abstraction.js

class SwimMixin {
   // prevents duplication of implementation;
   // can be a helper/util class as well
   swim(context) {
     // swim implementation
   }
}

class Animal {
   walk() {
     this.throwNotImplemented();
   }
  
   fly() {
     this.throwNotImplemented();
   }

   swim() {
     this.throwNotImplemented();
   }

   throwNotImplemented() {
     throw new Error('Not implemented');
   }
}

class Fish extends Animal {
   swim() {
     swimMixinInstance.swim(this);
   }
}

const fish = new Fish();

fish.swim(); // the fish swims
fish.fly(); // sorry not implemented in this kind of object