// This example is from the book _JavaScript: The Definitive Guide_.    
// Written by David Flanagan.  Copyright (c) 1996 O'Reilly & Associates.
// This example is provided WITHOUT WARRANTY either expressed or implied.
// You may study, use, modify, and distribute it for any purpose.        

// Define a constructor method for our class.
// Use it to initialize properties that will be different for
// each individual circle object.
function Circle(x, y, r) 
{
    this.x =index.html x;  // the X coordinate of the center of the  circle
    this.y = y;  // the Y coordinate of the center of the circle
    this.r = r;  // the radius of the circle
}

// Create and discard an initial Circle object.
// Doing this forces the prototype object to be created
new Circle(0,0,0);

// Now define a constant; a property that will be shared by
// all circle objects. Actually, we could just use Math.PI,
// but we do it this way for the sake of example.
Circle.prototype.pi = 3.14159;

// Now define some functions that perform computations on circles
// Note the use of the constant defined above
function Circle_circumference() { return 2 * this.pi * this.r; }
function Circle_area() { return this.pi * this.r * this.r; }

// Make these functions into methods of all Circle objects by
// setting them as properties of the prototype object.
Circle.prototype.circumference = Circle_circumference;
Circle.prototype.area = Circle_area;

// Now define a default property. Most Circle objects will share this 
// default value, but some may override it by setting creating their 
// own unshared copy of the property.
Circle.prototype.url = "images/default_circle.gif";

// Now, create a circle object, and use the methods defined
// by the prototype object
c = new Circle(0.0, 0.0, 1.0);
a = c.area();
p = c.circumference();