Filling an MVC dropdown based on selections from a textbox (datepicker) .NET/Jquery/JS/MVC

Here is a simple example:  https://jsfiddle.net/zephyr_hex/cvmL4o01/

HTML

Start:
<input type="text" class="datepicker start"> End:
<input type="text" class="datepicker end">
<select id="mySelect"></select>

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jQuery

$(document).ready(function() {
  $('#mySelect').hide();
  $(".datepicker").datepicker({
    "onClose": function(dateText, inst) {
      var string1 = $('.start').val();
      var string2 = $('.end').val();
      if (string1 !== "" && string2 !== "") {
        //do some validation to make sure they are properly formatted dates
        //then...
        var dateStart = new Date(string1);
        var dateEnd = new Date(string2);
        var days = parseInt((dateEnd - dateStart) / (24 * 3600 * 1000));
        if (days > 0) {
          var ddl = $('#mySelect');
          for (var i = 1; i <= days; i++) {
            var option = new Option(i, i);
            ddl.append(option);
          }
          ddl.show();
        }
      }
    }
  });

});

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And to explain my example:  Add a listener to the datepickers that triggers when the datepicker closes.  In that event, check to see if both datepickers have values.  If they do, then convert the values to JavaScript Date objects, which can be subtracted from each other and converted to an integer to get the number of days.  If the number of days is greater than 0, do a loop which creates an "option" element and add the options to the select, then show the select element.

Note:  You may want to clear out all existing option elements from the select before appending new ones.

var ddl = $('#mySelect');
ddl.empty();  //add this line
//do the loop that adds the new options

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Does jquery version matter here? I literally copied your code verbatim to test and the only part that is working is the hiding of the dropdown. Other ideas?

jQuery version should not matter.  There's nothing fancy going on with the code I provided.
Have you checked F12 console for errors?  Or perhaps you need to insert some console.log() lines to see where things are not going as expected.  As you can see from the Fiddle Demo I provided, the code does work.

I added the line where I clear the option from the select to the Fiddle Demo.  Here's that version:  https://jsfiddle.net/zephyr_hex/cvmL4o01/1/

Yup, you're right. I just did what you did in notepad with the basic hosted library refs and it worked fine. No errors but I have a bunch of other crap going on. I'll keep playing with it

I have some other functionality on the date textboxes that is causing the issue. Really struggling to combine the two.

   <script>
        $(function () {
            var dateFormat = "mm/dd/yy",
                from = $("#StartDate")
                    .datepicker({
                        changeMonth: true,
                        numberOfMonths: 1
                    })
                    .on("change", function () {
                        to.datepicker("option", "minDate", getDate(this));
                    }),
                to = $("#EndDate").datepicker({
                    changeMonth: true,
                    numberOfMonths: 1
                })
                .on("change", function () {
                    from.datepicker("option", "maxDate", getDate(this));
                });

            function getDate(element) {
                var date;
                try {
                    date = $.datepicker.parseDate(dateFormat, element.value);
                } catch (error) {
                    date = null;
                }

                return date;
            }
        });
    </script>  
    <script>
        $(document).ready(function () {
            $(".datepicker").datepicker({
                "onClose": function (dateText, inst) {
                    var string1 = $('.start').val();
                    var string2 = $('.end').val();
                    if (string1 !== "" && string2 !== "") {
                        //do some validation to make sure they are properly formatted dates
                        //then...
                        var dateStart = new Date(string1);
                        var dateEnd = new Date(string2);
                        var days = parseInt((dateEnd - dateStart) / (24 * 3600 * 1000));
                        if (days > 0) {
                            var ddl = $('#mySelect');
                            ddl.empty();
                            for (var i = 1; i <= days; i++) {
                                var option = new Option(i, i);
                                ddl.append(option);
                            }
                        }
                    }
                }
            });

        });
    </script>

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So this works fine (but I lose my previous functionality)...

        <input type="text" class="form-control datepicker start" id="fboy"> End:
        <input type="text" class="form-control datepicker end" id="fface">
        <select id="mySelect"></select>

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But this doesn't

        <input type="text" class="form-control datepicker start" id="StartDate"> End:
        <input type="text" class="form-control datepicker end" id="StartDate">
        <select id="mySelect"></select>

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Anyway, that's where I'm at. Still plugging away over here. Any thoughts are welcome.

So, you already are instantiating the datepicker, which is also what my code does.  You'll want to add the OnClose event to your current instantiation.  You instantiate based on element ID, which means you do it once for StartDate and once for EndDate.  That means you need to add the OnClose event to each one.

Also, I noticed on your HTML markup that doesn't work:  you've assigned the same id to both inputs.  One should be End Date.  But the core of the problem is that you need to include the OnClose event in your existing instantiations.  I'll work up a demo.

Wow, thank you so much. I was going crazy over here. That worked perfectly!