renumbering ordered session variables when deleting one - ajax

Question

I'm updating a classic ASP application, written in jScript, for a local pita restaurant. I've created a new mobile-specific version of their desktop site, which allows ordering for delivery and lots of customization of the final pita (imagine a website for Subway, which would allow you to add pickles, lettuce, etc.). Each pita is stored as a string of numbers in a session variable. The total number of pitas is also stored. The session might look like this:
PitaCount = 3
MyPita1 = "35,23,16,231,12"
MyPita2 = "24,23,111,52,12,23,93"
MyPita3 = "115,24"
I know there may be better ways to store the data, but for now, since the whole thing is written, working , and live (and the client is happy), I'd like to just solve the problem I have. Here's the problem...
I've got buttons on the order recap page which allow the customer to delete pitas from the cart. When I do this, I want to renumber the session variables. If the customer deletes MyPita1, I need to renumber MyPita2 to MyPita1, renumber MyPita3 to MyPita2, and then decrement the PitaCount.
The AJAX button sends an integer to an ASP file with the number of the pita to be deleted (DeleteID). My function looks at PitaCount and DeleteID. If they're both 1, it just abandons the session. If they're both the same, but greater than one, we're deleting the most recently added pita, so no renumbering is needed. However, if PitaCount is greater then DeleteID, we need to renumber the pitas. Here's the code I'm using to do that:
for (y=DeleteID;y<PitaCount;y++) {
Session("MyPita" + y) = String(Session.Contents("MyPita" + (y+1)));
};
Session.Contents.Remove("MyPita" + PitaCount);
PitaCount--;
Session.Contents("PitaCount") = PitaCount;
This works for every pita EXCEPT the one which replaces the deleted one, which returns 'undefined'. For example, if I have 6 pitas in my cart, and I delete MyPita2, I end up with 5 pitas in the cart. Number 1, 3, 4, and 5 are exactly what you'd expect, but MyPita2 returns undefined.
I also tried a WHILE loop instead:
while (DeleteID < PitaCount) {
Session("MyPita" + DeleteID) = String(Session.Contents("MyPita" + (DeleteID+1)));
DeleteID++;
};
Session.Contents.Remove("MyPita" + PitaCount);
PitaCount--;
Session.Contents("PitaCount") = PitaCount;
This also returns 'undefined', just like the one above.
Until I can get this working I'm simply writing the most recent pita into the spot vacated by the deleted pita, but this reorders the cart, and I consider that a usability problem because people expect the items they added to the cart to remain in the same order. (Yes, I could add some kind of timestamp to the sessions and order using that, but it would be quicker to fix the problem I'm having, I think).
I'm baffled. Why (using the 6 pita example above) would it work perfectly on the second, third, and fourth iteration through the loop, but not on the first?

Answer 1

I can't be sure, but I think your issue may be that the value of DeleteID is a string. This could happen you assign its value by doing something like:
var DeleteID = Session("DeleteID");
Assuming this is true, then in the first iteration of your loop (which writes to the deleted spot), y is a string, and the expression y+1 is interpreted as a string concatenation instead of a numeric addition. If, for example, you delete ID 1, you're actually copying the value from id 11 ("1" + 1) into the deleted spot, which probably doesn't exist in your tests. This can be tested by adding at least 11 items to your cart and then deleting the first one. On the next iteration, the increment operator ++ forces y to be a number, so the script works as expected from that point on.
The solution is to convert DeleteID to a number when initializing your loop:
for (y = +DeleteID; y < PitaCount; y++) {
There may be better ways to convert a string to a number, but the + is what I remember.