I am moving selected item from a list to another. 

 i want to increase the quantity of a field it the item already exists.
Please see my existing code is below

       <script type="text/javascript">
        function AddToCart() {
            var context = SP.ClientContext.get_current();
            //var siteUrl = 'https://xxx.sharepoint.com/sites/Developer';
            //you don't use clientContext
            //var clientContext = new SP.ClientContext(siteUrl);
            var web = context.get_web();
            context.load(web);
    
            var sourceId = SP.ListOperation.Selection.getSelectedList();
            var source = web.get_lists().getById(sourceId);
    
    
            var oList = context.get_web().get_lists().getByTitle('Chart');
            context.load(oList);
    
            var selectedItems = SP.ListOperation.Selection.getSelectedItems(context);
            var spItems = [];
            //  alert('debut');
            for (i = 0; i < selectedItems.length; i++) {
                var currentItem = source.getItemById(selectedItems[i].id);
                //alert(selectedItems[i].id);
                context.load(currentItem);
                spItems.push(currentItem);
    
            }
            context.executeQueryAsync(Function.createDelegate(this, function (sender, args) {
                // alert('test');
                for (var j = 0; j < spItems.length; j++) {
                    if (spItems[j].get_item('Product_x0020_Name') == "Thoshiba")
                    {
                        alert("Item already exists in the chart");
                    }
                    else
                    {
                        var itemCreateInfo = new SP.ListItemCreationInformation();
                        var oListItem = oList.addItem(itemCreateInfo);
                        oListItem.set_item('Title', spItems[j].get_item('Title'));
                        //alert(spItems[j].get_item('Title'));
                        oListItem.set_item('Item', spItems[j].get_item('Item'));
                        oListItem.set_item('Product_x0020_Name', spItems[j].get_item('Product_x0020_Name'));
                        oListItem.set_item('Description', spItems[j].get_item('Description'));
                        oListItem.set_item('Price', spItems[j].get_item('Price'));
                        oListItem.set_item('Department', spItems[j].get_item('Department'));
                        oListItem.set_item('Branch', spItems[j].get_item('Branch'));
                    }
                   
                   // oListItem.set_item('QTY', spItems[j].get_item('QTY') + 1);
    
                    oListItem.update();
                    //alert(spItems[j].get_item('Product_x0020_Name'));
                    //oList.update();                
                }
                context.executeQueryAsync(success);
                //   alert('done');
    
            }), Function.createDelegate(this, error));
    
    
            function success(sender, args) {
                alert('Added to chart!');
                // window.location.reload(true);
                window.location.href = window.location.href;
            }
    
            function error(sender, args) { alert('error'); }
        }
        </script>
        <input name="btnApprove" onclick="AddToCart()" type="button" value="Add To Cart" />