index.md

---

title: sessions.onChanged
slug: Mozilla/Add-ons/WebExtensions/API/sessions/onChanged
page-type: webextension-api-event
tags:
  - API
  - Add-ons
  - Event
  - Extensions
  - Non-standard
  - Reference
  - WebExtensions
  - onChanged
  - sessions
browser-compat: webextensions.api.sessions.onChanged

---

{{AddonSidebar()}}

Fired whenever the list of closed tabs or windows changes.

Syntax

browser.sessions.onChanged.addListener(listener)
browser.sessions.onChanged.removeListener(listener)
browser.sessions.onChanged.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.

addListener syntax

Parameters

• callback

  • : Function that will be called when this event occurs. It's passed no parameters.

Browser compatibility

{{Compat}}

Examples

This very annoying extension listens for onChanged, then immediately restores the most recently closed session, thus making it impossible to close windows or tabs:

function restoreSession(sessionInfos) {
  if (!sessionInfos.length) {
    console.log("No sessions found")
    return;
  }
  let sessionInfo = sessionInfos[0];
  if (sessionInfo.tab) {
    browser.sessions.restore(sessionInfo.tab.sessionId);
  } else {
    browser.sessions.restore(sessionInfo.window.sessionId);
  }
}

function onError(error) {
  console.log(error);
}

function restoreMostRecent() {
  let gettingSessions = browser.sessions.getRecentlyClosed({
    maxResults: 1
  });
  gettingSessions.then(restoreSession, onError);
}

browser.sessions.onChanged.addListener(restoreMostRecent);

{{WebExtExamples}}

Note: This API is based on Chromium's chrome.sessions API.