Chrome extension: detect when popup window closes

Question

For my chrome extension, I want to perform an action when the browserAction popup window closes. I understand that there no built-in events are triggered when this happens.

Answer 1

onDisconnect is not an assignable property.
It's an object that provides addListener method to register a callback:

externalPort.onDisconnect.addListener(function() {
    var ignoreError = chrome.runtime.lastError;
    console.log("onDisconnect");
});