JavaScript------如何解决表单登录信息输入为空显示提示

<form name="fname" method="post" action="../Home/Login" onsubmit="return My();">
        <input type="text" name="user" />
        <input type="submit" value="提交"/>
</form>

function My() {
    var s = document.forms["fname"]["user"].value;
    if (s == null || s == "") {
        alert("不能为空");
        return false;
    }
}