windows.onRemoved

browser.windows.onRemoved.addListener(listener)
browser.windows.onRemoved.removeListener(listener)
browser.windows.onRemoved.hasListener(listener)

Events have three functions:

addListener(listener)

Adds a listener to this event.

removeListener(listener)

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

hasListener(listener)

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

listener

The function called when this event occurs. The function is passed this argument:

windowId

integer. ID of the window that was closed.

Log the IDs of windows as they are removed.

browser.windows.onRemoved.addListener((windowId) => {
  console.log(`Closed window: ${windowId}`);
});