freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/object-oriented-programming/add-methods-after-inheritance.english.md

id, title, challengeType, forumTopicId

id	title	challengeType	forumTopicId
587d7db1367417b2b2512b87	Add Methods After Inheritance	1	301315

Description

A constructor function that inherits its prototype object from a supertype constructor function can still have its own methods in addition to inherited methods. For example, Bird is a constructor that inherits its prototype from Animal:

function Animal() { }
Animal.prototype.eat = function() {
  console.log("nom nom nom");
};
function Bird() { }
Bird.prototype = Object.create(Animal.prototype);
Bird.prototype.constructor = Bird;

In addition to what is inherited from Animal, you want to add behavior that is unique to Bird objects. Here, Bird will get a fly() function. Functions are added to Bird's prototype the same way as any constructor function:

Bird.prototype.fly = function() {
  console.log("I'm flying!");
};

Now instances of Bird will have both eat() and fly() methods:

let duck = new Bird();
duck.eat(); // prints "nom nom nom"
duck.fly(); // prints "I'm flying!"

Instructions

Add all necessary code so the Dog object inherits from Animal and the Dog's prototype constructor is set to Dog. Then add a bark() method to the Dog object so that beagle can both eat() and bark(). The bark() method should print "Woof!" to the console.

Tests

tests:
  - text: <code>Animal</code> should not respond to the <code>bark()</code> method.
    testString: assert(typeof Animal.prototype.bark == "undefined");
  - text: <code>Dog</code> should inherit the <code>eat()</code> method from <code>Animal</code>.
    testString: assert(typeof Dog.prototype.eat == "function");
  - text: <code>Dog</code> should have the <code>bark()</code> method as an <code>own</code> property.
    testString: assert(Dog.prototype.hasOwnProperty('bark'));
  - text: <code>beagle</code> should be an <code>instanceof</code> <code>Animal</code>.
    testString: assert(beagle instanceof Animal);
  - text: The constructor for <code>beagle</code> should be set to <code>Dog</code>.
    testString: assert(beagle.constructor === Dog);
  - text: <code>beagle.eat()</code> should log <code>"nom nom nom"</code>
    testString: |
      console.log = function(msg){throw msg;}
      assert.throws(() => beagle.eat(),"nom nom nom");
  - text: <code>beagle.bark()</code> should log <code>"Woof!"</code>
    testString: |
      console.log = function(msg){throw msg;}
      assert.throws(() => beagle.bark(),"Woof!");

Challenge Seed

function Animal() { }
Animal.prototype.eat = function() { console.log("nom nom nom"); };

function Dog() { }

// Only change code below this line




// Only change code above this line

let beagle = new Dog();

Solution

function Animal() { }
Animal.prototype.eat = function() { console.log("nom nom nom"); };

function Dog() { }
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;
Dog.prototype.bark = function () {
  console.log('Woof!');
};
let beagle = new Dog();

beagle.eat();
beagle.bark();