app.js

// class product
Product = function(product) {
      // instantiation works fine
    var id = Object.getPrototypeOf(this).id++;
    this.id = function() {
        return id;
      };

    var title = product.title;
    this.title = function(productTitle) {
        if(productTitle !== undefined) title = productTitle;
            return title;

    };
    var price = product.price;

    this.price = function(productPrice) {
        if(productPrice !== undefined) price = productPrice;


            price = price.toString();
            // returns $ on single and chain call
            return "$" + price.substr(0, price.length-2).concat('.', price.substr(-2));



        };

    this.formatted_price = function() {
        return "$" + this.price(); // concat also works fine
    };

    this.discount = function() {
         // set price -1$ (100)
         price -= 100;

         // return my obj here
         return this;

    };


};

// static member - shared between object constructor and inherited in instances
Product.prototype.id = 0;

// nethod on class = static method

Product.unique_id = function() {
    return Product.prototype.id++;
};



// class SubProduct
SubProduct.prototype = new Product();
/*
SubProduct.prototype.unique_id = function() {
    return "sub_" + Product.prototype.id++;
};
*/




/* Simple test methods both on instance and static */
/*
console.log("Call 1: " + Product.unique_id() + "// 0");
console.log("Call 2: " + Product.unique_id() + "// 1");
console.log("Call 3: " + Product.unique_id() + "// 2");


console.log(p1.id());

console.log("Call 4: " + Product.unique_id() + "// 4");
*/

 var p1 = new Product({
    title: 'Keyboard',
    price: 10050
});
p1.id();

p1.id();                    // 1
p1.title();                 // 'Keyboard'
p1.title('Cool keyboard');  // 'Cool keyboard'
p1.title();                 // 'Cool keyboard'

p1.price(); // returns $ here too, which is BAD
p1.price(10050);
p1.formatted_price();
p1.discount().price();

var p2 = new SubProduct({
    title: 'Monitor',
    price: 500000
  });

p2.unique_id();