How to add (S.No)serial no column

ahieezer

I want to fetch data from server side script. but need to add one extra column to display as a row no.
How to change column order.

dataTable.on( 'order.dt search.dt', function () {
dataTable.column(4, {search:'applied', order:'applied'}).nodes().each( function (cell, i) {
cell.innerHTML = i+1;
} );
} ).draw();

when i use this code it replace the index column.
Thanks in advance.