Debugging a Mendix widget

I had to debug a custom Mendix widget that was failing. All that was shown on screen in red was “Could not create widget BHCCDropZone.widget.BHCCDropZone”.

We’d recently upgraded to Mendix 7.18.1, so something looked like it happened during the upgrade.

The first step was to make sure the appropriate access rights to the entity the widget uses were right. In this case they were.

Next was to up the log levels from INFO to DEBUG in the Mendix Modeller to see if that gave me any clues. I could see the widget’s constructor and postCreate being run as I had logger.debug calls in those functions being run. However, one of my debug calls was not being reached, so I had narrowed down the location of problem.

After this I added JavaScript breakpoints in the failing method using Chrome’s developer tools. The widget is only loaded the first time the page is reached. When I reached this page I singled stepped through, and found an exception being thrown by the JavaScript but being caught by Mendix. My error was…

"TypeError: Converting circular structure to JSON
    at JSON.stringify ()
    at http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:164930
    at ()
    at t.log (http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:164840)
    at http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:196250
    at http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:166406
    at Array.forEach ()
    at t.log (http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:166377)
    at t.debug (http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:165718)
    at Object.postCreate (http://localhost:8080/widgets/BHCCDropZone/widget/BHCCDropZone.js?636758982080189581:80:11)"

I could now see I had a circular data structure.

Working backwards through the stack trace, I could see the problem was in my widget’s postCreate function, and it was a debug function causing the problem. This was the faulty code causing the widget to fail.

My postCreate function looked like this…

postCreate: function () {
  console.log("BHCCDropZone session", mx.session);
  logger.debug( + ".postCreate");
  logger.debug("this", this);

The faulty line was this…

logger.debug("this", this);

Trying to send the contents of this to the debug logs was causing problems as it was a circular data structure. Removing this line of JavaScript fixed the problem, and the widget started to work again as expected.

I hope this helps others in future when debugging “Could not create widget” errors in Mendix.