右边选中的项目没有全部被删除？函数要怎么改进 - 编程论坛

程序代码：

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<html>
<head>
<script language="javascript">
function selectChange()
{
  var allNumber=document.selectForm.allItem.options.length;
  for(var i=0;i<allNumber;i++)
  {
    document.selectForm.allItem.options[0]=null;
  }
  if(document.selectForm.profession.options[document.selectForm.profession.selectedIndex].text=="学生")
  {
    document.selectForm.allItem.options[0]=new Option("数学","数学");
    document.selectForm.allItem.options[1]=new Option("语文","语文");
    document.selectForm.allItem.options[2]=new Option("英语","英语");
    document.selectForm.allItem.options[3]=new Option("Java基础","Java基础");
    document.selectForm.allItem.options[2]=new Option("JavaScript应用","JavaScript应用");
  }
  else if(document.selectForm.profession.options[document.selectForm.profession.selectedIndex].text=="运动员")
  {
    document.selectForm.allItem.options[0]=new Option("足球","足球");
    document.selectForm.allItem.options[1]=new Option("篮球","篮球");
    document.selectForm.allItem.options[2]=new Option("排球","排球");
    document.selectForm.allItem.options[3]=new Option("网球","网球");
    document.selectForm.allItem.options[4]=new Option("羽毛球","羽毛球");
    document.selectForm.allItem.options[5]=new Option("游泳","游泳");
    document.selectForm.allItem.options[6]=new Option("武术","武术");
    document.selectForm.allItem.options[7]=new Option("登山","登山");
  }
}
function checkData(selectedItem)/*检测项目是否已被添加*/
{
  var flag=true;
  for(var i=0;i<document.selectForm.selectedItem.options.length;i++)
  {
    if(document.selectForm.selectedItem.options[i].text==selectedItem.text)
    {
      flag=false;
      break;
    }
   }
  return flag;
}
function addSelectedItem()/*添加选中*/
{
  for(var i=0;i<document.selectForm.allItem.options.length;i++)
  {
    if(document.selectForm.allItem.options[i].selected)
    {
      if(checkData(document.selectForm.allItem.options[i]))
      {
        document.selectForm.selectedItem.options[document.selectForm.selectedItem.length]=new Option

(document.selectForm.allItem.options[i].text,document.selectForm.allItem.options[i].value);
      }
    }
  }
}
function QaddAll()/*添加全部*/
{
  for(var i=0;i<document.selectForm.selectedItem.length;i++)
  {
    document.selectForm.selectedItem.options[i]=null;
  }
  for(var i=0;i<document.selectForm.allItem.options.length;i++)
  {
    document.selectForm.selectedItem.options[i]=new Option(document.selectForm.allItem.options

[i].text,document.selectForm.allItem.options[i].value);
  }
}
function deleteSelectedItem()/*删除选中*/
{
  for(var i=0;i<document.selectForm.selectedItem.length;i++)
  {
    if(document.selectForm.selectedItem.options[i].selected)
    {
      document.selectForm.selectedItem.options[i]=null;
    }
  }
}
function deleteAll()/*删除全部*/
{
  var number=document.selectForm.selectedItem.options.length;
  for(var i=0;i<number;i++)
  {
    document.selectForm.selectedItem.options[0]=null;
  }
}
</script>
</head>
<body>
<form name="selectForm">
<h5><font size="3">Example:select对象的应用</font></h5>
<hr>
<b><font size="3"> 选择用户类别：</font></b>
<select size="1" name="profession" style="border-style:solid; width:150; height:1" onChange="selectChange()">

<option value="学生" selected>学生</option>

<option value="运动员">运动员</option>
</select>
<br><br>
<b><font size="3"> 请选择您曾经学过的课程或者参与过的项目：</font></b>
<table border="0" width="560" id="table1" height="210">

<tr>
  <td width="180" rowspan="6" height="204" align="center">
   <select size="13" name="allItem" multiple style="width:150;">
    <option value="数学">数学</option>
    <option value="语文">语文</option>
    <option value="英语">英语</option>
    <option value="Java基础">Java基础</option>
    <option value="JavaScript应用">JavaScript应用</option>
   </select>
  </td>
  <td width="200" height="30">
  </td>
  <td width="180" rowspan="6" height="204" align="center">
  <select size="13" name="selectedItem" multiple style="width:150;">
  </select>
  </td>

</tr>

<tr>
  <td width="200" height="30" align="center">
   <input type="button" value="添加选中" name="addSelect" style="width:100; height=:25" onClick="addSelectedItem()">
  </td>

</tr>

<tr>
  <td width="200" height="30" align="center">
   <input type="button" value="添加全部" name="addAll" style="width:100; height=:25" onClick="QaddAll()">
  </td>

</tr>

<tr>
  <td width="200" height="30" align="center">
   <input type="button" value="删除选中" name="delectSelect" style="width:100; height=:25"

onClick="deleteSelectedItem()">
  </td>

</tr>

<tr>
  <td width="200" height="30" align="center">
   <input type="button" value="删除全部" name="addAll" style="width:100; height=:25" onClick="deleteAll()">
  </td>

</tr>

<tr>
  <td width="145" height="37" align="center">
  </td>

</tr>
</table>
</form>
</body>
</html>

为什么“删除选中”按钮不尽人意，我选了几个，点“删除选中”，但选中的没有全部被删除？为啥，请指导下函数哪里需要改进。