第一种：
复制代码 代码如下:
<script type="text/javascript">
         function check(form) {

          if(form.userId.value=='') {
                alert("请输入用户帐号!");
                form.userId.focus();
                return false;
           }
       if(form.password.value==''){
                alert("请输入登录密码!");
                form.password.focus();
                return false;
         }
         return true;
         }
</script>

<form action="login.do?act=login" method="post">
用户帐号
  <input type=text name="userId" size="18" value="" >
<br>
 登录密码     
<input type="password" name="password" size="19" value=""/>     
 <input type=submit name="submit1" value="登陆" onclick="return check(this.form)"> 

</form>  
 
第二种
复制代码 代码如下:
<script type="text/javascript">
         function check(form) {

          if(form.userId.value=='') {
                alert("请输入用户帐号!");
                form.userId.focus();
                return false;
           }
       if(form.password.value==''){
                alert("请输入登录密码!");
                form.password.focus();
                return false;
         }
         return true;
         }
</script>

<form action="login.do?act=login" method="post" onsubmit="return check(this)">
用户帐号
  <input type=text name="userId" size="18" value="" >
<br>
 登录密码     
<input type="password" name="password" size="19" value=""/>     
 <input type=submit name="submit1" value="登陆"> 

</form> 

第三种：
复制代码 代码如下:
<script type="text/javascript">
         function check(form) {

          if(form.userId.value=='') {
                alert("请输入用户帐号!");
                form.userId.focus();
                return false;
           }
       if(form.password.value==''){
                alert("请输入登录密码!");
                form.password.focus();
                return false;
         }

          document.myform.submit();
}
</script>

<form action="login.do?act=login" name="myform" method="post">
用户帐号
  <input type=text name="userId" size="18" value="" >
<br>
 登录密码     
<input type="password" name="password" size="19" value=""/>     
<input type=button name="submit1" value="登陆" onclick="check(this.form)"> 

</form>