Read further in prototypes and saw ES6 classes. A lot of syntactic sugar but the concepts are identical.

class.js

class Dog {
  constructor(name, age) {
    this.name = name
    this.age = age
  }

  fetch(obj) {
    return `Yo ${this.name}, go fetch the ${obj}`
  }
}

class Breed extends Dog {
  constructor(name, age, breedName, color, mixed) {
    super(name, age)

    this.breedName = breedName
    this.color = color
    this.mixed = Boolean(mixed)
  }

  description() {
    return `${this.name} is a ${this.age} year old ${this.color} ${this.breedName}`
  }

  mixedBreed() {
    return this.mixed
  }

  static breedOf() {
    return 'This is a class that inherits from Dog. This is a class method and cannot be used on instantiated Dog/Breeds(new Dog / new Breed)'
  }
}

const boringDog = new Dog('Bob', 2)
console.log(boringDog); // Dog { name: 'Bob', age: 2 }
console.log(boringDog.fetch('stick')); // Yo Bob, go fetch the stick.
console.log(boringDog.description()); // Won't work, methods that are scoped in the subclass(Breed < Dog) will only work with instances of 'Breed'.

const ava = new Breed('Ava', 8, 'Border Collie', 'Black/White', false)
console.log(ava); // Breed { name: 'Ava', age: 8, breedName: 'Border Collie', color: 'Black/White', mixed: false }
console.log(ava.fetch('newspaper')); // Yo Ava, go fetch the newspaper.
console.log(ava.description()); // Ava is a 8 year old Black/White Border Collie.
console.log(ava.constructor.name); // Breed
console.log(ava.mixedBreed()); // False
console.log(Breed.breedOf()); // This is a class that inherits from Dog. This is a class method and cannot be used on instantiated Dog/Breeds(new Dog / new Breed)
console.log(Dog.breedOf()); // Won't work, will only work on the scoped class.