Get custom data-attribute in select2 with <select>

$(document).ready(function() {
    $('select#myselect').select2({
        templateResult: formatOutput
    });

});
function formatOutput (item) {
    var $state = $(item.element).data('id') + ' ' + item.text;
    return $state;
};

There is no direct method with select2, you can use a combination of select2 data and jQuery like following :

$("#example").select2().find(":selected").data("id");

First you get the select2 data then you find your selected option with jQuery and finally the data-attribute.

Select2 v4 relies on <select> tags instead of hidden <input> tags so it's now easier to get the selected option or options: you just refer to the original <select>. This may have also been the case with Select2 v3 but I've only used Select2 v4.

$('#example option:selected').attr('data-id')

jsfiddle demonstration

See also Get selected value in dropdown list using JavaScript?

Edit: I like this answer for general purpose data object access:

var d = $("#dropdown").select2("data");

d will be an array containing the selected item(s) as objects, so to access the id property of the first item:

var id = d[0].id;

After hours of trying to solve this, I have manage to pull out the attribute. I am using 3.5.4

$("#select2").select2('data').element[0].attributes[1].nodeValue

HTML

<select id="select2" name="course" class="form-control">
  <option></option>  
  <optgroup label="Alabama">  
    <option value="City 1" data-url="/alabama/city-1">City 1</option>  
    <option value="City 2" data-url="/alabama/city-2">City 2</option>  
  </optgroup>  
</select>

$("#select2").select2('data').element[0].attributes[0].nodeValue --> Value Attribute
$("#select2").select2('data').element[0].attributes[1].nodeValue --> Data-URl Attribute