browser.notifications.onShown.addListener(listener)
browser.notifications.onShown.removeListener(listener)
browser.notifications.onShown.hasListener(listener)

Events have three functions:

addListener(callback)

Adds a listener to this event.

removeListener(listener)

Stop listening to this event. The listener argument is the listener to remove.

hasListener(listener)

Check whether listener is registered for this event. Returns true if it is listening, false otherwise.

callback

Function that will be called when this event occurs. The function will be passed the following arguments:

notificationId

string. ID of the notification that has been shown.

Add a listener to the notifications.onShown event and log its details:

function logShown(itemId) {
  console.log(`shown: ${itemId}`);
  browser.notifications.getAll().then((all) => {
    console.log(all[itemId]);
  })
}

browser.notifications.onShown.addListener(logShown);