[etc]

Array.prototype.count = function() {
  var out_num = 0;
  var in_arr = this;
  for (var i=0;i<in_arr.length;i++) {
      if (isNaN(in_arr[i])) {
          return null;
      }
      out_num+=in_arr[i];
  }
  return out_num;
}
// USAGE:
var arr = [2,4,6,8,12];
trace(arr.count()); // 32

не проверял, сразу говорю