign(Base.prototype, mixin));
  return Base;
}

class Bird {}
class Fish {}

const FlyingFish = mixin(class extends Fish {}, Flyable, Swimmable);
const instance = new FlyingFish();
console.log(instance.swim());
console.log(instance.fly());
console.log(instance instanceof Fish);