jQuery UI DatePicker - Change Date Format

So I struggled with this and thought I was going mad, the thing is bootstrap datepicker was being implemented not jQuery so the options are different;

$('.dateInput').datepicker({  format: 'dd/mm/yyyy'})

You can add and try this way:

HTML file:

<p><input type="text" id="demoDatepicker" /></p>

JavaScript file:

$(function() {  
    $("#demoDatepicker").datepicker({
        dateFormat: 'dd/mm/yy',
        changeMonth: true,
        changeYear: true,
        constrainInput: false
    });
});

This works smoothly. Try this.

Paste these links in your head section.

<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script type="text/javascript"
        src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.4.1/js/bootstrap-datepicker.min.js"></script>
<link rel="stylesheet"
      href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.4.1/css/bootstrap-datepicker3.css"/>
<link rel="stylesheet" href="css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>
<script src="http://code.jquery.com/ui/1.11.0/jquery-ui.js"></script>
<link rel="stylesheet" href="http://code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">

Following is the JS coding.

 <script>

        $(document).ready(function () {
            $('#completionDate').datepicker({
                dateFormat: 'yy-mm-dd', // enter date format you prefer (e.g. 2018-10-09)
                changeMonth: true,
                changeYear: true,
                yearRange: "-50:+10",
                clickInput: true,

                onSelect: function (date) {
                    loadMusterChit();
                }

            });
        });

    </script>

Here is an out of the box future proof date snippet. Firefox defaults to jquery ui datepicker. Otherwise HTML5 datepicker is used. If FF ever support HTML5 type="date" the script will simply be redundant. Dont forget the three dependencies are needed in the head tag.

<script>
  src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js">
</script>
<link rel="stylesheet"href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">  
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>

<!--Form element uses HTML 5 type="date"-->
<div class="form-group row">
    <label for="date" class="col-sm-2 col-form-label"Date</label>
    <div class="col-sm-10">          
       <input type="date" class="form-control" name="date" id="date" placeholder="date">
    </div>
</div>

<!--if the user is using FireFox it          
autoconverts type='date' into type='text'. If the type is text the 
script below will assign the jquery ui datepicker with options-->
<script>
$(function() 
{
  var elem = document.createElement('input');
  elem.setAttribute('type', 'date');

  if ( elem.type === 'text' ) 
  {
    $('#date').datepicker();
    $( "#date" ).datepicker( "option", "dateFormat", 'yy-mm-dd' );     
  }
});

This is what worked for me:

$.fn.datepicker.defaults.format = 'yy-mm-dd'

If You want that format "dd-mm-yy" you have to change dateformat in Datepicker, this will work...

$("#date").
({
 dateFormar:'dd-mm-yy',
}).val();