Add property to DataTables,

Hi, I want to init columns outside $().DataTable. How can I do it? My code:

COLONNE='{data:"first_name"},{data:"last_name"}'
var  TABELLA = $('#example').DataTable( {

        dom: 'C<"clear">Tlfrtip',
        "columns":COLONNE
});

but it doesn't work. I tried eval(COLONNE) and window[COLONNE] but nothing. Help please