asp.net中在前台用js增删ListBox的items

asp.net中在前台用js增删ListBox的items(3)                         lbFieldsSelect.options[lbFieldsSelect.length-1].value=lbFields.options[i].value;
                        Hidden1.value = Hidden1.value + "#" + lbFields.options[i].text + "@" + lbFields.options[i].value;
                    }
                }
            }
           
            function UnSelectField()
            {
                var lbFields = document.getElementById("lbFields");               
                var lbFieldsSelect = document.getElementById("lbFieldsSelect");
                var Hidden1 = document.getElementById("Hidden1");
               
                for (i = 0;i<lbFieldsSelect.length; i++)
                {
                    if (lbFieldsSelect.options[i].selected)