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storage.sync

Represents the sync storage area. Items in sync storage are synced by the browser. The data is then available on all instances of the browser the user is logged into (for example, when using Firefox account on desktop versions of Firefox or a Google account on Chrome) across different devices.

For Firefox, a user must have Add-ons checked under the "Sync Settings" options in "about:preferences".

Note that the implementation of storage.sync in Firefox relies on the Add-on ID. If you use storage.sync, you must set an ID for your extension using the browser_specific_settings manifest.json key.

The main use case of this API is to store preferences about your extension and allow the user to sync them to different profiles.

Storage quotas for sync data

The browser enforces limits on the amount of data each extension is allowed to store in the sync area:

Name	Description	Value in bytes
Maximum total size	The maximum total amount of data that each extension is allowed to store in the sync storage area, as measured by the JSON stringification of every value plus every key's length.	102400
Maximum item size	The maximum size of any one item that each extension is allowed to store in the sync storage area, as measured by the JSON stringification of the item's value plus the length of its key.	8192
Maximum number of items	The maximum number of items that each extension can store in the sync storage area.	512

If an extension attempts to store items that exceed these limits, calls to storage.sync.set() are rejected with an error. An extension can use storage.sync.getBytesInUse() to find out how much of its quota is in use.

Synchronization process

In Firefox, extension data is synced every 10 minutes or whenever the user selects Sync Now in Settings > Sync or Firefox account. When the browser performs a sync, for each key stored, it:

compares the value on the server with the value at the last sync; if they are different, the value from the server is written to the key in the browser's sync storage.
compares the browser's sync storage values with the value on the server; if they are different, writes the browser's key value to the server.

This means that, for each key, a change on the server takes precedence over a change in the browser's sync storage.

This mechanism is generally OK for data such as user preferences or other global settings changed by the user.

However, a key's value can be updated on one browser and synchronized then updated on a second browser before the second browser is synchronized, resulting in the local update being overwritten during sync. This mechanism is, therefore, not ideal for data aggregated across devices, such as a count of page views or how many times an option is used. To handle such cases, use storage.sync.onChanged to listen for sync updates from the server (for example, a count of page views on another browser instance). Then adjust the value locally to take the remote value into account (for example, update the total views based on the remote count and new local count).

Methods

The sync object implements the methods defined on the storage.StorageArea type:

storage.sync.get(): Retrieves one or more items from the storage area.
storage.sync.getBytesInUse(): Gets the amount of storage space (in bytes) used for one or more items in the storage area.
storage.sync.set(): Stores one or more items in the storage area. If the item exists, its value is updated.
storage.sync.remove(): Removes one or more items from the storage area.
storage.sync.clear(): Removes all items from the storage area.

Events

The sync object implements the events defined on the storage.StorageArea type:

storage.sync.onChanged: Fires when one or more items in the storage area change.

Example extensions

favourite-colour

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`sync`	Yes	15	53 Before version 79, storage quota limits are not enforced.	?	No	14 ["Safari does not sync items saved in the `sync` storage area, i.e. it behaves the same as the `local` storage area.", "Safari 15 and 15.1 erroneously store sync items in the `local` storage area. If unable to locate sync items, check for sync items in the `local` storage area and do a one-time migration to the `sync` storage area."]	?	?	No	?	15 ["Safari does not sync items saved in the `sync` storage area, i.e. it behaves the same as the `local` storage area.", "Safari 15 and 15.1 erroneously store sync items in the `local` storage area. If unable to locate sync items, check for sync items in the `local` storage area and do a one-time migration to the `sync` storage area."]	?

Note: This API is based on Chromium's chrome.storage API. This documentation is derived from storage.json in the Chromium code.

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https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/sync