JS判断提交表单不能为空 等的验证

<script type="text/javascript">

function beforeSubmit(form){

if(form.username.value==''){

alert('用户名不能为空！');

form.username.focus();

return false;

}

if(form.password.value==''){

alert('密码不能为空！');

form.password.focus();

return false;

}

if(form.password.value.length<6){

alert('密码至少为6位，请重新输入！');

form.password.focus();

return false;

}

if(form.password.value!=form.password2.value) {

alert('你两次输入的密码不一致，请重新输入！');

form.password2.focus();

return false;

}

return true;

}

</script>

<fieldset>

<legend>用户注册</legend>

<form method="post" name="form" action="user.do?method=register" onSubmit="return beforeSubmit(this);">

<table border="1" width="100%" cellspacing="0" cellpadding="0">

<tr><td><label>用户名：<input type="text" name="username" value=""></label></td></tr>

<tr><td><label>密 码：<input type="password" name="password" value=""></label></td></tr>

<tr><td><label>重复密码：<input type="password" name="password2" value=""></label></td></tr>

<tr><td><input value="注册" type="submit"> <input type="reset" value="重置"></td></tr>

</table>

</form>

</fieldset>