I probably should have used the term "notification" rather than "alert", because a JavaScript alert would be a pain.

I think this would involve adjusting jeToggle() so that it sends a message toServer() when jeEnabled is changed. The message would probably need to hold the playerName of the person who toggled their JSON editor, and the state of their jeEnabled.

playerName changes would also need to send 2 messages, sending false for the old playerName, and jeEnabled for the new playerName. There would also need to be a message sent when someone leaves the game (if their JSON editor was open) - sending false for jeEnabled.

The toServer() message needs to be handled in messageReceived in server/player.mjs.

if(func == 'jeToggle')
        this.room.jeToggle(args); 

Then probably something like
this.broadcast('jeToggle', { ... })
in server/room.mjs

Then something like
onMessage('jeToggle', function() { ... })
on the client side, and that function would need to somehow (e.g. a variable) track who has their JSON editor open, and eliminate the names in that array when they close their editor (or leave, or change names). Then when the array isn't empty add a class to the editor menu icon, and when it is empty remove the class to trigger css color changes.

export function jeToggle() {
    null === jeEnabled && (jeInitTree(),
    jeAddCommands(),
    jeEmpty(),
    $("#jeText").addEventListener("input", jeColorize),
    $("#jeText").onscroll = (e => $("#jeTextHighlight").scrollTop = e.target.scrollTop)),
    jeEnabled = !jeEnabled,
    setJEenabled(jeEnabled),
    jeLoggingHTML = "",
    jeEnabled ? ($("body").classList.add("jsonEdit"),
    jeWidget && !widgets.has(jeWidget.id) && jeEmpty(),
    jeDebugViewing && (jeCallCommand(jeCommands.find(e => "je_toggleDebug" == e.id)),
    jeShowCommands())) : $("body").classList.remove("jsonEdit")
}

Also, I noticed that this seems redundant.

    jeEnabled = !jeEnabled,
    setJEenabled(jeEnabled),

function setJEenabled(v) {
jeEnabled = v;
 }

I don't really see the point of setJEenabled() if all the code to open the editor isn't in that function.

And if there is a purpose I am missing, then those two lines above should probably just be setJEenabled(!jeEnabled),.

On edit: It looks like toggleEditMode() and the edit variable were the ones to use rather than jeToggle() and jeEnabled, which are specific for the {} JSON setting in the editor.