old_contents: function something()
  {
  
    var x = window.localStorage.getItem('bbb'); // x = hh['bbb']
  
    x = x * 1 + 1; // x = x + 1
  
    window.localStorage.setItem('bbb', x); // hh['bbb'] = x
  
    alert(x);
  }
  
  function add_to_cart(id)
  {
    var key = 'product_' + id;
    
    var x = window.localStorage.getItem(key);
    x = x * 1 + 1;
    window.localStorage.setItem(key, x);
  }
  
new_contents: function something()
  {
  
    var x = window.localStorage.getItem('bbb'); // x = hh['bbb']
  
    x = x * 1 + 1; // x = x + 1
  
    window.localStorage.setItem('bbb', x); // hh['bbb'] = x
  
    alert(x);
  }
  
  function add_to_cart(id)
  {
    var key = 'product_' + id;
  
    var x = window.localStorage.getItem(key);
    x = x * 1 + 1;
    window.localStorage.setItem(key, x);
    alert ('Items in your cart: ' + cart_get_number_of_items());
  }
  
  function cart_get_number_of_items()
  {
    var cnt = 0;
  
    for(var i = 0; i < window.localStorage.length; i++)
    {
      var key = window.localStorage.key(i);
      var value = window.localStorage.getItem(key);
  
      if (key.indexOf('product_') == 0)
      {
        cnt = cnt + value * 1;
      }
    }
    return cnt;
  }
  
subject: Add cart_get_number_of_items function