Microsoft JScript runtime error: 'Sys' is undefined

"Microsoft JScript runtime error: 'Sys' is undefined" is one of the common error we get when your web.config is not configured properly.

To resolve this issue, we need to have below lines under <system.web> tag.

  <httpHandlers>
   <remove verb="*" path="*.asmx"/>
   <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
   <add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
   <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
  </httpHandlers>
  <httpModules>
   <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
  </httpModules>

If you are getting this error after modifying thw web.config also then you have a another solution in below link,

http://encosia.com/2007/08/16/updated-your-webconfig-but-sys-is-still-undefined/