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AJAX Error: 'Sys' is undefined

If you are using AJAX enabled web site or project, you may come across this error message. I got this error message too and spent few hours to fix this problem. I tried finding all possible solutions on Google and finally concluded with following three steps.

    • Web.Config: Make sure your web.config is correctly configured to handle AJAX enabled request. You might have created a project which was not AJAX enabled earlier and now you want to Ajaxfy. The best solution is creating a new project ASP.NET AJAX Enabled Web Site. It will create a Web.config file by default. Compare this Web.config file with your other project’s Web.config file which is casuing the error. Make sure that you are not missing any new section. You can simply copy and paste it at appropriate place. For detail information you can also visit at
    •  Using Master Page: If your ASP.NET web page is using a Master Page, make sure your Script Manager Control is placed in Master Page only. Content page should not be using Script Manager Control.
    • Regular Web Page: If you are not using a Master Page then each ASP.NET web page must have its own Script Manager Control.

Comments (16) -

  • Anand

    7/5/2007 3:48:20 PM | Reply

    Thanks a lot, pyaare Vishamohan. Your article was the only way I could solve this Sys not defined bug. Thanks again. & keep posting such queer bug-fixes. They help a lot!

  • Anju

    9/20/2007 7:57:30 AM | Reply


    Thanks a lot. Its very helpful.

  • Ryan

    2/3/2008 12:56:13 PM | Reply

    Wonderful post! I have been piling through blog after blog and posting everywhere I know and have not been able to resolve this error.

    "If your ASP.NET web page is using a Master Page, make sure your Script Manager Control is placed in Master Page only. Content page should not be using Script Manager Control."

    Brilliant!!!! Thank you!

  • Anil Soni

    3/2/2008 7:22:16 AM | Reply

    Hi dear

    The above define 3 option is very right, I was fcing a javascript problem "Sys Undifine"  then i did follow your 1st point and modify my Web config and my problem got solve.


  • Ammar

    5/22/2008 5:20:09 AM | Reply

    Hi Vishwa, I have tried it all but still can't get it to work !!! I have copied the Web.Config from the AjaxEnabled website but still I can't get rid of this DUMB ERROR

  • Chris

    7/25/2008 1:01:07 PM | Reply

    Thank you your tip on creating a correct AJAX web.config saved me a lot of time

  • Dil

    8/24/2008 7:24:22 PM | Reply

    Excellent!! Thanks Man.!!

  • Internet marketing

    9/6/2009 1:39:53 PM | Reply

    Admiring the time and effort you put into your blog and detailed information you offer!

  • Orlando A

    9/16/2009 11:28:23 PM | Reply

    I wanted to thank you for this great read!! I definitely enjoying every little bit of it I have you bookmarked to check out new stuff you post

  • Orlando A

    9/19/2009 7:04:57 AM | Reply

    Nice information, many thanks to the author. It is incomprehensible to me now, but in general, the usefulness and significance is overwhelming. Thanks again and good luck!

  • Name Change UK

    10/1/2009 8:43:39 PM | Reply

    Whew! Never thought that I would find the solution here on your blog. Works like a charm dude, thanks!

  • yogesh

    11/16/2009 9:27:01 PM | Reply

    Hello Dear,

       I got the solution bro thanks a lot.

  • Sebastian

    3/31/2010 11:27:15 AM | Reply

    Thank you very much. It was very helpful.

  • numeros des casinos

    5/18/2010 9:53:30 PM | Reply

    If you’re going to be generating a lot of code each time, like in my example, the AJAX route makes more sense because you can have a separate page (PHP for me, but you could use a straight HTML page) to write that content in HTML, then use an AJAX call to include that page in your main page.. Any time you want to change the teensiest thing, you have two places in which you need to change it: the original HTML, and the JavaScript that creates the same chunk of HTML before inserting it into your page. Going the AJAX way here would be best.

  • Jun

    5/26/2010 7:22:19 PM | Reply

    You are the best one. Thanks a lot !!