下面代码里为什么要有hidden，hidden的作用，hidden返回的值是什么？

function login(){ var hidden=document.getElementById("hidden").value; if(document.getElementById("username"+hidden).value ==""){ alert("用户名不能为空"); return false; }else if(document.getElementById("password"+hidden).value ==""){ alert("密码不能为空"); return false; } else { return true; } } function adminlogin(){ document.getElementById("username1").style.display="none"; document.getElementById("password1").style.display="none"; document.getElementById("username2").style.display="block"; document.getElementById("password2").style.display="block"; document.myform.action="admin/login" } function init(){ document.getElementById("username1").style.display="block"; document.getElementById("password1").style.display="block"; document.getElementById("username2").style.display="none"; document.getElementById("password2").style.display="none"; document.myform.action="user/user_login"; } function change(){ var select=document.myform.type.value; if(select=="0") { var username2=document.getElementById("username2").value; var password2=document.getElementById("password2").value; init(); document.getElementById("username1").value=username2; document.getElementById("password1").value=password2; } if(select=="1") { var username1=document.getElementById("username1").value; var password1=document.getElementById("password1").value; adminlogin(); document.getElementById("username2").value=username1; document.getElementById("password2").value=password1; } }






            </tr>
            <tr>
            <td>&nbsp;用户名：</td>
            <td>&nbsp; 
                <input id="username1" type="text" name="account.username" >
                <input id="username2" type="text" name="admin.username" >

            </td>
            </tr>
            <tr>
            <td>&nbsp;密码：</td>
            <td>&nbsp; 
                <input id="password1" type="password" name="account.password">
                <input id="password2" type="password" name="admin.password">

            </td></tr>
            <tr>
            <td>
                &nbsp;用户类型：
            </td>
            <td>
                <select name="type" onchange="change()">
                    <option value="0" selected>客户</option>
                    <option value="1">管理员</option>
                </select>
            </td>
            </tr>
            <tr>
            <td>

            </td>
            <td>&nbsp; 
                <input type="submit" value="登录" id="login">
                <input type="hidden" id="hidden">
            </td></tr>
            </tbody>
        </table>