How To Fix: ‘Sys’ is undefined or ‘AjaxControlToolkit’ is undefined javascript errors
I developed an ASP.NET application for my company that is configured on several laptops. Yesterday I was given the task to fix an error that was causing the ASP.NET Ajax features to flake and not work. I spent hours on Google searching for an answer. If you did before reading this article, I wont be asking you to edit your web.config file or setting the CombineScripts property for the ToolkitScriptManager to false. I tried all of those and alas the problem remained. You are free to try those if my solution does not work for you.
I fixed this issue by pure chance. While troubleshooting, I realized that the clock was showing the incorrect date and time. I merely corrected that and realized soon after that everything worked flawlessly. There were no more ‘Sys’ is undefined or ‘AjaxControlToolkit’ is undefined javascript errors. I figured this error occurs since the last modified date on the assemblies associated with ‘Sys’ and ‘AjaxControlToolkit’ would have had a future date and caused a problem. What problem specifically? I am unable to answer even after some research on Google.
I hope that my solution will save you some time. Please let me know in the comments if it helped!