Intro to Object-Oriented JavaScript

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Some "rules":

• We are here for you
• Every question is important
• Help each other
• Have fun!

Tell us about yourself.

• Who are you?
• What do you hope to get out of the class?
• Fun Question Of The Day ...

• What is an object?
• What are attributes and methods?
• What is class-based inheritance?
• What is prototype-based inheritance?
• How do we create objects using inheritance in JavaScript?
• How can we extend a prototype in JavaScript?
• How do we write maintanable code?
• How can we refactor a website's code with Object-Oriented JavaScript?

Goal: Understanding objects

• Create an object with a couple properties. For example, you could make a "student" object with the properties "name" and "hometown".
• Use console.log() to print out a sentence that includes the properties of that object. For example, "[name] is from [hometown]."

Goal: Understanding functions

• Create a function that takes at least one argument. For example, you could make an "announce" function that alerts "Important announcement:" followed by the argument.
• Call your function with an argument of your choice. For example, you could announce "Dogs are cute".

Goal: Understanding "this"

• In HTML, create a link to the website of your choice. Using a JavaScript "onclick" event, make clicking on the link alert the link’s "href". [Hint: never used the "onclick" event before, and not sure how to use it? Check out w3schools.com for some documentation and examples]

In computer science, an object is a location in memory having a value and possibly referenced by an identifier. An object can be a variable, function, or data structure. [source: Wikipedia]

	var carObject = {
	    make: 'Volkswagen',
	    model: 'Rabbit',
	    drive: function() {
	        // Code that makes car go forward
	    }
	};
		

When we talk about objects, we're interested in two main concepts:

• Attributes - object characteristics
• Methods - object functionality

	var carObject = {
	    make: 'Volkswagen',    // attribute
	    model: 'Rabbit',       // attribute
	    drive: function() {    // method
	        // Code that makes car go forward
	    }
	};
		

Think about these geometric shapes:

• What are some attributes shared by all shapes?
• What are some methods that can be applied to all shapes?
• Are there any attributes or methods that are unique to only one shape? ... to some shapes?

Imagine going shopping for a new car.

• You go to the dealer with a concept of a "car" is.
• You find a particular car with qualities you like.
• You like the way it handles and drives.
• You purchase this particular car.

In object-oriented terms, we say that you have purchased an instance of the class of objects known as cars.

Let's imagine another way of shopping for a new car.

• You go to the dealer with an old car you already own, and you ask the dealer for a newer model of the same car.
• The color, make, and model of the car are the same, but the handling is better than your old car.
• You purchase this particular car.

In object-oriented terms, we say that you have purchased an instance of a car based off of a prototype, from which it inherited some attributes and methods.

In JavaScript, we use a constructor function to initialize new objects, and we use the new keyword to call the constuctor.

	// Create a constructor function for new cars
	function Car() {
	    // Set attributes of all new cars
	    this.odometer = 0;
	}

	var myCar = new Car();

	// myCar can access properties we defined in Car
	console.log(myCar.odometer);    // 0
		

	// Create a constructor function for new cars
	function Car(make, model, year) {
	    // Set the given attributes of this object
	    this.make = make;
	    this.model = model;
	    this.year = year;

	    // Set attributes of all new cars, independent of variables
	    this.odometer = 0;
	}

	var myCar = new Car('Volkswagen', 'Rabbit', 2006);

	// myCar can access properties we defined in Car
	console.log(myCar.odometer);    // 0
	console.log(myCar.make);        // Volkswagen
		

The prototype is the collection of all the attributes and methods that the object knows about.

The prototype property of an object is used primarily for inheritance: methods and attributes on a function's prototype property are available to instances of that function.

	function Car() {
	    this.odometer = 0;
	}

	// Extend the prototype to define methods for new Cars
	Car.prototype.drive = function() {
	    this.odometer = this.odometer + 1;
	};

	// Instantiate a new Car
	var myCar = new Car();

	// myCar can access methods defined in the prototype
	console.log(myCar.odometer);    // 0
	myCar.drive();
	console.log(myCar.odometer);    // 1
		

It's common practice for constructor function names to start with an uppercase letter.

It's common practice for regular function names to start with a lowercase letter.

