Call Code behind function from JavaScript in ASP.Net

Hi ahmadsubuhanlubis,

Please refer below sample.

HTML

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
    <title></title>
    <script type="text/javascript">
        function ShowText() {
            PageMethods.cobatest(OnSuccess);
        }
        function OnSuccess(response) {
            alert(response +' '+document.getElementById("<%=txtUserName.ClientID%>").value );
        }
    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
    </asp:ScriptManager>
    <div>
        Your Name :
        <asp:TextBox ID="txtUserName" runat="server"></asp:TextBox>
        <input id="btnGetText" type="button" value="Show Text" onclick="ShowText()" />
    </div>
</asp:Content>

Code

C#

[System.Web.Services.WebMethod()]
public static string cobatest()
{
    return "Hello";
}

VB.Net

<System.Web.Services.WebMethod()>
Public Shared Function cobatest() As String
    Return "Hello"
End Function

 Screenshot