You can do:

function formatDate(date) {
    var d = new Date(date),
        month = '' + (d.getMonth() + 1),
        day = '' + d.getDate(),
        year = d.getFullYear();

    if (month.length < 2) 
        month = '0' + month;
    if (day.length < 2) 
        day = '0' + day;

    return [year, month, day].join('-');
}
 
console.log(formatDate('Sun May 11,2014'));

Usage example:

console.log(formatDate('Sun May 11,2014'));

Output:

2014-05-11

Demo on JSFiddle: http://jsfiddle.net/abdulrauf6182012/2Frm3/

Some answers don't quite solve the issue. They print the date formatted as mm/dd/yyyy but the question was regarding MM/dd/yyyy. Notice the subtle difference? MM indicates that a leading zero must pad the month if the month is a single digit, thus having it always be a double digit number.

i.e. whereas mm/dd would be 3/31, MM/dd would be 03/31.

I've created a simple function to achieve this. Notice that the same padding is applied not only to the month but also to the day of the month, which in fact makes this MM/DD/yyyy:

function getFormattedDate(date) {
  var year = date.getFullYear();

  var month = (1 + date.getMonth()).toString();
  month = month.length > 1 ? month : '0' + month;

  var day = date.getDate().toString();
  day = day.length > 1 ? day : '0' + day;
  
  return month + '/' + day + '/' + year;
}

Update for ES2017 using String.padStart(), supported by all major browsers except IE.

function getFormattedDate(date) {
    let year = date.getFullYear();
    let month = (1 + date.getMonth()).toString().padStart(2, '0');
    let day = date.getDate().toString().padStart(2, '0');
  
    return month + '/' + day + '/' + year;
}

Sorry, I cannot help you with that.

I honestly suggest that you use moment.js. Just download moment.min.js and then use this snippet to get your date in whatever format you want:

<script>
$(document).ready(function() {

     // set an element
     $("#date").val( moment().format('MMM D, YYYY') );

     // set a variable
     var today = moment().format('D MMM, YYYY');

});
</script>

Use following chart for date formats: