How To Pass Values From Controller To A TextBox Using Ajax

Few changes I have done on the accepted answer and here is my version:

$(document).ready(function() {
  $(".ItemId").change(function () {
    var itemId = $(this).val();
    getAmountFromItem(itemId);
  });

  function getAmountFromItem(itemId) {
    $.get({
      url: '@Url.Action("GetAmountFromItem", "Stock")',
      data: { itemId: itemId },
      dataType: 'json',
      success: function(data) {
        $(".Avail").val(data);
      },
      error: function() {
        alert("Error while processing.");
      }
    });
  }
});

I have changed the AJAX method from $.ajax to the shorthand method $.get, which simplifies the syntax. Also I have removed the async: true option, since it's the default behavior for AJAX requests in jQuery. Removed the contentType option, since it's not necessary for a GET request.

Hi, Please try below

var GetAmountFromItem = function () {
    $.ajax({
        url: '@Url.Action("GetAmountFromItem","Stock")',
        type: 'GET',
        data: {
            ItemId: $('.ItemId').val(),
        },
        success: function (data) {
            $('.Avail').val(data.Amount); // Update the value of the Available textbox
        },
        error: function () {
        }
    });
}

Modify your controller action GetAmountFromItem

public JsonResult GetAmountFromItem(int ItemId)
{
    var ac = (from c in cdc.Stock
              where c.ID == ItemId
              select c).FirstOrDefault();
    return Json(new { Amount = ac.Amount }); // Return the Amount property as a JSON object
}

Thanks