Javascript: Object-Oriented Programming Basics

Alright, so now we can finally learn the basics of OOP! The first thing I'm going to introduce is a class.    When you make a constructor, you are actually defining a new class! A class can be thought of as a type, or a category of objects -- remember numbers and strings? Those are types too!

/* take a look at the example below, jenny and alex are two separate objects, but they both belong to the class Person */
function Person(name, age) {
    this.name = name;
    this.age = age;
};

var jenny = new Person("Jenny", 18);
var alex = new Person("Alex", 13);

A prototype keeps track of what a given class can or cannot do. Javascript automatically defines the prototype for class with a constructor.

// for example, this dog constructor ensures that the dog prototype has a breed property
function Dog (breed) {
    this.breed = breed;
};

Classes are really useful because they tell us helpful information about objects. You can actually think of an object as a particular instance of a class. Look at our Person class. We know that any Person will have a  name and age because they are in they are in the constructor. Let's create a function like printPersonName--this will take a Person as an argument and print out their name.

// recall our Person class
function Person(name, age) {
    this.name = name;
    this.age = age;
};

// a function that prints out the name of any given person
function printPersonName(p) {
    console.log(p.name);
};

var jenny = new Person("Jenny", 18);
printPersonName(jenny);

If you want to add a method to a class such that all members of the class can use it, we use the following syntax to extend the prototype:

className.prototype.newMethod = 
function() {
statements; 
}; 

// here's another example using the dog class:
function Dog (breed) {
    this.breed = breed;
};

// here we make fifi and teach her how to bark
var fifi = new Dog("pomeranian");
Dog.prototype.bark = function() {
    console.log("Arf!");
};
fifi.bark();

// here we make maya
var maya = new Dog("Pug");
maya.bark();

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