primes.js

require(['prime/prime'], function(prime){
    var Rectangle = prime({

        constructor: function(width, height){
            return this.setwidth(width).setHeight(height);
        },

        setWidth: function(width){
            this.width = width;
            return this; // allow chaining
        },

        setHeight: function(height){
            this.height = height;
            return this;
        },

        squareRoot: function(){
            return this.height * this.width;
        }

    });

    var Square = prime({

        // subclass of Rectangle
        extend: Rectangle,

        constructor: function(side){
            return this.setSide(side);
        },

        setSide: function(side){
            // both sides are the same
            this.width = this.height = side;
            return this;
        },

        setWidth: function(width){
            return this.setSide(width);
        },

        setHeight: function(height){
            return this.setSide(height);
        }

    });

    var square = new Square(30);
    square.setWidth(5); // local
    console.log(square.height); // 5
    console.log(square.squareRoot()); // from parent proto of Rectangle, 25
    console.log(square instanceof Rectangle); // true
});