freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/object-oriented-programming/override-inherited-methods.md

id, title, challengeType, forumTopicId

id	title	challengeType	forumTopicId
587d7db1367417b2b2512b88	Override Inherited Methods	1	301322

Description

In previous lessons, you learned that an object can inherit its behavior (methods) from another object by referencing its prototype object:

ChildObject.prototype = Object.create(ParentObject.prototype);

Then the ChildObject received its own methods by chaining them onto its prototype:

ChildObject.prototype.methodName = function() {...};

It's possible to override an inherited method. It's done the same way - by adding a method to ChildObject.prototype using the same method name as the one to override. Here's an example of Bird overriding the eat() method inherited from Animal:

function Animal() { }
Animal.prototype.eat = function() {
  return "nom nom nom";
};
function Bird() { }

// Inherit all methods from Animal
Bird.prototype = Object.create(Animal.prototype);

// Bird.eat() overrides Animal.eat()
Bird.prototype.eat = function() {
  return "peck peck peck";
};

If you have an instance let duck = new Bird(); and you call duck.eat(), this is how JavaScript looks for the method on duck’s prototype chain:

1. duck => Is eat() defined here? No.
2. Bird => Is eat() defined here? => Yes. Execute it and stop searching.
3. Animal => eat() is also defined, but JavaScript stopped searching before reaching this level.
4. Object => JavaScript stopped searching before reaching this level.

Instructions

Override the fly() method for Penguin so that it returns "Alas, this is a flightless bird."

Tests

tests:
  - text: <code>penguin.fly()</code> should return the string "Alas, this is a flightless bird."
    testString: assert(penguin.fly() === "Alas, this is a flightless bird.");
  - text: The <code>bird.fly()</code> method should return "I am flying!"
    testString: assert((new Bird()).fly() === "I am flying!");

Challenge Seed

function Bird() { }

Bird.prototype.fly = function() { return "I am flying!"; };

function Penguin() { }
Penguin.prototype = Object.create(Bird.prototype);
Penguin.prototype.constructor = Penguin;

// Only change code below this line



// Only change code above this line

let penguin = new Penguin();
console.log(penguin.fly());

Solution

function Bird() { }

Bird.prototype.fly = function() { return "I am flying!"; };

function Penguin() { }
Penguin.prototype = Object.create(Bird.prototype);
Penguin.prototype.constructor = Penguin;
Penguin.prototype.fly = () => 'Alas, this is a flightless bird.';
let penguin = new Penguin();
console.log(penguin.fly());