The storage API has an onChanged event to allow the observation of storage changes.

Since Chrome 73, there is also an onChanged event on individual StorageArea methods, such as browser.storage.local.omChanged.

Motivations were given in more detail in this resolved feature request in Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=863277

And the lack of support has been reported as a documentation issue at https://github.com/mdn/content/issues/11411

As I wrote in that issue, storage.sync does support this now, so only local and managed are impacted.

(In reply to Mark Hammond [:markh] [:mhammond] from comment #1)

As I wrote in that issue, storage.sync does support this now, so only local and managed are impacted.

I don't see a definition or implementation of storage.sync.onChanged.

There is storage.onChanged, but not an event on individual StorageArea objects:

• storage.local.onChanged
• storage.sync.onChanged
• storage.managed.onChanged

storage.sync's API is defined at https://searchfox.org/mozilla-central/rev/61fcf20da23053d54f9a057e208dd5e7407a977c/toolkit/components/extensions/schemas/storage.json#159-287

storage.sync is implemented at https://searchfox.org/mozilla-central/rev/61fcf20da23053d54f9a057e208dd5e7407a977c/toolkit/components/extensions/child/ext-storage.js#281-296 and https://searchfox.org/mozilla-central/rev/61fcf20da23053d54f9a057e208dd5e7407a977c/toolkit/components/extensions/parent/ext-storage.js#145-166, but there is no onChanged event there.

It's implemented at https://searchfox.org/mozilla-central/rev/b0779bcc485dc1c04334dfb9ea024cbfff7b961a/toolkit/components/extensions/ExtensionStorageSync.jsm#72-80 and https://searchfox.org/mozilla-central/rev/b0779bcc485dc1c04334dfb9ea024cbfff7b961a/toolkit/components/extensions/ExtensionStorageSync.jsm#161-183 - but maybe that's not hooked up for extension to actually use?

That logic is hooked up with the storage.onChanged event at https://searchfox.org/mozilla-central/rev/b0779bcc485dc1c04334dfb9ea024cbfff7b961a/toolkit/components/extensions/parent/ext-storage.js#204

Note that StorageArea.onChanged and storage.onChanged are two different things. Although the implementation can use similar/shared logic, the interfaces exposed to extensions are distinct.

I'm going to refactor the storage event handler soon, so I may as well take this bug too.

This will need documentation, plus a BCD entry. Support is Chrome 73+, Safari, Opera+Edge (both Chromium based), Firefox 101+.

https://hg.mozilla.org/mozilla-central/rev/ab8202d430a8

Documentation addressed in Support for StorageArea.onChanged and Session #16450 (BCD) and https://github.com/mdn/content/pull/16873 (docs)