Ytukyg

I'm working on a drawer cart populated via ajax for a shopify site. There's also a quantity field with plus and minus buttons within this cart.
This is the code for the plus button:

// This button will increment the value

$('.ajax-cart-form').on("click",'.qtyplus.qtyplus-2', function(){

    // Get the field name

    fieldName = $(this).attr('field');

    // Get its current value

    var currentVal = parseInt($('input[id='+fieldName+']').val());

    // If is not undefined

    if (!isNaN(currentVal)) {

        // Increment

        $('input[id='+fieldName+']').val(currentVal + 1);

    } else {

        // Otherwise put a 0 there

        $('input[id='+fieldName+']').val(0);

    }

    var currentVal2 = parseInt($('input[id='+fieldName+']').val());

    var currentProd = $('input[id='+fieldName+']').attr('id');

    $.ajax({

  type: 'POST',

  url: '/cart/update.js',

  data: { updates: { currentProd : currentVal2 }},

  dataType: 'json',

  success: function() {

    $.ajax({

      type: 'GET',

      url: '/cart.js',

      async: false,

      cache: false,

      dataType: 'json',

      success: function() {

          $.getJSON(_config.shopifyAjaxCartURL, function(cart) {

            $('.drawer-subtotal').remove();

            $('.ajax-subtotal').css('display','none');

            $(function(index, cartItem) {

                  var line2= index +1;

                  var cents2 = "";



          if (cart.original_total_price % 100 < 10) {

          cents2 = "0";

          }

          var price2 = parseInt(cart.original_total_price/100) + "." + cents2 + cart.original_total_price % 100;

            $('.ajax-subtotal2').append("<div class='drawer-subtotal'><h3>Subtotal <span class='money'>£"+ price2 +" GBP</span></h3></div>");

            });

              $('#item_count').text(currentVal2);

      });

    }

    });

  }

});

});

As you can see I'm getting the value of the quantity field to update the quantity in the ajax post request via the variable currentVal2. This works fine. I'm also getting the product ID via the id of the input field.
I've tried

alert(currentProd);

and this outputs the ID correctly. However, when I'm trying to use this variable in the update function like this

 data: { updates: { currentProd : currentVal2 }},

I'm getting the error that update.js is not found so I'm assuming it's not pulling in the Product ID correctly.

If instead I try

data: { updates: { 35450129542: currentVal2 }},

so input the product ID directly it works.

Any ideas how to fix this?