Hello,
your JavaScript code-element does not matter offline (locally). Offline the python-part is of relevance, online it is the JavaScript-part.
Whenever to change the setting from JS to Py, your JS code it “deleted”. For this reason, I either leave the setting set to both at all times or simply insert a JS-only code-element.
See the following example which has a python and JS code-element:
content JS code-element
content python code-element
As you can see, both code-elementes contain quite different code. I do not need to detect a mobile device offline, for instance 
I need the python vpkode-code to run the expeiment locally because vpkode will be used later in the experiment. But as a matter of fact I would not need it if I run the experiment only offline.
I do not understand what you mean with
There was an issue.
caused an error.
Best Jens