Unique Property Object

issues = [{"borrowernumber":"16595","itemnumber":"406731","biblionumber":"97756","date_due":"2013-02-05","branchcode":"BUP8","issuingbranch":"","returndate":"2013-01-24","lastreneweddate":"","return":"","renewals":"0","timestamp":"2013-01-24 16:56:07","issuedate":"2013-01-22","inhouse_use":"0","sex":"M","dateofbirth":"1991-07-18","categorycode":"3BUP8","Ufr":"ARTS","Niveau":"L3","Etape":"L3APLA","$$hashKey":"06T"},{"borrowernumber":"61204","itemnumber":"406731","biblionumber":"97756","date_due":"2013-02-08","branchcode":"BUP8","issuingbranch":"","returndate":"2013-02-08","lastreneweddate":"","return":"","renewals":"0","timestamp":"2013-02-08 14:58:13","issuedate":"2013-01-25","inhouse_use":"0","sex":"M","dateofbirth":"1975-03-17","categorycode":"3BUP8","Ufr":"ARTS","Niveau":"M2","Etape":"AGAPLA","$$hashKey":"06U"},{"borrowernumber":"22172","itemnumber":"131352","biblionumber":"97756","date_due":"2013-02-15","branchcode":"BUP8","issuingbranch":"","returndate":"2013-02-11","lastreneweddate":"","return":"","renewals":"0","timestamp":"2013-02-11 15:48:47","issuedate":"2013-01-18","inhouse_use":"0","sex":"M","dateofbirth":"1986-10-19","categorycode":"3BUP8","Ufr":"ZZ","Niveau":"ZZ","Etape":null,"$$hashKey":"06V"}]

var unique = function(data, key) {
   var result = [];
   for(var i=0;i<data.length;i++) {
      var value = data[i][key];
      if (result.indexOf(value) == -1) {
         result.push(value);
      }
   }
   return result;
};
var uniqueItem = unique(issues, 'itemnumber');
// ["406731", "131352", "131351", "131350", "131355", "131358", "131354", "131356"]