There are two ways to call a constructor from within a constructor. One way is to use the call() method of the superclass constructor. Here is the superclass Shape() and the subclass SquareB():

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

function SquareB() {
  Shape.call(this);
}

After we define shape1B and shape2B, and set shpae1B's area to 100, shape2B's area is not affected (stays at 50). Here is the code:

var shape1B = new SquareB();
var shape2B = new SquareB();

shape1B.setArea(100);

Try it. The second way to call a constructor from within a constructor is by defining the superclass constructor as a method of the subclass constructor. Here is the superclass Shape() and the subclass SquareA():

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

function SquareA() {
  this.Shape = Shape;
  this.Shape();
}

After we define shape1A and shape2A, and set shape1A's area to 100, shape2A's area is not affected (stays at 50). Here is the code:

var shape1A = new SquareA();
var shape2A = new SquareA();

shape1A.setArea(100);

Try it

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