(UXP script) ScriptMenuActions method addEventListener rejects handler as not function

[vnh68]

I am trying to convert a working ESTK script to InDesign UXP one. It should use the menu.
A new menu item can be added without errors.
The error occurs when registering the "beforeDisplay" and "onInvoke" event handlers with the addEventListener method.
Error:
Invalid value for the "handler" parameter of the add method. Expected File or JavaScript Function but got () .

Then I took the BeforeDisplay.jsx file from the latest version of the SDK for testing and renamed it to BeforeDisplay.idjs. The line #targetengine had to be commented.

The error is the same. The event handler could not be registered.

[Peter Kahrel · Accepted solution]

UXP scripting is still in development -- the lack of support for event handlers is one of the (still) missing features, apparently, For production scripts you'd better stick with ExtendScript.

[vnh68]

Thank you for your answer! Indeed, everything is just getting started and
it will be a long time to use ExtendScript...

I already thought that I missed the moment and started looking for
instructions on this topic.