Public/Private Variables and Methods

* private variables are declared with the 'var' keyword inside the object, and can only be accessed by private functions and privileged methods.
* private functions are declared inline inside the object's constructor (or alternatively may be defined via var functionName=function(){...}) and may only be called by privileged methods (including the object's constructor).
* privileged methods are declared with this.methodName=function(){...} and may invoked by code external to the object.
* public properties are declared with this.variableName and may be read/written from outside the object.
* public methods are defined by Classname.prototype.methodName = function(){...} and may be called from outside the object.
* prototype properties are defined by Classname.prototype.propertyName = someValue
* static properties are defined by Classname.propertyName = someValue

function Person(n,race){ 
	this.constructor.population++;

	// ************************************************************************ 
	// PRIVATE VARIABLES AND FUNCTIONS 
	// ONLY PRIVELEGED METHODS MAY VIEW/EDIT/INVOKE 
	// *********************************************************************** 
	var alive=true, age=1;
	var maxAge=70+Math.round(Math.random()*15)+Math.round(Math.random()*15);
	function makeOlder(){ return alive = (++age <= maxAge) } 

	var myName=n?n:"John Doe";
	var weight=1;


	// ************************************************************************ 
	// PRIVILEGED METHODS 
	// MAY BE INVOKED PUBLICLY AND MAY ACCESS PRIVATE ITEMS 
	// MAY NOT BE CHANGED; MAY BE REPLACED WITH PUBLIC FLAVORS 
	// ************************************************************************ 
	this.toString=this.getName=function(){ return myName } 

	this.eat=function(){ 
		if (makeOlder()){ 
			this.dirtFactor++;
			return weight*=3;
		} else alert(myName+" can't eat, he's dead!");
	} 
	this.exercise=function(){ 
		if (makeOlder()){ 
			this.dirtFactor++;
			return weight/=2;
		} else alert(myName+" can't exercise, he's dead!");
	} 
	this.weigh=function(){ return weight } 
	this.getRace=function(){ return race } 
	this.getAge=function(){ return age } 
	this.muchTimePasses=function(){ age+=50; this.dirtFactor=10; } 


	// ************************************************************************ 
	// PUBLIC PROPERTIES -- ANYONE MAY READ/WRITE 
	// ************************************************************************ 
	this.clothing="nothing/naked";
	this.dirtFactor=0;
} 


// ************************************************************************ 
// PUBLIC METHODS -- ANYONE MAY READ/WRITE 
// ************************************************************************ 
Person.prototype.beCool = function(){ this.clothing="khakis and black shirt" } 
Person.prototype.shower = function(){ this.dirtFactor=2 } 
Person.prototype.showLegs = function(){ alert(this+" has "+this.legs+" legs") } 
Person.prototype.amputate = function(){ this.legs-- } 


// ************************************************************************ 
// PROTOTYOPE PROERTIES -- ANYONE MAY READ/WRITE (but may be overridden) 
// ************************************************************************ 
Person.prototype.legs=2;


// ************************************************************************ 
// STATIC PROPERTIES -- ANYONE MAY READ/WRITE 
// ************************************************************************ 
Person.population = 0;



// Here is the code that uses the Person class 
function RunGavinsLife(){ 
	var gk=new Person("Gavin","caucasian");       //New instance of the Person object created. 
	var lk=new Person("Lisa","caucasian");        //New instance of the Person object created. 
	alert("There are now "+Person.population+" people");

	gk.showLegs(); lk.showLegs();                 //Both share the common 'Person.prototype.legs' variable when looking at 'this.legs' 

	gk.race = "hispanic";                         //Sets a public variable, but does not overwrite private 'race' variable. 
	alert(gk+"'s real race is "+gk.getRace());    //Returns 'caucasian' from private 'race' variable set at create time. 
	gk.eat(); gk.eat(); gk.eat();                 //weight is 3...then 9...then 27 
	alert(gk+" weighs "+gk.weigh()+" pounds and has a dirt factor of "+gk.dirtFactor);

	gk.exercise();                                //weight is now 13.5 
	gk.beCool();                                  //clothing has been update to current fashionable levels 
	gk.clothing="Pimp Outfit";                    //clothing is a public variable that can be updated to any funky value 
	gk.shower();
	alert("Existing shower technology has gotten "+gk+" to a dirt factor of "+gk.dirtFactor);

