javascript cookie问题

Question

function int()
              { 
               var userName="";
                if(document.cookie!="")
                {
                    userName=getCookie();
                    document.getElementById("username").value=userName;
                }
                else
                {
                   document.getElementById("username").value="";
                }
                   document.getElementById("submit").onclick=setCookie;
               }
         function setCookie()
        {
          var expireDate=new Date();
          expireDate.setMonth(expireDate.getDate()+6);
          var userName=document.getElementById("username").value;
          if(strcmp())
          document.cookie="userName="+userName+";expires="+expireDate.toGMTString();
        }
        function getCookie()
        {
          var allcookies=document.cookie;
          var pos=allcookies.indexOf("userName=");
          if(pos!=-1)
          {
            var start=pos+9;
            var end=allcookies.indexOf(";",start);
            if(end==-1) end=allcookies.length;
            var value=allcookies.substring(start,end);
            value=decodeURIComponent(value);
            return value;
          }
        }
       问题是，当我在text框里面什么也不输入的时候，点击提交后，下次登陆的时候text框里面显示的是undefined，我想在这种情况下什么也不显示，希望高手执教，谢谢！

Answer 1

<form id="loginForm" action="secretdb.php" method="post">
    <table border="0">
       <tr>
      <td>用户名:</td>
      <td> <input type="text" id="username" name="username"></td>
   </tr>
   <tr>
  <td>密 码:</td>
  <td><input type="password" id="pswd" name="pswd"></td>
   </tr>
   <tr>
  <td colspan="2" align="center">  
  <input type="submit" id="submit"  value="登录">
  <input type="reset" value="重置">
  </td>
   <tr/>
</table>   
</form>这个是帖子代码。