>>Tim
>
>I'm working my way through this book right now and it seems to be pretty exhaustive in its coverage.
>
>http://www.amazon.com/JavaScript-Definitive-Guide-David-Flanagan/dp/0596101996/ref=sr_1_1?ie=UTF8&s=books&qid=1242828609&sr=1-1

Yeah, I've got an older version of this book and it was OK. To be honest, I tend to just Google things now.

To do what you're describing:

<script language="javascript" type="text/javascript"
   function SetValue()
   {
      var myControl = document.getElementById('txtHidden');
      if (myControl)
         document.forms[0].submit();
        // Instead of this you can use ASP.NET's __doPostback code to fire the postback. If you use it, it's then possible to intercept this info in the page
        // __doPostBack('ControlOrEventCausingPostback', 'AnyParameters');
        // In that case, you don't need the hidden input - pass the parameter as the second parameter of __doPostBack
   }
</script
<input type="hidden" name="txtHidden" /

// In your ASP.NET code-behind:
if (Page.IsPostBack)
{
    string hiddenValue = Request.Form["txtHidden"];
    // Do something
    // Or if you are using ASP.NET's __doPostBack you can do this in Page_Load:
    // string eventName = Request.Form["__EVENTTARGET"];
    // string arg = Request.Form["__EVENTARGUMENT"];
    // if (eventName == "ControlOrEventCausingPostback")
    //    this.DoSomething(arg);
}

You can't easily persist variables between postback w/o just recreating them. What I normally do is just inject some JavaScript after the postback which calls the function you want, ex:

string script = "<script type=\"text/javascript\">JSFunctionToRun();</script>";
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "JustNameOfScript", script);
// You can actually call RegisterClientScriptBlock with a 4th parameter of true - that will add the <script tag stuff for you.

This just tells ASP.NET to inject this JS code into the page. As soon as the browser loads it should run this code.