professional_javascript_for_web/ch22/ScopeSafeConstructorsExample03.htm

<!DOCTYPE html>
<html>
<head>
    <title>Scope-Safe Constructors Example</title>
</head>
<body>
    <script type="text/javascript">
            
        function Polygon(sides){
            if (this instanceof Polygon) {
                this.sides = sides;
                this.getArea = function(){
                    return 0;
                };
            } else {
                return new Polygon(sides);
            }
        }
        
        function Rectangle(width, height){
            Polygon.call(this, 2);
            this.width = width;
            this.height = height;
            this.getArea = function(){
                return this.width * this.height;
            };
        }
        
        var rect = new Rectangle(5, 10);
        alert(rect.sides);   //undefined



    </script>
</body>
</html>