How to show Random values in Textbox JSON

Here is my option code and text box

  <select id="sel" class="form-control input-lg" data-live-search="true">
    <option  value="">-- Select Your Country--</option>
    </select><br/><br/><br/>

<input type = "text" id = "txtName" class="form-control input-lg" />
</div>

This is my JSON code

[  
    {  
      "country":"First",
      "coupon":["1","10","11"]
    },
    {  
      "country":"Second",
      "coupon":"2"
    },
    {  
      "country":"third",
      "coupon":"3"
    },
    {  
      "country":"fourth",
      "coupon":"4"
    },
    {  
      "country":"fifth",
      "coupon":"5"
    }
  ]

i have populated JSON to dropdown and shown to text (input) box

$('#sel').append('<option value="' + value.coupon + '">' + value.country + '</option>');

        $('#sel').change(function () {
var str = $("#sel").val();
        $("#txtName").val(str);
}

What i need is when i select the value "First" in dropdown which has 3 numbers ["1","10","11"] i need it to be shown in random order 1 at a time in text box

I used below code to randomly generate it but i couldn't get desired result.Could anyone help me ?

Array.prototype.random = function (length) {
       return this[Math.floor((Math.random()*length))];
 }
// var randomcoupon = value.coupon.random(value.coupon.length);