确保最初隐藏你的警告框。此外，您似乎将JavaScript变量放入HTML中。字符串bag.session.username也可能总是评估为true。也许这个？

Make sure to initially hide your alert box. Also it looks like your putting a JavaScript variable right into your HTML. Also the string bag.session.username may always evaluate to true. Maybe this?

<script type="text/javascript">
$(document).ready(function () {
    if (typeof bag.session.username != "undefined") {
        $(".alertBox span").text(bag.session.username);
        $(".alertBox").show();
    }
    else {
        $(".alertBox").hide();
    }
});
</script>