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Storage Extension

The storage extension adds the ability to sync store state to/from localStorage or sessionStorage.

Getting Started

Follow the steps below to get started using the storage extension.


Before installing this extension make sure you have installed harlem.

yarn add @harlem/extension-storage
# or
npm install @harlem/extension-storage


To get started simply register this extension with the store you wish to extend.

import storageExtension from '@harlem/extension-storage';

import {
} from 'harlem';

const STATE = {
    firstName: 'Jane',
    lastName: 'Smith'

const {
} = createStore('example', STATE, {
    extensions: [
            type: 'local',
            prefix: 'harlem',
            sync: true,
            restore: false,
            include: '*',
            exclude: [],
            branch: state => state.firstName,
            serialiser: state => JSON.stringify(state),
            parser: value => JSON.parse(value)

The storage extension adds several new methods to the store instance (highlighted above).



The storage extension method accepts an options object with the following properties:

  • type: string - The type of storage interface to use. Acceptable values are local or session. Default value is local.
  • prefix: string - The prefix to use on the storage key. The storage value will be in the form ${prefix}:${storeName}. Default value is harlem.
  • sync: boolean - Whether to automatically sync changes from the storage interface back to the store. Default value is true.
  • restore: boolean - Whether to automatically restore the state back from the storage on load. Default is false.
  • include: (string | regex)[] - A matcher to specify which mutations trigger a storage sync event
  • exclude: (string | regex)[] - A matcher to specify mutation names to exclude from triggering a storage sync event.
  • branch: state => any - A function to a specific sub-branch of state to store as opposed to the whole state tree.
  • serialiser: unknown => string - A function to serialise the store to string. The default behaviour is JSON.stringify.
  • parser: string => unknown - A function to serialise the storage string to a state structure. The default behaviour is JSON.parse.


The default behaviour for serialising/parsing only supports JSON-compatible types. For non-JSON-compatible types please specify a custom serialiser/parser.

See the MDN documentation on JSON serialisation for more information: MDN docs

Manually starting/stopping sync

The startStorageSync and stopStorageSync methods can be used to start or stop sync behaviour.

Clearing storage

Use the clearStorage method to clear all stored data relating to this store.

Restoring storage

Use the restoreStorage method to manually restore the state from storage.

MIT Licensed