javascript - alter array object property then save back to localStorage? -

i'm doing cart , want increment qty if item_id exising in localstorage.

<button data-item_id="1" data-item_name="cake" data-item_price="2 dollar">add cart</button> 

not sure why below code doesn't work, added new object instead of increment qty value.

   $(function(){          $('button').click(function(){            var item_id = $(this).attr('data-item_id');            var item_name = $(this).attr('data-item_name');            var item_price = $(this).attr('data-item_price');              var arr = json.parse(localstorage.getitem('cart')) || [];             var obj = {};              if(arr.length === 0){ // first time insert                 obj.item_id = item_id;                 obj.item_name = item_name;                 obj.item_price = item_price;                  obj.item_qty = 1;             }else{                 $(arr,function(i){                 //doesn't work here                    obj.item_qty[i] = parseint(this.qty) + 1;                 });             }              arr.push(obj);             localstorage.setitem('cart',json.stringify(arr));          });      }); 

debug few hours couldn't solved.

use {} instead of [] making arr contain 1 object per item_id , each object 1 added qty each time called. there no remove in code

$(function(){     $('button').on("click",function(e){        var item_id    = $(this).data("item_id");        var item_name  = $(this).data("item_name");        var item_price = $(this).data("item_price");        var cart = json.parse(localstorage.getitem('cart')) || {};        var currentobj = cart[item_id];        if(currentobj){           currentobj.qty++; // increase qty 1        }        else { // item_id had not been stored before, create          cart[item_id]= {             "item_name" :item_name,            "item_price":item_price,            "item_qty"  :1          }        }        localstorage.setitem('cart',json.stringify(cart));     }); });