How do I make my code be able to stack items properly?

And how I check the first columm with 0 amount? I saw other forum posts and didn't saw anything

You should be able to use indexof "0" since you used that for empty slots, so at the top of the add item function set cur slot ID to array.indexof("0"). If you have a problem you can send the file again.

I found something wrong on the new code: The items stopped being able to stack, now when I try stacking, the amount turns into the ID of the items on the inventory. Also, when I click on a potion item that amount is over 2, all are used, but I wanted to make only 1 item get removed.