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Tab caching utility for Firefox

This script can be used to maintain a addon-local copy of the browser tabs' state primarily to avoid performing browser.tabs.query, browser.tabs.get, and browser.sessions.getTabValue function calls.

The script hasn't been tested on chrome.


Depends on js-syncqueue (see Queue).

Required manifest permissions: tabs, sessions.

The tabs.Tab objects that are returned from the cache methods, or passed to the cache event handlers must not be mutated.


Creating the cache: let cache = newCache(myConfig);

Initializing the cache: await cache.init(myInitFunction);

function myOnCreatedHandler(tab) { /*  */ }

// Example configuration
let myListeners = {
	onCreated: myOnCreatedHandler

let myConfig = {
	listeners: myListeners,
	auto: true,
	queue: null,
	tabValueKeys: ['foo', 'bar'],

The listeners object may contain any combination of the following values: onActivated, onAttached, onCreated, onMoved, onRemoved, and onUpdated, corresponding to browser.tabs events. The values are registered as cache event handlers (see Cache Event Handlers).

auto is a boolean which can be used to avoid manually adding the required browser event listeners. If true, it will also create the required queue object (see Queue). This may be set as false if some browser events are not desired to be listened to, but hooking the cache listeners must be done manually.

queue is required only when configuration doesn't have auto: true. The value is the js-syncqueue object used for processing asynchronous browser events in a synchronous manner.

tabValueKeys is an array of keys (strings) used for the sessions api tabValue functions. Values for keys defined in the array are fetched and stored by the cache. This prefetching occurs when initializing the cache itself and in the cache onCreated handler. The fetched values are guaranteed to be available in the cache for myInitFunction and onCreated cache event handler functions.

Cache Events

The cache object has onActivated, onAttached, onCreated, onMoved, onRemoved, and onUpdated keys which provide a similar interface as the corresponding browser.tabs events. Each key supports the following:

  • addListener(myListenerFunction): the function passed as a parameter will be invoked by the cache as described in the Cache Event Handlers section.
  • removeListener(myListenerFunction): removes the specified function from the list of functions invoked when the event is fired.
  • hasListener(myListenerFunction): returns true if the parameter is currently registered as a listener of the event, false otherwise.

Cache Event Handlers

The cache event handlers are the functions passed to the cache in the listeners object, and any function that was added as a listener for one of the cache events. They will be invoked after the cache state has been updated following a browser event. They may be asynchronous and will be awaited on before the cache continues processing further browser events.

  • onActivated will be invoked with arguments tabs.Tab, activeInfo.
  • onAttached will be invoked with arguments tabs.Tab, attachInfo. There will not be a corresponding onDetached event. Instead, the attachInfo contains the values of the detachInfo and the cache will immediately reflect the browser state after the onDetached event. In addition, the cache will re-submit any cached tabValues before invoking the cache event handlers.
  • onCreated will be invoked with the argument tabs.Tab. Any values (if defined) for keys in tabValueKeys will be available in the cache before the cache event handlers are invoked. If prefetching one of the values fails the cache will not invoke any events related to the tab, as the tab will have been closed.
  • onMoved will be invoked with the arguments tabs.Tab, moveInfo.
  • onRemoved will be invoked with arguments tabs.Tab, removeInfo, values. values is an object consisting of the key-value pairs which were present in the cache when the tab was removed. The values may not correspond to the actual values stored by the browser, for example, when the cache values were updated after a tab was removed.
  • onUpdated will be invoked with arguments tabs.Tab, updateProperties.



  • cache.setValue(tabId, key, value) will store a tabValue in the cache. If the argument value doesn't match the currently cached value then browser.sessions tabValue will be updated.
  • cache.getValue(tabId, key) returns the cached tabValue. If the cache was configured with tabValueKeys any keys for which there is a defined value will be available in the cache, otherwise the value must be stored manually with the setValue function.
  • cache.removeValue(tabId, key) removes a value from the cache and browser.sessions.

Tab queries

  • cache.get(tabId) returns the corresponding tabs.Tab object in the cache.
  • cache.getIndexed(windowId, index) returns the corresponding tabs.Tab object in the cache.
  • cache.getActive(windowId) returns the tabs.Tab of the active tab in the window.


  • cache.forEach(callback, windowId, filter) (asynchronous) will invoke callback for every cached tab, passing the tabs.Tab object as the only argument. If windowId is defined, only tabs in the specified window are iterated. If defined, filter will be invoked for every iterated tab with the tabs.Tab as the only argument. If filter returns a value that coerces to false the callback will not be invoked for that tab. callback may be asynchronous, and will be awaited on as a group.
  • cache.forEachWindow(callback) (asynchronous) will invoke callback for every cached window, passing the windowId value as the only argument. callback may be asynchronous, and will be awaited on as a group.


  • cache.init(myInitFunction) (asynchronous) causes the cache to initialize, querying for the current tab data from the browser, updating its internal state, invoking myInitFunction function with the cache itself as the parameter, and finally beginning to process browser events. myInitFunction is optional, may be asynchronous, and will be awaited on.
  • cache.debug() returns an object containing the internal variables in the cache.

Known intentional behaviour differences to Firefox

  • The cache event handlers for tabs.onUpdated will not be invoked before the tabs.onCreated event for a given tab.
  • The index and windowId of tabs.Tab passed to onUpdated cache event handler may differ from the values given to the tabs.onUpdated event callback, particularly when tabs are moved from one window to another.


Internally the script maintains a js-syncqueue object to queue pending browser events while its internal state is updating and the cache event handlers are being awaited on. This also guarantees all browser events are handled, even the ones occurring during cache initialization.

src.js of js-syncqueue must be in scope when using auto:true.

Accessing the queue when using auto: true can be done via let cacheQueue = cache.debug().queue as soon as the cache has been created (not necessarily initialized).

auto: false

For the cache to function correctly, at least the cache.cacheOnCreated, cache.cacheOnRemoved, cache.ocacheOnMoved, and cache.cacheOnAttached listeners must be hooked to the corresponding browser events.

let myQueue = newSyncQueue({ enabled: false });
let cache = newCache({ myQueue });

browser.tabs.onCreated.addEventListener(info => {, info);

/* other listeners */

await cache.init(myInitFunction);



Tiled Tab Groups

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