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

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

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

class Bird {}
class Fish {}

applyMixins(Bird, Flyable, Swimmable);
applyMixins(Fish, Swimmable);

const eagle = new Bird();
const shark = new Fish();

console.log(eagle.swim());
console.log(shark.fly?.() || 'undefined method');