inheritance.js

class Animal {
  swim() {
    console.log('swimming');
  }
  eat() {
    console.log('eating');
  }
  sleep() {
    console.log('sleeping');
  }
}

class Duck extends Animal {
  quack() {
    console.log('quacking');
  }
  fly() {
    console.log('flying');
  }
}

class Chicken extends Animal {
  cluck() {
    console.log('clucking')
  }
}

var d = new Duck();
d.fly();
d.sleep();

var c = new Chicken();
c.cluck();
c.sleep();