	gk.muchTimePasses();                          //50 Years Pass 
	Person.prototype.shower=function(){           //Shower technology improves for everyone 
		this.dirtFactor=0;
	} 
	gk.beCool=function(){                         //Gavin alone gets new fashion ideas 
		this.clothing="tinfoil";
	};

	gk.beCool(); gk.shower();
	alert("Fashionable "+gk+" at " 
		+gk.getAge()+" years old is now wearing " 
		+gk.clothing+" with dirt factor " 
		+gk.dirtFactor);

	gk.amputate();                                //Uses the prototype property and makes a public property 
	gk.showLegs(); lk.showLegs();                 //Lisa still has the prototype property 

	gk.muchTimePasses();                          //50 Years Pass...Gavin is now over 100 years old. 
	gk.eat();                                     //Complains about extreme age, death, and inability to eat. 
}

Inheritance

* You cause a class to inherit using ChildClassName.prototype = new ParentClass();.
* You need to remember to reset the constructor property for the class using ChildClassName.prototype.constructor=ChildClassName.
* You can call ancestor class methods which your child class has overridden using the Function.call() method.
* Javascript does not support protected methods.

function Mammal(name){ 
	this.name=name;
	this.offspring=[];
} 
Mammal.prototype.haveABaby=function(){ 
	var newBaby=new Mammal("Baby "+this.name);
	this.offspring.push(newBaby);
	return newBaby;
} 
Mammal.prototype.toString=function(){ 
	return '[Mammal "'+this.name+'"]';
} 


Cat.prototype = new Mammal();        // Here's where the inheritance occurs 
Cat.prototype.constructor=Cat;       // Otherwise instances of Cat would have a constructor of Mammal 
function Cat(name){ 
	this.name=name;
} 
Cat.prototype.toString=function(){ 
	return '[Cat "'+this.name+'"]';
} 


var someAnimal = new Mammal('Mr. Biggles');
var myPet = new Cat('Felix');
alert('someAnimal is '+someAnimal);   // results in 'someAnimal is [Mammal "Mr. Biggles"]' 
alert('myPet is '+myPet);             // results in 'myPet is [Cat "Felix"]' 

myPet.haveABaby();                    // calls a method inherited from Mammal 
alert(myPet.offspring.length);        // shows that the cat has one baby now 
alert(myPet.offspring[0]);            // results in '[Mammal "Baby Felix"]'

Every object instance in JS has a property named constructor that points to its parent class. For example, someAnimal.constructor==Mammmal is true. Armed with this knowledge, we can remake the haveABaby() method like this:

Mammal.prototype.haveABaby=function(){ 
	var newBaby=new this.constructor("Baby "+this.name);
	this.offspring.push(newBaby);
	return newBaby;
} 
...
myPet.haveABaby();                    // Same as before: calls the method inherited from Mammal 
alert(myPet.offspring[0]);            // Now results in '[Cat "Baby Felix"]' 

Calling 'super' methods

Javascript does not have any sort of 'super' property, which would point to its parent class. Instead, you use the call() method of a Function object, which allows you to run a function using a different object as context for it. If you needed to pass parameters to this function, they would go after the 'this'.

Cat.prototype.haveABaby=function(){ 
	Mammal.prototype.haveABaby.call(this);
	alert("mew!");
}

Rather than having to know that Cat inherits from Mammal, and having to type in Mammal.prototype each time you wanted to call an ancestor method, wouldn't it be nice to have your own property of the cat pointing to its ancestor class?

Cat.prototype = new Mammal();
Cat.prototype.constructor=Cat;
Cat.prototype.parent = Mammal.prototype;
...
Cat.prototype.haveABaby=function(){ 
	var theKitten = this.parent.haveABaby.call(this);
	alert("mew!");
	return theKitten;
} 

Spoofing pure virtual classes

Some OOP languages have the concept of a pure virtual class...one which cannot be instantiated itself, but only inherited from. For example, you might have a LivingThing class which Mammal inherited from, but you didn't want someone to be able to make a LivingThing without specifying what type of thing it was. You can do this in JS by making the virtual class an object instead of a function.

LivingThing = { 
	beBorn : function(){ 
		this.alive=true;
	} 
} 
...
Mammal.prototype = LivingThing;
Mammal.prototype.parent = LivingThing;   //Note: not 'LivingThing.prototype' 
Mammal.prototype.haveABaby=function(){ 
	this.parent.beBorn.call(this);
	var newBaby=new this.constructor("Baby "+this.name);
	this.offspring.push(newBaby);
	return newBaby;
} 

With the above, doing something like var spirit = new LivingThing() would result in an error, since LivingThing is not a function, and hence can't be used as a constructor.

