Sharing Private Data Among Subclasses

An object is an encapsulation of data (properties) and functions (methods). They are called public data members, because they are accessible in any instance of the same class. JavaScript also allows you to keep local variables within an object. They are called private data members, because they are not directly accessible. The following function has two methods (setArea() and getArea()) and one local variable (area):

function Shape() {
  var area = 50;
  this.setArea = function(a) {area = a;};
  this.getArea = function() {return area;};
}

The setArea() method sets the local variables, while the getArea() method returns it. But keeping a local variable within an object may get very tricky when you inherit the class to two other classes. Once you set the local variable area by one subclass, it is also set for the other subclass. The following example has two constructors: Shape() and Square(), where Square()'s prototype is Shape(). We then define two instances of the Square class, shape1 and shape2. When shape1 sets the value of area to 100, shape2 will see the new area as well. Here is the code:

function Shape() {
  var area = 50;
  this.setArea = function(a) {area = a;};
  this.getArea = function() {return area;};
}

function Square() {
}

Square.prototype = new Shape();

var shape1 = new Square();
var shape2 = new Square();

shape1.setArea(100);

Now examine what shape2 is getting: the 100 that shape1 stored before.

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