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Earlier today he released an update to his popular WebDevHelper utility. Included within this release is support for easily monitoring the HTML updates that are returned from ASP.NET when using the <asp:updatepanel> control. His utility allows you to log and inspect all of the HTTP traffic between the browser and the server - making it much easier to debug AJAX code. For example, in the picture below you can see the HTML returned from an UpdatePanel when partial rendering is enabled on the page:
Nikhil has also recently added new support for inspecting JSON network requests and drilling into the objects serialized on the wire. For example, in the picture below you can see the properties for an "EXIF" picture object returned from the server via the ASP.NET AJAX Networking stack. Nikhil's utility automatically sniffs these on the client and allows you to easily inspect them:
You can learn more about this free utility here, and read Nikhil's excellent documentation on how to use it here.
Hope this helps,