Datatables sort images on title tag -

i have question using custom sorting option datatables. have table images in last column. images use title tag, want sort them on.

so, use code set sorting:

<script type="text/javascript">              jquery.extend( jquery.fn.datatableext.osort, {                 "title-string-pre": function ( ) {                     return a.match(/title="(.*?)"/)[1].tolowercase();                 },                  "title-string-asc": function ( a, b ) {                     return ((a < b) ? -1 : ((a > b) ? 1 : 0));                 },                  "title-string-desc": function ( a, b ) {                     return ((a < b) ? 1 : ((a > b) ? -1 : 0));                 }             } );              $(document).ready(function() {                         $('#customertable').datatable( {                              "aocolumndefs": [                               { "bsortable": false, "atargets": [ 0,1 ] },                              { "stype": "title-string-asc",   "atargets": [ 4 ] }                            ],                              "bpaginate": false,                             "blengthchange": false,                             "bfilter": false,                             "binfo": false,                             "bautowidth": false,                             "bsortcellstop": true                          } );                     } ); </script> 

however, when click on header of column 4, not work , chrome reports errors "property 'title-string-asc-desc' of object # not function" , "property 'title-string-asc-asc' of object # not function"

what doing wrong?

found answer, don't understand it.

changed { "stype": "title-string-asc", "atargets": [ 4 ] } { "stype": "title-string", "atargets": [ 4 ] }

now works. why? "title-string" defined nowhere. how can javascript link string content of title tag?