JavaScript不验证文本框

Question

在标题中，我在asp.net中创建了一个登录页面，并想要检查登录名和密码复选框。如果文本框为空，则应弹出警报，但没有任何反应。

JS代码：

<script type="text/javascript">
        function log() {
            var x = document.getElementById("tb_login").value;
            if (x == null || x.trim() == "") {
                alert("please type login");
                return false;
            }
            var z = document.getElementById("tb_pwd").value;
            if (z == null || z.trim() == "") {
                alert("please type pass");
                return false;

            }
        }
    </script>

和asp登录页面代码：

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage2.master" AutoEventWireup="true" CodeFile="Login.aspx.cs" Inherits="Login" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
    <style type="text/css">
        .auto-style1 {}
    </style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <p>
        <br />
    </p>
    <asp:Label ID="Label2" runat="server" Text="Login"></asp:Label>
    <br />
    ;<asp:TextBox ID="tb_login" runat="server" CssClass="auto-style1" Width="180px"></asp:TextBox>
    <br />
    <br />
    <asp:Label ID="Label1" runat="server" Text="Password"></asp:Label>
    <br />
    <asp:TextBox ID="tb_pwd" runat="server" TextMode="Password" Width="180px"></asp:TextBox>
    <br />
    <br />
    <asp:Label ID="Label3" runat="server" Font-Bold="True" Font-Overline="True" ForeColor="Red"></asp:Label>
    <p>
   <asp:Button ID="btn_login" runat="server" OnClick="btn_login_Click" Text="Log In" OnClientClick="return log();" />
        <asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl="~/Register.aspx">Register new account !</asp:HyperLink>
    </p>
    <p>
    </p>
</asp:Content>