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If you're using Svelte v3 or v4 to make a WebExtension for Firefox or Chrome, you can use this to create writable stores that are backed by your extension's storage. Handy features are provided to you right out of the box:

  • Flexible: This package can work with a part of or the entirety of any area of, including areas from 3rd-party packages.
  • Automatic batching: Save on storage writes when all your store changes are batched up to be sent out at the next Svelte tick.
  • Live updates: If you'd like, this package will handle listening for storage changes for you, even in 3rd-party area (with their support).

This project has a Code of Conduct. By participating in the Git repo or issues tracker, you agree to be as courteous, welcoming, and generally a lovely person as its terms require. 😊

Default export: webextStorageAdapter()

Parameters: storageArea (string or StorageArea), keys (string, array, object, or null), optional options (object)
Returns a store group

Creates a group of Svelte v3 writable stores, populated from & persisted to It will immediately request the needed values; this is asynchronous, but the store group will be returned synchronously with all stores in place. You can use the store group's ready promise to determine when values from storage are available.

// Example

import webextStorageAdapter from "svelte-webext-storage-adapter";
var storeGroup = webextStorageAdapter("sync", ["thisKey", "thatKey"]);
storeGroup.ready.then( () => {
	// You can now interact with data in storage
} );

Parameter: storageArea

string or StorageArea

The object that will be read from and written to. If it's a string, it's used to look up the object from Objects from outside may be used if they meet the implementation requirements.

// Example: These are equivalent
var group1 = webextStorageAdapter(, null);
var group2 = webextStorageAdapter("local", null);

Parameter: keys

string, array, object, or null

This can be any of the same values accepted by StorageArea.get. Using null will allow the store group to read and write any key in storage. Unlike StorageArea.get, default values specified in the object form will survive the round trip regardless of type.

// Example: Specifying keys with default values
var storeGroup = webextStorageAdapter("sync", {
	i: "he",
	you: "me",
	we: "all together",
storeGroup.stores.we.subscribe(console.log); // logs "all together"

Parameter: options


This parameter and all its properties are optional. There is currently only one option; more may be added later.


Default: true

If true, webextStorageAdapter will listen for changes to storageArea and propagate them to the stores. If a key is deleted from storage, this will set the key's corresponding store to its default value if one was specified in the keys parameter, or undefined otherwise.

This will prevent the store group from being garbage-collected. If this is a concern, you can call the group's unLive method when you're done with it.

This throws an error if used with a 3rd-party area that doesn't support it.

// Example

// In your extension's tab page / browser action popup / etc.
import webextStorageAdapter from "svelte-webext-storage-adapter";
// The live option defaults to true, but we explicitly set it here for demonstration purposes
var storeGroup = webextStorageAdapter("sync", "noise", {live: true});
storeGroup.stores.noise.subscribe( (value) => console.log(value) );
chrome.runtime.sendMessage("Sound the alarm!");

// In your extension's background page
chrome.runtime.onMessage.addListener( (message) => {
	if (message == "Sound the alarm!") {{noise: "awooga"}); // The above script will eventually log "awooga"
} );

Store groups


Store groups are returned synchronously from webextStorageAdapter.

Property: stores

Object (prototype: null)

This has a property for every key specified in keys, and each property's value is a Svelte writable store containing the corresponding value in storageArea. If keys was null, then stores is a Proxy that allows getting any property to obtain a usable store, but operations such as Object.keys(),, etc. will only expose keys that are known to have values.

New values introduced via the stores' set and update methods are batched up into a single storageArea.set() call that happens when Svelte's tick() resolves. The stores themselves will have their new values immediately.

To help avoid bugs in your code, directly setting any property of stores is disallowed, and will throw an error in strict mode. Remember to use stores.example.set(value) instead of stores.example = value!

// Example

import webextStorageAdapter from "svelte-webext-storage-adapter";
var storeGroup = webextStorageAdapter("sync", null);
// Since storeGroup.ready hasn't resolved yet, no data will be loaded...
console.log( Object.keys(storeGroup.stores) ); // []
// ... but we can set data and have it sent to extension storage anyways
storeGroup.stores.tooBad.set("Waluigi time");
console.log( Object.keys(storeGroup.stores) ); // ["tooBad"]

// For non-null keys parameters, the specified keys are always present
var storeGroup2 = webextStorageAdapter("sync", ["thing1", "thing2"]);
console.log( Object.keys(storeGroup2.stores) ); // ["thing1", "thing2"]

Property: ready


Resolves with true after all stores have been set to values received from the initial storageArea.get call, or rejects with a reported error object.

