Why Does Firebase "on" "value" Not Run The Listener Function When The Reference Doesn't Exist, But "once" "value" Does?

Say I'm listening to a widget: const ref = firebase.database().ref(`widgets/${widgetId}`); ref.on('value', function(snapshot){ console.log(snapshot.val()); // <-- I never see

Solution 1:

I needed to upgrade Firebase to the lastest version (3.0.5 -> 3.2.0)

npm install --save firebase