How to use dynamic variable in update.js shopify

I want yo use test variable in update.js but it shows error when I use as variable but when I pass this value directly it works can someone please tell me how can use dynamic variable to change quantity of existing products in cart

I have updated my code It will allow user to add only 5 items more than 5 items will be removed It will create string which will look like this 32082238341235:0,39470423048307:0,32164693278835:0,32164693835891:1

and finally the all IDs and qunatity will be updated by update.js

I have got error in last step its shows {“status”:404,“message”:“Cart Error”,“description”:“Cannot find variant”}

when i try to update all products

jQuery.getJSON('/cart.js', function(cart) {
  var items_new = cart.items;
  var count = 0;
  count = cart.item_count;
  var item_to_remove = count - 5;

  if (count > 5) {
    var item_to_remove = count - 5;
    var combine = ""
    if (item_to_remove > 0) {
      for (var i = 0; i < items_new.length; i++) {
        if (count > 5) {
          var c_id = items_new[i].variant_id;
          var c_quantity = items_new[i].quantity;

          if (c_quantity >= item_to_remove) {

            var q = c_quantity - item_to_remove
            var data_multiple = c_id + ":" + q + ",";

            debugger;

            count = count - item_to_remove;
            console.log(data_multiple);
            var combine = combine + data_multiple;

          } else {

            var data_single = c_id + ":" + 0 + ",";

            count = count - c_quantity;
            item_to_remove = item_to_remove - c_quantity
            console.log(data_single)

            var combine = combine + data_single;
          }

        }

      }

      console.log(combine.slice(0, -1));
      var test = combine.slice(0, -1);

      console.log({
        updates: {
          test
        }
      });
      jQuery.post('/cart/update.js', {
        updates: {
          test
        }
      });

    }
    t._rerenderCart()

  }
  t.sidebarDrawer.open()
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

Where is this getJSON function being called from?