JSPM

var HelloWorld = Class.extend(function() {
    this.hello = 'Hello '; // public property

    var world = 'World!'; // private/privileged property

    this.constructor = function() { // constructor method (default name can be changed via constant)
        // do nothing
    };

    this.say = function() { // public method
        return privileged.call(this);
    };

    function privileged() { // private/privileged method
        return this.hello + world;
    }
});

var HelloWorldTwo = HelloWorld.extend(function(parent) {
    this.hello = 'Hi ';

    this.constructor = function() {
        parent.constructor.apply(this, arguments); // call parent contructor
    };
});

var helloWorld = new HelloWorld()
helloWorld.say() // 'Hello World!'
helloWorld instanceof HelloWorld // true

var helloWorldTwo = new HelloWorldTwo()
helloWorldTwo.say() // 'Hi World!'
helloWorldTwo instanceof HelloWorldTwo // true
helloWorldTwo instanceof HelloWorld // true