Convenient Inheritance

Rather than writing 3 lines every time you want to inherit one class from another, it's convenient to extend the Function object to do it for you:

Function.prototype.inheritsFrom = function( parentClassOrObject ){ 
	if ( parentClassOrObject.constructor == Function ) 
	{ 
		//Normal Inheritance 
		this.prototype = new parentClassOrObject;
		this.prototype.constructor = this;
		this.prototype.parent = parentClassOrObject.prototype;
	} 
	else 
	{ 
		//Pure Virtual Inheritance 
		this.prototype = parentClassOrObject;
		this.prototype.constructor = this;
		this.prototype.parent = parentClassOrObject;
	} 
	return this;
} 
//
//
LivingThing = { 
	beBorn : function(){ 
		this.alive = true;
	} 
} 
//
//
function Mammal(name){ 
	this.name=name;
	this.offspring=[];
} 
Mammal.inheritsFrom( LivingThing );
Mammal.prototype.haveABaby=function(){ 
	this.parent.beBorn.call(this);
	var newBaby = new this.constructor( "Baby " + this.name );
	this.offspring.push(newBaby);
	return newBaby;
} 
//
//
function Cat( name ){ 
	this.name=name;
} 
Cat.inheritsFrom( Mammal );
Cat.prototype.haveABaby=function(){ 
	var theKitten = this.parent.haveABaby.call(this);
	alert("mew!");
	return theKitten;
} 
Cat.prototype.toString=function(){ 
	return '[Cat "'+this.name+'"]';
} 
//
//
var felix = new Cat( "Felix" );
var kitten = felix.haveABaby( ); // mew! 
alert( kitten );                 // [Cat "Baby Felix"] 

Just make sure you call this method immediately after your constructor, before you extend the prototype for the object.

Pattern

    function Constructor(...) {

        this.membername = value;

    }
    Constructor.prototype.membername = value;

A privileged method is able to access the private variables and methods, and is itself accessible to the public methods and the outside. It is possible to delete or replace a privileged method, but it is not possible to alter it, or to force it to give up its secrets.

Privileged methods are assigned with this within the constructor.

    function Container(param) {

        function dec() {
            if (secret > 0) {
                secret -= 1;
                return true;
            } else {
                return false;
            }
        }

        this.member = param;
        var secret = 3;
        var that = this;

        this.service = function () {
            if (dec()) {
                return that.member;
            } else {
                return null;
            }
        };
    }

service is a privileged method. Calling myContainer.service() will return 'abc' the first three times it is called. After that, it will return null. service calls the private dec method which accesses the private secret variable. service is available to other objects and methods, but it does not allow direct access to the private members.

Pattern:

    function Constructor(...) {

        this.membername = function (...) {...};

    }

Container makes three private instance variables: param, secret, and that. They are attached to the object, but they are not accessible to the outside, nor are they accessible to the object's own public methods. They are accessible to private methods. Private methods are inner functions of Container.

    function Container(param) {

        function dec() {
            if (secret > 0) {
                secret -= 1;
                return true;
            } else {
                return false;
            }
        }

        this.member = param;
        var secret = 3;
        var that = this;
    }

By convention, we make a private that parameter. This is used to make the object available to the private methods. This is a workaround for an error in the ECMAScript Language Specification which causes this to be set incorrectly for inner functions.

Private methods cannot be called by public methods. To make private methods useful, we need to introduce a privileged method.

Pattern

    function Constructor(...) {

        var that = this;
        var membername = value;

        function membername(...) {...}

    }

    Note: The function statement

        function membername(...) {...}

    is shorthand for

        var membername = function membername(...) {...};

Some languages like C++ support the concept of static variables, they are local variables that retain their values between function calls, JavaScript doesn't have a static keyword or direct support to this technique, however, the fact that functions are also objects makes simulating this feature possible, the idea is storing the static variable as a property of the function, suppose that we want to create a counter function, here is a code snippet that shows this technique in action:

function count() {
    if (typeof count.i == 'undefined') {
        count.i = 0;
    }
    return count.i++;
}

When count is called for the first time, count.i is undefined, so the if condition is true and count.i is set to 0, because we are storing the variable as a property, it's going to retain its value between function calls, thus it can be considered a static variable.

We can introduce a slight performance improvement to the above function, by removing that if check and initialize count.i after defining the function:

function count() {
    return count.i++;
}
count.i = 0;