DataTables - Buttons / Index column

HTML

<script src="https://cdn.datatables.net/r/dt/jszip-2.5.0,pdfmake-0.1.18,dt-1.10.9,b-1.0.3,b-html5-1.0.3/datatables.min.js"></script>
<link rel="stylesheet" href="https://cdn.datatables.net/r/dt/jszip-2.5.0,pdfmake-0.1.18,dt-1.10.9,b-1.0.3,b-html5-1.0.3/datatables.min.css">
<table id="example" class="display" cellspacing="0" width="100%">
    <thead>
        <tr>
            <th>Name</th>
            <th>Position</th>
            <th>Office</th>
            <th>Age</th>
            <th>Start date</th>
            <th>Salary</th>
        </tr>
    </thead>
    <tfoot>
        <tr>
            <th>Name</th>
            <th>Position</th>
            <th>Office</th>
            <th>Age</th>
            <th>Start date</th>
            <th>Salary</th>
        </tr>
    </tfoot>
    <tbody>
        <tr>
            <td>Tiger Nixon</td>
            <td>System Architect</td>
            <td>Edinburgh</td>
            <td>61</td>
            <td>2011/04/25</td>
            <td>$320,800</td>
        </tr>
        <tr>
            <td>Garrett Winters</td>
            <td>Accountant</td>
            <td>Tokyo</td>
            <td>63</td>
            <td>2011/07/25</td>
            <td>$170,750</td>
        </tr>
        <tr>
            <td>Ashton Cox</td>
            <td>Junior Technical Author</td>
            <td>San Francisco</td>
            <td>66</td>
            <td>2009/01/12</td>
            <td>$86,000</td>
        </tr>
        <tr>
            <td>Bradley Greer</td>
            <td>Software Engineer</td>
            <td>London</td>
            <td>41</td>
            <td>2012/10/13</td>
            <td>$132,000</td>
        </tr>
        <tr>
            <td>Dai Rios</td>
            <td>Personnel Lead</td>
            <td>Edinburgh</td>
            <td>35</td>
            <td>2012/09/26</td>
            <td>$217,500</td>
        </tr>
        <tr>
            <td>Jenette Caldwell</td>
            <td>Development Lead</td>
            <td>New York</td>
           ...

JavaScript

$(document).ready(function (){
    var table = $('#example').DataTable({
       dom: 'Bfrtip',
       buttons: [ 'pdfHtml5', 'csvHtml5', 'copyHtml5', 'excelHtml5' ],
       "columnDefs": [ {
          "searchable": false,
          "orderable": false,
          "targets": 0
        } ],        
        "order": [[ 1, 'asc' ]]
    });
    
    table.on( 'order.dt search.dt', function () {
       table.column(0, {search:'applied', order:'applied'}).nodes().each( function (cell, i) {
          cell.innerHTML = i + 1;
          table.cell(cell).invalidate('dom'); 
       } );
    } ).draw();
});