How to Create Protected Object Properties in JavaScript

Question

Is there a JavaScript pattern which mimics \"Protected\" object properties like what you see in languages like C++ ??

Basically, I\'d like to create an Object A whic

Answer 1

I was interested to find a way to answer your question, and here's what I was able to do.

You'll need this helper:

var ProtectedHandler = (function () {
    /// 
    /// Tool to handle the protected members of each inheritance.
    /// 
    /// Current protected variable.
    /// The arguments variable of the object.
    /// The function to initialise the variable in the 'object'.
    /// Is this the ultimate base object.
    function ProtectedHandler(current, args, callback, isParent) {
        this.child = getChild(args);
        if (callback)
            this.callback = callback;

        if (isParent)
            this.overrideChild(current);
    }

    // Get the ProtectedHandler from the arguments
    var getChild = function (args) {
        var child = null;
        if (args.length > 0 && (child = args[args.length - 1]) && child.constructor === ProtectedHandler)
            return child;
    };

    // Chain Initialise the protected variable of the object and its inheritances.
    ProtectedHandler.prototype.overrideChild = function (newValue) {
        if (this.callback != null) {
            this.callback(newValue);
        }
        if (this.child != null) {
            this.child.overrideChild(newValue);
        }
    };

    // Static function to create a new instance of the protectedHandler object.
    ProtectedHandler.handle = function (protected, arguments, callback, isParent) {
        return new ProtectedHandler(protected, arguments, callback, isParent);
    };

    return ProtectedHandler;
})();

This helper will allow you to handle multiple inheritances. The trick is to copy the protected variable from the base object to your new object (child).

To prove you it's working, here's an example:

// That's the default extends function from typescript (ref: http://www.typescriptlang.org/)
var __extends = this.__extends || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    __.prototype = b.prototype;
    d.prototype = new __();
};

var BaseClass = (function () {        
    function BaseClass() {
        // Members
        var private = {},
            protected = {},
            public = this;

        // Constructor
        ProtectedHandler.handle(protected, arguments, function () {
            protected.type = "BaseClass";
        }, true);

        // Methods
        protected.saySomething = function () {
            return "Hello World";
        };

        public.getType = function () {
            return protected.type;
        };
    }

    return BaseClass;
})();



var Person = (function (_super) {
    __extends(Person, _super);

    function Person(name) {
        // Members
        var private = {},
            protected = {},
            public;

        // Constructor
        _super.call(public = this, ProtectedHandler.handle(protected, arguments, function (p) {
            protected = p; //This is required to copy the object from its base object.
            protected.name = name;
            protected.type = "Person";
        }));

        //Method
        public.getName = function () {
            return protected.name;
        };

        public.saySomething = function () {
            return protected.saySomething();
        };
    }
    return Person;
})(BaseClass);


var Child = (function (_super) {
    __extends(Child, _super);

    function Child(name) {
        // Members
        var private = {},
            protected = {},
            public;

        // Constructor
        _super.call(public = this, name, ProtectedHandler.handle(protected, arguments, function (p) {
            protected = p; //This is required to copy the object from its base object.
            protected.type = "Child";
        }));

        //Method
        public.setName = function (value) {
            return protected.name = value;
        };
    }
    return Child;
})(Person);

And here's the tests:

var testBase = new BaseClass();
testBase.getType(); //"BaseClass"
testBase.saySomething; //undefined

var testPerson = new Person("Nic");
testPerson.getType(); //"Person"
testPerson.saySomething(); //"Hello World"
testPerson.name; //undefined
testPerson.getName() //"Nic"
testPerson.setName; //undefined

var testChild = new Child("Bob");
testChild.getType(); //"Child"
testChild.saySomething(); //"Hello World"
testChild.name; //undefined
testChild.getName(); //"Bob"
testChild.setName("George");
testChild.getName(); //"George"