Javascript OOP public and private variable scope

There's no private or public, there's variables and object properties.

Variables and object properties are different in many more ways than the one of variables having a variable scope and object properties not having a variable scope. Variable scope is not the same as private property of an object, because it's not a property but a variable.

Variables do not belong to any object but they can be sustained through closures. You can invoke those closures as a property of any object or without any object at all and the supposed private properties will work:

function A() {
    var private = 0;

    this.setPrivate = function( value ) {
        private = value;    
    };

    this.getPrivate = function() {
        return private;
    };
}

var a = new A();

a.getPrivate() //0;

var b = [];

b.fn = a.setPrivate; //The function is fully promiscuous, especially since the data is closed over by it,
                    //so it doesn't matter at all where or how it's invoked.

b.fn(1);

a.getPrivate(); //1

You are redefining functions in a prototype object every time the constructor is called. The whole point of prototypes is that you only have to create certain function objects just once. You are assigning methods to the prototype object inside a function, so every time that function is called, the functions are recreated and form new closures that refer to specific state.

I showed above that closures, because they hold state in the closed over variables, don't care about how they are invoked. So when you assign a closure as a property to the prototype, all the instances you have refer to the latest closure assigned, and you are getting its state.

I recommend using the standard way of defining "classes" in JS and not mixing it up with closures:

function A() {
    this._private = 1;
}
//Note, this code is outside any function
//The functions assigned to prototype are therefore only defined once.
A.prototype.getPrivate = function() {
    return this._private;
};

A.prototype.setPrivate = function( value ) {
    this._private = value;
};

var a = new A();

You can find a good tutorial here: https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Details_of_the_Object_Model

Actually, I advocate using a non-standard approach to defining javascript classes. The following coding convention makes code easy to read and understand for anyone with an object-oriented background; it is also very easy to maintain unlike the Method.prototype=function(){}; method which sucks anytime you want to rename a class, add more methods, understand the hierarchy of a class or even re-interpret what your own code is doing.

Instead, you can declare object-oriented structures using the following architecture:

/**
* public class Animal
**/
(function(namespace) {
    var __class__ = 'Animal';

    /**
    * private static:
    **/
    var animalCount = 0;

    /**
    * public Animal(string name)
    **/
    var constructor = function(name) {

        // here you can assert arguments are correct
        if(arguments.length == 0) {
            return global.error('needs a name');
        }

        /**
        * private:
        **/
        var animalIndex = animalCount++;

        /**
        * public:
        **/
        var operator = {
            speak: function() {
                console.log('?');
            },
            getName: function() {
                return name;
            },
            getAnimalIndex: function() {
                return animalIndex;
            },
        };

        return operator;
    };

    /**
    * public static Animal()
    **/
    var global = namespace[__class__] = function() {
        // new Animal();
        if(this !== namespace) {
            // construct a new instance of this class
            instance = constructor.apply(this, arguments);
            return instance;
        }
        // Animal();
        else {
            // return the last instantiation of this class
            return instance; // or do whatever you want
        }
    };

    /**
    * public static:
    **/
    // overrides the default toString method to describe this class from a static context
    global.toString = function() {
        return __class__+'()';
    };

    // prints a message to the console's error log
    global.error = function() {
        var args = Array.prototype.slice.apply(arguments);
        args.unshift(__class__+':');
        console.error.apply(console, args);
    };
})(window);

/**
* publc class Dog extends Animal
**/
(function(namespace) {
    var __class__ = 'Dog';

    /**
    * private static:
    **/
    var dogCount = 0;

    /**
    * public Dog()
    **/
    var construct = function(name) {

        /**
        * private:
        **/
        var dogIndex = dogCount++;

        /**
        * public operator() ();
        **/
        var operator = new Animal(name);

        /**
        * public:
        **/

        // overrides parent method 'speak'
        operator.speak = function() {
            console.log(operator.getName()+': bark!');
        };

        // method returns value of private variable
        operator.getSpeciesIndex = function() {
            return dogIndex;
        };

        return operator;
    };

    /**
    * public static Dog()
    **/
    var global = namespace[__class__] = function() {

        // new Dog();
        if(this !== namespace) {
            // construct a new instance of this class
            instance = construct.apply(this, arguments);
            return instance;
        }

        // Dog();
        else {
            // return the last instantiation of this class
            return instance; // or do whatever you want
        }
    };
})(window);


/**
* publc class Cat extends Animal
**/
(function(namespace) {
    var __class__ = 'Cat';

    /**
    * private static:
    **/
    var catCount = 0;

    /**
    * public Cat()
    **/
    var construct = function(name) {

        // here you can assert arguments are correct
        if(arguments.length == 0) {
            return global.error('needs a name');
        }

        /**
        * private:
        **/
        var catIndex = catCount++;

        /**
        * public operator() ();
        **/
        var operator = new Animal(name);

        /**
        * public:
        **/

        // overrides parent method 'speak'
        operator.speak = function() {
            console.log(name+': meow!');
        };

        // method returns value of private variable
        operator.getSpeciesIndex = function() {
            return catIndex;
        };

        return operator;
    };

    /**
    * public static Cat()
    **/
    var global = namespace[__class__] = function() {

        // new Cat();
        if(this !== namespace) {
            // construct a new instance of this class
            instance = construct.apply(this, arguments);
            return instance;
        }

        // Cat();
        else {
            // return the last instantiation of this class
            return instance; // or do whatever you want
        }
    };
})(window);

Now with the above classes declared: Animal, Dog extends Animal, and Cat extends Animal... We get the following:

new Dog(); // prints: "Animal: needs a name" to error output

var buddy = new Dog('Buddy');
buddy.speak(); // prints: "Buddy: bark!"

var kitty = new Cat('Kitty');
kitty.speak(); // prints: "Kitty: meow!"

var oliver = new Dog('Oliver');
oliver.speak(); // prints: "Oliver: bark!"


buddy.getSpeciesIndex(); // returns 0;
buddy.getAnimalIndex(); // returns 0;

kitty.getSpeciesIndex(); // returns 0;
kitty.getAnimalIndex(); // returns 1;

oliver.getSpeciesIndex(); // returns 1;
oliver.getAnimalIndex(); // returns 2;

I provide this javascript coding convention solely as a means to maintain organized object-oriented structures. I do not boast the performance of such coding style over other conventions, but if you want performance from your code I strongly suggest using Google's Closure Compiler which will optimize the same.

I have derived this javascript coding style from many years of coding experience on my own and the assimilation of critiquing other's code. I swear by it's robustness and modularity and welcome any comments regarding otherwise.

You goofed. Constructors should not change the prototype. Either:

function subObject(oName)
{
    var myName = oName;
    this.myName = "My Secret SubName!";

}

subObject.prototype.revealName  = function()
{
    console.info("My Property Name is: " + this.myName);
    console.info("OK, my real name is: " + myName + ", yeah and we also say: " + global);
}

Or:

function subObject(oName)
{
    var myName = oName;
    this.myName = "My Secret SubName!";

    subObject.revealName  = function()
    {
        console.info("My Property Name is: " + this.myName);
        console.info("OK, my real name is: " + myName + ", yeah and we also say: " + global);
    }
}

Blake's answer inspired me, but I found it not doing everything that I wanted, so I hacked away at it until I have something that covers most of the OOP features of C++ in a simple and elegant syntax.

The only things not supported at the moment (but it's a matter of implementing them):

• multiple inheritance
• pure virtual functions
• friend classes

See the github repo for examples and a serious readme:

• https://github.com/najamelan/TidBits_Javascript_OoJs