表單判斷值是否為空

<script type="text/javascript">
   function check(fm){
     	 
    	 if(document.fm.username.value==""||document.fm.passwd.value==""){
    	 alert("用戶名或者密碼不能為空!");
    	 document.fm.username.focus();
    	 return false;
    	 }
    	 return true;
   }
   </script>

 <center>
    <h2>用戶登錄</h2> 
    <form action="servlet/login" name="fm" method="post" onsubmit="return check(this)">
    <label>用戶名:</label><input type="text" name="username" id="username" style="width:140px"/><br/>
    <label>密&nbsp;&nbsp;&nbsp;&nbsp;碼:</label><input type="password" name="passwd" id="passwd" style="width:140px"/><br/><br/>
    <bt><input type="submit" value="登錄" />
    <input type="button" value="注冊" 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("注冊內容不能為空哦!");
    	   fm.elements[i].focus(); 
    	   return false;
    	 }
    	 return true;
  }
 </script>


<form action="servlet/register" name="fm" method="post" onsubmit="return check(this)">
    <h2>用戶注冊</h2> 
            用戶名:<input type="text" name="username" style="width:140px"/></br>
    <label>密碼:</label><input type="password" name="passwd" style="width:140px"/></br>
    <span>性別:</span><input type="radio" name="sex" checked="checked" value="1"/>男<input type="radio" name="sex" value="2"/>女</br>
    <label>年齡:</label><input type="text" name="age" style="width:140px"/><br/>
    &nbsp;&nbsp;<span>專業:</span><select name="major"><option value="1">軟件工程</option><option value="2">冶金工程</option></select></br>
            手機號:<input type="text" name="phone" style="width:140px"/></br>
    <label>郵件:</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="返回登錄" onclick="window.location.href='login.jsp'"> 
    </form>