JavaScript functional inheritance

gistfile1.js

function Shape(x, y) {
    var that = this;

    this.x = x;
    this.y = y;

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

function Circle(x, y, r) {
    var that = this;

    Shape.call(this, x, y);
    this.r = r;

    var _baseToString = this.toString;
    this.toString = function() {
        return 'Circular ' +  _baseToString(that) + ' with radius ' + that.r;
    };
};

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

Taken from http://stackoverflow.com/questions/1595611/how-to-properly-create-a-custom-object-in-javascript/1598077#1598077