表单判断值是否为空

<script type="text/javascript">
   function check(fm){
         
         if(document.fm.username.value==""||document.fm.passwd.value==""){
         alert("用户名或者密码不能为空!");
         document.fm.username.focus();
         return false;
         }
         return true;
   }
   </script>

 <center>
    <h2>用户登录</h2> 
    <form action="servlet/login" name="fm" method="post" onsubmit="return check(this)">
    <label>用户名:</label><input type="text" name="username" id="username" style="width:140px"/><br/>
    <label>密&nbsp;&nbsp;&nbsp;&nbsp;码:</label><input type="password" name="passwd" id="passwd" style="width:140px"/><br/><br/>
    <bt><input type="submit" value="登录" />
    <input type="button" value="注册" onclick="window.location.href='register.jsp'">
    <input type="reset" value="重置"/></bt>
    </form>
    </center>




<script type="text/javascript">
   function check(fm){
         for(i=0;i<fm.length;i++){
           if(fm.elements[i].value=="")
           alert("注册内容不能为空哦!");
           fm.elements[i].focus(); 
           return false;
         }
         return true;
  }
 </script>


<form action="servlet/register" name="fm" method="post" onsubmit="return check(this)">
    <h2>用户注册</h2> 
            用户名:<input type="text" name="username" style="width:140px"/></br>
    <label>密码:</label><input type="password" name="passwd" style="width:140px"/></br>
    <span>性别:</span><input type="radio" name="sex" checked="checked" value="1"/>男<input type="radio" name="sex" value="2"/>女</br>
    <label>年龄:</label><input type="text" name="age" style="width:140px"/><br/>
    &nbsp;&nbsp;<span>专业:</span><select name="major"><option value="1">软件工程</option><option value="2">冶金工程</option></select></br>
            手机号:<input type="text" name="phone" style="width:140px"/></br>
    <label>邮件:</label><input type="text" name="email" style="width:140px"/></br></br>
    <input type="submit" value="提交"/>
    <input type="reset" value="重置"/>
    <input type="button" value="返回登录" onclick="window.location.href='login.jsp'"> 
    </form>