@basos9 thanks for creating this issue. Are you using the Matomo JavaScript tracker? Or how do you track the data?

Hello,
Yes we are using the Javascript tracking client. The url part mentioned in my comment above, is extracted via the Network tab of the browser's (Chrome) inspect tools (F12)

And I think I have found a clue.
By looking at the url we can see that the cvar url parameter when url decoded is

`cvar={"5":"[\"PAGEVAR\", \"http://test.com/test.html\"]"}`

and this leads to error in tracking

Now if we change by removing the quotes and make the lvalue of the hash a list (instead of a string), like this

`cvar={"5":["PAGEVAR", "http://test.com/test.html"]}`

And url encode and hit, no error is logged and data starts to appear in the database

               custom_var_k5: PAGEVAR
               custom_var_v5: http://test.com/test.html

So i can assume that it might be a javascript client json encoding issue.

Thanks for this @basos9 . Do you know in what browser this happens and whether that site maybe overwrites JSON or JSON.stringify?

Hello,
It happens on major browsers (Chrome, Firefox).
It seems to be related with something in the web page (maybe some JS library), since I was able to successfully track variables by creating a simple html web page with only the tracking code. We are investigating the issue.

Thanks for investigating. Be great to let us know what you find out.

For this webpage we tracked to the inclusion of prototype.js library which was included in our web application.

The affected file has a header

Prototype JavaScript framework, version 1.6.0.3

The problem is fixed when we removed this library.

More specifically this library modified (monkey patches) Builtin in Objects like String and Array, defining among others a .toJSON() method.

Another example using the Chrome's browser console on our site

"test".toJSON()
""test""

JSON.stringify(["king","kong"]);
""[\"king\", \"kong\"]""

and on youtube

"test".toJSON()
VM529:1 Uncaught TypeError: "test".toJSON is not a function
    at <anonymous>:1:8

JSON.stringify(["king","kong"]);
"["king","kong"]"

That version was released in 2008 meaning it is older than the first release of Internet Explorer 8.

So I guess there is not really much to do as one would need to specifically check for this case which would be weird considering we just dropped support for IE<8.

See eg #2165 prototype is kind of known actually to have caused issues with JSON and there isn't anything we can do unfortunately. Ideally the project would look at removing this dependency but I understand this is a big ask and not easily done. AFAIK the only other way would be that you overwrite again some of these changes that prototype does. Not sure if we can maybe close this issue but document somewhere this incompatibility or so?

It seems that this is a library incompatibility issue and the best would be the things mentioned

• either remove the library altogether
• or unpatch the patched buildin objects.

Thank you for helping sort this out.

Noticed we haven't actually documented the requirements / supported browsers anywhere for the JS tracker (at least I couldn't find it) so created matomo-org/developer-documentation#392