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OSweb error when using JATOS in Firefox browser

Hi there!

I'm trying to prepare my first OSweb experiment to run it with JATOS. My experiment is running fine in the test mode (started from OpenSesame directly) and also when I use JATOS (the local version) in Safari or Chromse as a browser. However, when I use Firefox, I get the following error:

Application Error: TypeError: cannot use 'in' operator to search for "data" in " "

This issue has only popped up now after I updated to the latest JATOS version today. I also made a tiny change to my OSweb experiment (but really only the wording of the instruction, so nothing that should cause a problem) and imported the updated experiment. Before that, I had no problems with Firefox.

Any idea what the reason could be or how to solve it? I could probably ask my participants to not use firefox, but I'd rather solve the underlying issue, if possible.

Thanks a lot!


  • Same here! I could start the experiment nonetheless, but it crashed after a couple of runs of the first loop. In Chrome it works just fine, but since many of my participants are likely to use Firefox as a standard browser, I would be very happy if I could make it run in Firefox as well.

  • Hi everyone, I do not get those errors in any browser (incl FF). Can you please send me an JATOS .jzip so I'm able to reproduce the issue?



  • edited January 2022

    Hi Kristian!

    Hm, I cannot upload it here, it seems. Not sure why. You should be able to download it from this link:

    Also, it's a .zip that OpenSesame creates for me, not a jzip. Could that be part of the problem?

    Thanks for your efforts!

  • I got your JATOS .zip/.jzip (.zip is fine too). I tried and had the same error message in 3.7.1 and not in 3.6.1. It looks like difficult to debug. But I won't have time today to look at it. Hopefully tomorrow.



  • @JaHa, I think I found a fix. It seems to have been an issue with Webpack that is used to pack jatos.js since the last version 3.7.1. But can you please confirm that it works on your side. I made a fixed jatos.js and load it in your study from the assets:

    Then if it works for you too I will to a new JATOS release (3.7.2) with the fixed jatos.js.



  • @kri, sorry for the late reply. I didn't check back here for a couple of days and only got around to try it out now. It works perfectly! I also do not have any problems with transferring the data (as the others report in the other thread

    So, did I get it right that it was a general problem (not specifically only for my experiement) and you will fix it in a new JATOS release? Because I realized I need to make some further changes to my experiement, so I can't just continue with the fixed zip you uploaded.

    Thanks a lot!

  • @JaHa The new JATOS version 3.7.2 is already out and you can use it. But if you want you can continue using the fixed zip I send you. It basically contains the same fix as in the new version.

  • Perfect! Thanks a lot!

  • Hi there!

    Reviving this thread because I'm encountering a similar problem like more than a year ago :(

    I wanted to implement a new experiment and again my experiment is running fine in test mode (started from OpenSesame directly in an external browser). When I upload it on my local JATOS installation it works with Safari or Chrome, but when using Firefox, I get the error:

    Application Error: TypeError: cannot use 'in' operator to search for "data" in " "

    In addition, it says "Sending data. Please wait..." on the screen the whole time (sceenshot attached). Nonetheless, the experiment works, I can do it an give answers and everything, but it is a bit annoying.

    I updated my JATOS version to 3.8.2, because @kri, you said above that newer versions include a fix. However, I still encounter the same problem.

    I also tried to run an older experiment on my local JATOS (one that worked before) and I also encounter the problem there. It only happens with OSWeb Components, my questionnaire components using lab.js html forms works fine.

    I would be happy if anyone could help my fix this or at least point me in the correct direction.

    Or can I assume that it's a problem specific for my local JATOS and should disappear when I upload it to the mindprobe server? In this case, I could also just ignore it, I guess)

    Any help very much appreciated! Thanks!


  • Hi Jana,

    I doubt that this issue is a JATOS one. Your error msg "Application Error: TypeError: cannot use 'in' operator to search ..." puts it more in the JavaScript/browser side and there, since it only happens with OpenSesame components, I'd search in OpenSesame/OSWeb first. Unfortunately I'm not able to help you much there.



  • Hi @kri,

    thanks a lot for the JavaScript hint. I re-installed both JATOS and Java from scratch (instead of just updating) and it seems to be working now. So I don't know what exactly the problem was, but it's solved now!



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