JavaScript prototypal inheritance

gistfile1.js

function Shape(x, y) {
    this.x = x;
    this.y = y;
}

Shape.prototype.toString = function() {
    return 'Shape at ' + this.x + ', ' + this.y;
};

function Circle(x, y, r) {
    Shape.call(this, x, y); // invoke the base class's constructor function to take co-ords
    this.r = r;
}

Circle.prototype = new Shape();

Circle.prototype.toString = function() {
    return 'Circular ' + Shape.prototype.toString.call(this) + ' with radius ' + this.r;
}

var c = new Circle(1, 2, 3);
console.log(c.toString()); // "Circular Shape at 1, 2 with radius 3"