Here is an example of how to execute JavaScript after a PostBack in a WebForms page (this specific case is not using ASP.NET Ajax, but rather a synchronous PostBack). This will allow you to run custom JavaScript after the server side code has executed.
VB.Net
Page.Validate
If Page.IsValid = True Then
Page.ClientScript.RegisterClientScriptBlock(Me.GetType, "script", js, True)
End If
C#
Page.Validate();
if (Page.IsValid)
{
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "script", js, true);
}