Javascript date format is not working

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Javascript

My screen as follows

From Date Datepicker

when i select the date less than the today date shows the message and date format of datepicker date dd-mm-yyyy.

for that i written a code

(function () {
             ("#datepicker").datepicker();
             ("#datepicker").on('change', function () {
                 var date = Date.parse($(this).val());
                 if (date < Date.now()) {
                     alert('Selected date must be greater than today date');
                     $(this).val('');
                 }
             });
         });

Date picker date format code as follows

(function () {

             ("#datepicker").datepicker({ dateFormat: "dd-mm-yy" }).val()

         });

in run mode when i select the date less than today date means message shows correctly Selected date must be greater than today date. but date format is not changing , date format i get as
02/14/2018. the date format must 14-02-2018.

what is the mistake in my above code.

What I have tried:

My screen as follows

From Date Datepicker

when i select the date less than the today date shows the message and date format of datepicker date dd-mm-yyyy.

for that i written a code

$(function () {
             ("#datepicker").datepicker();
             ("#datepicker").on('change', function () {
                 var date = Date.parse($(this).val());
                 if (date < Date.now()) {
                     alert('Selected date must be greater than today date');
                     $(this).val('');
                 }
             });
         });

Date picker date format code as follows

(function () {

             ("#datepicker").datepicker({ dateFormat: "dd-mm-yy" }).val()

         });

in run mode when i select the date less than today date means message shows correctly Selected date must be greater than today date. but date format is not changing , date format i get as
02/14/2018. the date format must 14-02-2018.

what is the mistake in my above code.