求一个事件！关于文本框文字改变的信息！

<html>
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=gb2312">
  <title></title>
  <link href="/KFWebApp/css/stylehq.css" rel="stylesheet" type="text/css">
  
</head>
<body >
<form id="theForm"  method="POST" >
<table width="740" border="0" cellpadding="0" cellspacing="1" class="table-border1" align="center" id="objTable">

<TR valign="middle">
<TD width="6%" align="center" class="table-content2">序号</TD>
<TD width="9%" align="center" class="table-content2">状态</td>
<TD width="14%" align="center" class="table-content2">操作</TD>
</TR>
        <tr>
          <td align="center" class="table-content4">1</td>
          <td align="center" class="table-content4">
    <input type="text" name="state" value="已有" size="6" class="input" readonly>
  </td>
         
          <td align="center" class="table-content4" onclick="mm(this)"> 取消 </td>
        </tr>
      
        <tr>
          <td align="center" class="table-content4">2</td>
          <td align="center" class="table-content4">
    <input type="text" name="state" value="已有" size="6" class="input" readonly>
  </td>          
          <td align="center" class="table-content4" onclick="mm(this)"> 取消 </td>
        </tr>
</TABLE>
</form>
<SCRIPT LANGUAGE="JavaScript">
function mm(td)
{
  var tr = td.parentNode;
  var a = tr.getElementsByTagName("INPUT");
  for(var i=0; i<a.length; i++)
    if (a[i].name=="state")
      a[i].value = a[i].value=="已有" ? "取消" : "已有";
}
</SCRIPT>
</body>
</html>