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(2)                
                for (i = 0;i<lbFields.length; i++)
                {
                    if (lbFields.options[i].selected)
                    {                       
                        if  (Test(lbFieldsSelect,lbFields.options[i].value))
                        {
                            //已经选择该字段
                            continue;
                        }
                       
                        lbFieldsSelect.options.add(document.createElement("OPTION"));
                        lbFieldsSelect.options[lbFieldsSelect.length-1].text=lbFields.options[i].text;