// Example

// In a Svelte component's <script> block
import webextStorageAdapter from "svelte-webext-storage-adapter";
var { stores, ready, unLive } = webextStorageAdapter("sync", null);
var preparations = Promise.all([ready, ...otherAsyncWork]);
<!-- In the same component's markup -->
{#await preparations}

Property: onWrite()

Parameter: subscriber (function with signature (write, setItems))
Returns a function

Functions passed to this method are subscribed to be called when data starts getting written to storage. The return value can be called to unsubscribe the subscriber. Subscribers are called with two parameters:

  • write is a Promise that resolves with true when the write is finished, or rejects with an object if there's an error. The rejection object has properties error with the error reported from chrome.runtime.lastError, and setItems which is the same as below.
  • setItems is an object of key/value pairs, the same one passed to storageArea.set.
// Example: Save indicator
/* Save indicators usually aren't needed for WebExtensions, as you're typically
saving small amounts of data to a local device. */

// In a Svelte component's <script> block
import { onDestroy } from "svelte";
var activeWriteCount = 0;
var unsubscribe = storeGroup.onWrite( (write) => {
	activeWriteCount += 1;
	write.finally( () => { activeWriteCount -= 1; } );
} );
<!-- In the same component's markup -->
{#if activeWriteCount}

Handling write errors with unhandledrejection

The write promise described above is created whether or not onWrite has subscribers. If no subscriber adds a rejection handler to it, a storage write error causes an unhandledrejection event. If this event also has no handlers, or none of them call event.preventDefault(), the rejection is logged to the browser console.

// Example

window.addEventHandler("unhandledrejection", (event) => {
	if ("setItems" in event.reason) {
		let {error, setItems} = event.reason;

Property: unLive()

No parameters

Reverses the effects of the live: true option so that the store group can be garbage-collected.

// Example

// In a Svelte component's <script> block
import webextStorageAdapter from "svelte-webext-storage-adapter";
import { onDestroy } from "svelte";

var { stores, ready, unLive } = webextStorageAdapter("sync", "myKey");
onDestroy( unLive );

Methods don't use this

The onWrite and unLive methods of store groups don't use this, and may be freely assigned to other variables or properties without breaking their connection to their original store group.

// Example

var stores = webextStorageAdapter("sync", "myKey");
var { unLive } = stores;
// These do the same thing

Implementation requirements for StorageArea

Any object you pass in via the storageArea parameter will work if it implements the following interface:

get and set must accept callbacks; any returned Promise will not be used. They may indicate errors either by setting chrome.runtime.lastError (⚠ but they need to delete it afterwards), or by passing an extra parameter to the callbacks.

Tip: Use with svelte-writable-derived

Full disclosure: I, Pixievolt, am the author of svelte-writable-derived. If you find this tip helpful, now you have two reasons to support me!

Chrome's implementation of can't safely store anything besides booleans, numbers, strings, and arrays. If you need something more complex, you can use the svelte-writable-derived package to translate your data to & from its storage-safe form.

import webextStorageAdapter from "svelte-webext-storage-adapter";
import writableDerived from "svelte-writable-derived";

var {stores, ready} = webextStorageAdapter({
	"key": `{"storedAsJson":true}`,
exportedStores = Object.assign({}, stores, {
	"key": writableDerived(
		(json) => JSON.parse(json),
		(data) => JSON.stringify(data)
export { exportedStores as stores, ready, exportedStores as default };

Browser compatibility

This package officially supports Firefox and Chrome. Other browsers that support WebExtensions aren't tested, but bug reports and pull requests for them are welcome. Firefox version 101+ and Chrome version 84+ support all additional requirements listed below.

Support for ECMAScript 2020 is required (see caniuse data for optional chaining). Transpilers & polyfills are supported provided keys is not null.

If keys is null, support for WeakRef is additionally required.

If live is true, support for StorageArea.onChanged is additionally required. A polyfill may be used.

💖 Support the developer

I muchly appreciate any way you'd like to show your thanks - knowing people are helped gives me warm fuzzies and makes it all worthwhile!

💸 ... with money

At my Ko-Fi page, you can make a one-time or monthly donation, or commission work on an issue.

You can also support all your dependencies at once using StackAid!

💌 ... with kind words

Current contact info is on this page - or you can create an "issue" on this repo just to say thanks! Thank-you "issues" will be closed right away, but are treasured regardless~

🤝 ... with a job

Want me to sling some JavaScript for you? Look over my other work and contact me!

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