How to detect where JavaScript alert is coming from?

Question

I am trying to debug a very complex IE Intranet application. I am getting an alert with a custom message stating that an exception has occurred. I would like to find out more information about this exception as the message is not very helpful. There is a master page which contains a lot of iFrames (and .htc files if that makes a difference) so I don't think that I can try and hijack window.alert. My last resort will be to try my luck with a file search. Using IE 8, is there anyway I can detect where this alert is coming from? The ideal solution would be to somehow create a "breakOnAlert" function which inserts a debbuger statement at the correct alert location. To clarify: The master page contains many iframes and I believe that the error+alert is coming from one of these. Each iframe is an aspx page (sometimes with dynamic html/javascript from the user) and contains inline and external JavaScript. Before posting I did try overriding alert in my page (a child page inside an iframe) bu…

Accepted Answer

Since it's a real chore to do it in all iframes, I'd probably use Fiddler and programatically replace `alert(` with something like:

```
(function(n){alert(n);debugger;})(
```

IE should support the `debugger` statement, so you'd have a call-stack

This page explains how to do a text-replace in Fiddler

Example Fiddler custom rule to add to `OnBeforeResponse`:

```
if (oSession.oResponse.headers.ExistsAndContains("Content-Type", "html"))
{
    oSession.utilDecodeResponse();
    var oBody = System.Text.Encoding.UTF8.GetString(oSession.responseBodyBytes);
    oBody = oBody.replace(/alert\(/gi, "(function(n){alert(n);debugger;})(");
    oSession.utilSetResponseBody(oBody);
}
```

How to detect where JavaScript alert is coming from?

Solution

Problem

Related problems