Question regarding error code

I added a script to the scriptmanager that cancels a request (a button click) if there current outstanding async request. Below the questions is the javascript that is being used

Questions:
1. I got a random 12004 error as a message box. Its only happened once, and I am not sure how to track it as it was "an unknown error".

2. Is there a way to tell if the way to tell if the the button caused the postback currently being processed, and the control requesting one?

3. In the javascript (taken fromhttp://www.asp.net/AJAX/Documentation/Live/tutorials/CancelAsyncPostback.aspx) there is this symbol: !== what does this do?

Sys.Application.add_load(ApplicationLoadHandler)
function ApplicationLoadHandler(sender, args)
{
Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(CheckStatus);
}

function CheckStatus(sender, args)
{
var prm = Sys.WebForms.PageRequestManager.getInstance();
if (prm.get_isInAsyncPostBack() & args.get_postBackElement().id == 'Button1')
{
args.set_cancel(true);
}

}

if(typeof(Sys) !== "undefined") Sys.Application.notifyScriptLoaded();

!== and === are strictly typed comparisons, instead of the dynamic typing that JavaScript normally uses. For example, 3 == "3" is true, but 3 === "3" is false.

For checking the sender against current request, I'd suggest making sure the button is disabled when a request is made instead. That way you don't even have to worry about getting the same control that you're processing for as a new request (which I assume is what you're trying to handle with the check).

The issue is that javascript such as: document.getElementById('Button1').click() can still fire even if the button is disabled.