求助：验证输入框是否为空的JS调用了两遍，我不要！

Question

function Check() 
        { 
            if(document.getElementById("UserName").value == "") 
            { 
                alert("请输入用户名！"); 
                document.getElementById("UserName").focus(); 
                return false; 
            } 
            else if(document.getElementById("UserPwd").value == "") 
            { 
                alert("请输入密码！"); 
                document.getElementById("UserPwd").focus(); 
                return false; 
            } 
            
                return true;         } <asp:ImageButton ID="ImgBtnOK" runat="server" ImageUrl="~/Admin/Image/ALogin/button_r2_c2.jpg" OnClientClick="return Check();" onclick="ImgBtnOK_Click" />