	// capitalized function "Car"? This must be a constructor
	function Car() {
	    this.odometer = 0;
	}

	// lowercase function "drive"? This must be a regular function
	Car.prototype.drive = function() {
	    this.odometer = this.odometer + 1;
	};
		

	// Create a constructor function for new cars
	function Car(make, model, year) {
	    this.make = make;
	    this.model = model;
	    this.year = year;

	    this.odometer = 0;
	}

	// Extend the prototype to define methods for new cars
	Car.prototype.drive = function() {
	    this.odometer = this.odometer + 1;
	};

	// Instantiate a new car
	var myCar = new Car('Volkswagen', 'Rabbit', 2006);
		

Let's make our own constructors and objects! We'll be using an exercise put together by GDI Chicago:

Suppose you wanted an easy way to talk about different types of cars.

• All cars start with a make, model, year, and odometer.
• Sedan-type cars have 4 doors
• Hatchback-type cars can have 3 doors or 5 doors

We can create a constructor function for each sub-type of Car that inherits attributes and methods from Car.

	// Assume Car() is defined as before
	function Hatchback(make, model, year, doors) {
	    // Call the Car constructor on the attributes
	    // Important: "this" is the first argument
	    Car.call(this, make, model, year);

	    // Set Hatchback-specific properties
	    this.doors = doors;
	}

	// Tell the Hatchback to extend the Car object
	Hatchback.prototype = Object.create(Car.prototype);
		

We can also create methods specific to Hatchback.

	function Hatchback(make, model, year, doors) {
	    Car.call(this, make, model, year);
	    this.doors = doors;
	}

	Hatchback.prototype = Object.create(Car.prototype);

	// Now add a Hatchback-specific function
	Hatchback.prototype.openHatch = function() {
	    console.log('You opened the hatch!');
	}

	var myHatch = new Hatchback('Volkswagen', 'Rabbit', 2006, 3);

	myHatch.drive();
	myHatch.openHatch();

		

Substitutability is a principle of object-oriented programming introduced by BarbaraLiskov in a 1987 conference keynote entitled "Data abstraction and hierarchy".

In a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S without altering any of the desirable properties of that program.

Let's extend our Video to make a MusicVideo! We'll be using another exercise put together by GDI Chicago:

• Model physical concepts more clearly in your code
• Make your code easier to read
• Create code that is more easily maintainable and reusable

When creating an object using our Car constructor, we have to pass in our arguments in a very specific order: make, then model, then year.

	var myCar = new Car('Volkswagen', 'Rabbit', 2006);
		

This is annoying and hard to maintain.

There is a better way.

Replace your list of arguments with a single argument.

	// The better way
	function Car(args) {
	    this.make = args.make;
	    this.model = args.model;
	    this.year = args.year;
	}

	// Instantiate your object
	// NOTE: You can now list the arguments in any order!
	var myCar = new Car({
	    model: 'Rabbit',
	    year: 2006,
	    make: 'Volkswagen'
	});
		

You can also provide default values for your arguments within the constructor.

	// Now, with defaults! 
	function Car(args) {
	    args = args || {};
	    this.make = args.make || 'Unknown';
	    this.model = args.model || 'Unknown';
	    this.year = args.year || 0;
	}

	// Instantiate your object
	// NOTE: Now there's a backup if you don't know a value
	var myCar = new Car({
	    make: 'Volkswagen',
	    year: 2006
	});
		

Let's clean up our constructors using what we learned! We'll be using another exercise put together by GDI Chicago:

You can create objects that know how to interact with your website.

In a real-world situation, you would probably create your car objects using data imported from a database!

Once your website code is working correctly, it's a good idea to go back and refactor it. Refactoring is a process developers use to restructure their existing code to make it easier to read.

Although it takes time to refactor your code now, right now is when you're most familiar with what you wrote.

Let's refactor some JavaScript using objects! We'll be using another exercise put together by GDI Chicago:

• Lots of practice!
• Intro to Model-View-Controller (MVC) frameworks
  • Your data goes in the "model"
  • The way to display that data goes in the "view"
  • The model and view talk to each other and your server-side data using the "controller"
• Intro to Backbone, a particular MVC framework that provides an amazing infrastructure for routine tasks like message passing, event handling, and working with multiple models