Question

How can I go about making a child class override a privileged method of a base class?

If its not possible, is there another way to achieve what I am trying to accomplish in the simple code example below?

I cannot convert the baseclass function parseXML() to public because it requires access to private variables

    function BaseClass()
    {
        var map = {};

        // I cannot make this function public BECAUSE it accesses & changes private variables
        this.parseXML = function( key, value )
        {
            alert("BaseClass::parseXML()");
            map[key] = value;
        }
    }

    function ChildClass()
    {
        BaseClass.call(this);
        this.parseXML = function( key, value, otherData )
        {
            alert("ChildClass()::parseXML()");

            // How can I call the base class function parseXML()?
            //this.parseXML();  // calls this function not the parent function
            //MyClass.prototype.doStuff.call
            BaseClass.prototype.parseXML.call(this, key, value);  // fails
            //BaseClass.prototype.parseXML(); // fails

            // perform specialised actions here with otherData
        }
    }

    ChildClass.prototype = new BaseClass;

    var a = new ChildClass();
    a.parseXML();

Answer 1

function BaseClass() {
    var map = {};
    this.parseXML = function(key, value) {
        alert("BaseClass::parseXML()");
        map[key] = value;
    }
}

function ChildClass() {
    BaseClass.call(this);
    var parseXML = this.parseXML;
    this.parseXML = function(key, value, otherData) {
        alert("ChildClass()::parseXML()");
        parseXML.call(this, key, value);
    }
}

ChildClass.prototype = new BaseClass;

var a = new ChildClass();
a.parseXML();

Live Example

Basically you cache the privileged method (which is only defined on the object) and then call it inside the new function you assign to the privileged method name.

However a more elegant solution would be:

function BaseClass() {
    this._map = {};
};

BaseClass.prototype.parseXML = function(key, value) {
    alert("BaseClass::parseXML()");
    this._map[key] = value;
}

function ChildClass() {
    BaseClass.call(this);
}

ChildClass.prototype = Object.create(BaseClass.prototype);
ChildClass.prototype.parseXML = function(key, value, otherData) {
    alert("ChildClass()::parseXML()");
    BaseClass.prototype.parseXML.call(this, key, value);
}

var a = new ChildClass();
a.parseXML();

Live Example

Also bonus implementation using pd