Something

        <script type="text/javascript">            function start() {                function ClassA() {                    var i = 20;                }                function ClassB() {                    var j = 20;                }                                ClassB.prototype = new ClassA();                                alert(ClassB.prototype instanceof ClassA); // false                alert(ClassA.prototype instanceof ClassB); // false                alert(ClassA isntanceof ClassB);  // false                alert(ClassB instanceof ClassA);  // false            }        </script>