Sometimes while you’re developing an application you need to track potential errors that can creep in post-deployment. Debugging and tracing are two features that come in handy for tracking such errors. In the case of ASP.NET AJAX applications, debugging is a challenge because these applications contain a mix of server-side and client-side code. The client-side code can contain scripts, styles, HTML markups, etc.
However, you can overcome these obstacles and continue to debug and trace your ASP.NET AJAX applications much the same way you debug and trace an ASP.NET application. The Sys.Debug class in the Sys namespace contains a list of methods that you can use to debug your ASP.NET AJAX application, handle trace output, and display the trace messages.
In his WebReference article, Joydip Kanjilal explains these built-in debugging and tracing features and how you can work with them in your ASP.NET Ajax applications.