JSPM

var HelloWorld = Class.extend(function() { // default name of .extend() can be changed via constant
    this.greeting = 'Hello '; // public property

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

    this.constructor = function(greeting) { // default name of .constructor() can be changed via constant
        if (typeof greeting != 'undefined') {
            this.greeting = greeting;
        }
    };

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

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

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

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

function isNonEmptyString(value) {
    return typeof value == 'string' && value.length > 0;
}

var Base = Class.extend(function() {
    this.name = '';

    this.constructor = function(name) {
        this.name = name;
    };

    // check name
    this.isValid = function() {
        return isNonEmptyString(this.name);
    };
});


var Extended = Base.extend(function(parent) {
    this.address = '';

    this.constructor = function(name, address) {
        parent.constructor.call(this, name); // call parent constructor
        // could be also written as: this.name = name;

        this.address = address;
    };

    // check name and address
    this.isValid = function() {
        return parent.isValid.call(this) && isNonEmptyString(this.address);
        // could be also written as: return isNonEmptyString(this.name) && isNonEmptyString(this.address);
    };
});

var emptyExample = new Extended(null, null);
emptyExample.isValid() // false

var nameExample = new Extended('John', null);
nameExample.isValid() // false

var addressExample = new Extended(null, 'London');
addressExample.isValid() // false

var validExample = new Extended('John', 'London');
validExample.isValid() // true