ASP.NET

<asp:UpdatePanel runat="server" ID="updPanel">
                                        <ContentTemplate>
                                            <div class="visibleDiv">
                                                <asp:Label ID="lblTimer" runat="server" Font-Bold="true" Font-Size="14"></asp:Label>
                                            </div>
                                            <asp:Timer ID="Timer1" runat="server" Enabled="true" Interval="500" OnTick="Timer1_Tick">
                                            </asp:Timer>
                                        </ContentTemplate>
                                        <Triggers>
                                            <asp:AsyncPostBackTrigger ControlID="Timer1" EventName="Tick" />
                                        </Triggers>
                                    </asp:UpdatePanel>

<ajaxToolkit:ToolkitScriptManager runat="server" ID="ScriptManager1" EnablePageMethods="true">
   <Scripts>
      <asp:ScriptReference Path="~/javascript/myscript.js" />
   </Scripts>
</ajaxToolkit:ToolkitScriptManager>
...
...
<a href="javascript:myFunction();">Click to run script</a>
...
...
<div id='theDiv'></div>

[System.Web.Services.WebMethod]
public static string ReturnTheString()
{
   return "This is the string returned";
}

function myFunction()
{
   PageMethods.ReturnTheString(OnSucceeded, OnFailed);
}

function OnSucceeded(result)
{
   var myDiv = document.getElementById('theDiv');
   myDiv.innerHTML = result;
}

function OnFailed(error)
{
   alert(error.get_message());
}