Factories and Constructors

both.js

function makeInstance (constructor, context) {
	return context === global || context === undefined ?
		Object.create(constructor.prototype) :
		context;
}

function Rectangle (length, width) {
	var rectangle = makeInstance(Rectangle, this);
	
	rectangle.area = function () {
		return length * width;
	};
	
	return rectangle;
}

function Square (side) {
	var square = makeInstance(Square, this);
	
	Rectangle.call(square, side, side);
	
	return square;
}

Square.prototype = Object.create(Rectangle.prototype);