添加checkbox 排它问题？

Question

运行至alert(answer[i].value)网页报“vlaue为空或不是对象”，我这种取值方式有问题吗？还是那里写法有错误？
          <select size=1 name="sd_carboncopy" id="sd_carboncopy">  
  <option value="">无</option>
   <%do while not rs.eof%>     
          <option value="<%=rs("name")%>"><%=rs("name")%></option>
  <%
  rs.movenext
  loop
  rs.close
          %>
          </select> 
<input  type="button" value="add" onClick="setname('td_carboncopy');">
function setname(obj)
{
    var copy=document.getElementById("sd_carboncopy").value;
var answer = document.getElementsByName("cp");
    if (copy != ""){
          var i = answer.length
  if (i > 0 )
   {
   alert(answer[i].value);
   }
   // for (var i = answer.length-1; i >= 0; i--) {
         //if (copy != answer[i].value )
                {
                 document.getElementById(obj).innerHTML += "<input type='checkbox' name='cp' />" + copy ;
                 }
                 // else
    // {
    // alert("对不起,你选择了重复的对象!");
// }
//}
}
 }

Answer 1

var i = answer.length
                  if (i > 0 )
                   {
                   alert(answer[i].value);
                   }
数组超界了

Answer 2

document.getElementsByName("cp"); 执行时候 cp存在？

Answer 3

这个就是添加 “cp”的啊
                        {
                         document.getElementById(obj).innerHTML += "<input type='checkbox' name='cp' />" + copy ;
                         }

Answer 4

<select size=1 name="sd_carboncopy" id="sd_carboncopy">  
            <option value="">无</option>
            <option value="1">无1</option>
            <option value="2">无2</option>
            <option value="3">无3</option>
          </select> 
          <div id="td_carboncopy">
            
          </div>
        <input  type="button" value="add" onclick="setname('td_carboncopy');"/>
        <script type="text/javascript">
            function setname(obj)
            {
                var copy=document.getElementById("sd_carboncopy").value;
                var answer = document.getElementById(obj).getElementsByTagName("input");
                var isRepeat = false;
                    if (copy != ""){
                              var i = answer.length
                              if (i > 0) {
                                  //alert(answer[i].value);
                              }
                              for (var i = answer.length - 1; i >= 0; i--) {
                                  if (answer[i].name == "cp") {
                                      if (copy != answer[i].value) {
                                          // document.getElementById(obj).innerHTML += "<input type='checkbox' name='cp' />" + copy ;
                                      }
                                      else {
                                          isRepeat = true;
                                          break;
                                          // alert("对不起,你选择了重复的对象!");
                                      }
                                  }
                              }
                              if (!isRepeat) { //如果没有重复
                                     document.getElementById(obj).innerHTML += "<input type='checkbox' name='cp' value='" + copy + "' />" + document.getElementById("sd_carboncopy").options[document.getElementById("sd_carboncopy").selectedIndex].text;
                                 }
                    }        
             }
             </script>

Answer 5

var i = answer.length;alert(answer[i].value);当然会报错了，js中数组下标从0开始到length - 1;answer[i]已经数组越界了。。answer[i] = undefined,当然没有valuealert(answer[i - 1].value);

Answer 6

还有你的checkbox没有设置value属性所以alert(answer[i - 1].value);可能不是你想要得结果，而是'on'

Answer 7

找到我的写法原因了，7楼方法也可以
document.getElementById(obj).innerHTML += "<input type='checkbox' name='cp' value='"+copy+"'/>" + copy ;