browser.notifications.onClosed.addListener(listener)
browser.notifications.onClosed.removeListener(listener)
browser.notifications.onClosed.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 closed.

byUser

boolean. true if the notification was closed by the user, or false if it was closed by the system. This argument is not supported in Firefox.

In this simple example we add a listener to the notifications.onClosed event to listen for system notifications being closed. When this occurs, we log an appropriate message to the console.

browser.notifications.onClosed.addListener((notificationId) => {
  console.log(`Notification ${notificationId} has closed.`);
});