Can't call Javascript on Safari through Applescript

Question

I'm trying to determine if a tab is playing audio via javascript, which is being called by the following applescript:

tell application "Safari"
    set theScript to "!!Array.prototype.find.call(document.querySelectorAll('audio,video'),function(elem){return elem.duration > 0 && !elem.paused});"
    set ret to do JavaScript theScript in current tab of first window
    return ret
end tell

However, I get the following error:

error "The variable ret is not defined." number -2753 from "ret"

This javascript works fine when called through the Safari Console. I have tried investigating missing imports that are loaded in Console but not via the call from Applescript, but can't seem to solve this. Please could someone help me?

Edit:

Running the following works, so the problem lies with the specific javascript I am trying to run

tell application "Safari"
    set theScript to "numbervalue=3;fred=numbervalue+4"
    set ret to do JavaScript theScript in current tab of first window
    return ret
end tell

returns 7 as expected

Answer 1

For some strange reason do JavaScript has trouble returning booleans, so after hours of trying things this ended up working:

tell application "Safari"
    set theScript to "a = !!Array.prototype.find.call(document.querySelectorAll('audio,video'),function(elem){return elem.duration > 0 && !elem.paused}); a.toString();"
    set ret to do JavaScript theScript in current tab of first window
    return ret
end tell

BUT this is only a problem when calling in Safari! Hope this helps a poor soul out