当做登陆页面时，JS判断用户名密码为空时需要阻止form表单提交。
<script type="text/javascript">
var a = true;
function validate(){
var username = document.getElementById("username").value;
var password = document.getElementById("password").value;
if(username==""||password==""){
alert("Can't be empty");
a= false;
return;
}else{
a=true;
}
}
function checkinput(){
return a;
}
</script>
<body>
<form method="post" action="hello.do" onsubmit="return checkinput();">
Username:
<input type="text" name="username" id="username"><br><br>
Password:
<input type="password" name="password" id="password"><br><br>
<center>
<input type="submit" value="Login" name="login" onclick="validate()">
</center>
</form>
</body>
点击submit后验证为空，form里的onsubmit获取到为false时就停止提交。