var fields = new Array();
 fields[0] = 'f1';
 fields[1] = 'f2';
 fields[2] = 'f3';

 var result = new Array();

 var len = fields.length;

 for (var i=0; i < len; i++) {
      if (document.getElementById(fields[i]).checked) {
          result[fields[i]] = true;
      } else {
          result[fields[i]] = false;
      }
 }