Nano Stores Persistent

A smart store for Nano Stores state manager to keep data in localStorage and synchronize changes between browser tabs.

• Small. from 281 bytes (minified and brotlied). Zero dependencies. It uses Size Limit to control size.
• It has good TypeScript.
• Framework agnostic. It supports SSR. localStorage can be switched to another storage.

import { persistentAtom } from '@nanostores/persistent'

export const locale = persistentAtom('locale', 'en')

---

Made in Evil Martians, product consulting for developer tools.

---

Install

npm install nanostores @nanostores/persistent

Usage

See Nano Stores docs about using the store and subscribing to store’s changes in UI frameworks.

Primitive Store

The store with primitive value keeps the whole data in the single localStorage key.

import { persistentAtom } from '@nanostores/persistent'

export const shoppingCart = persistentAtom<Product[]>('cart', [], {
  encode: JSON.stringify,
  decode: JSON.parse,
})

This store will keep its value in cart key oflocalStorage. An empty array [] will be initial value on missed key in localStorage.

You can change store value by set method.

shoppingCart.set([...shoppingCart.get(), newProduct])

You can store the object in a primitive store too. But Persistent Map store is better, because map store will update value if you add a new key to the initial value.

Map Store

There is a special key-value map store. It will keep each key in separated localStorage key.

import { persistentMap } from '@nanostores/persistent'

export type SettingsValue = {
  sidebar: 'show' | 'hide',
  theme: 'dark' | 'light' | 'auto'
}

export const settings = persistentMap<SettingsValue>('settings:', {
  sidebar: 'show',
  theme: 'auto'
})

This store will keep value in settings:sidebar and settings:theme keys.

You can change the key by setKey method:

settings.setKey('sidebar', 'hide')

Sync between Browser Tabs

By default, the store changes will be synchronized between browser tabs.

There is a listen option to disable synchronization.

import { persistentAtom } from '@nanostores/persistent'

export const draft = persistentAtom('draft', '', { listen: false })

Value Encoding

encode and decode options can be set to process a value before setting or after getting it from the persistent storage.

import { persistentAtom } from '@nanostores/persistent'

export const draft = persistentAtom('draft', [], {
  encode (value) {
    return JSON.stringify(value)
  },
  decode (value ) {
    try {
      return JSON.parse(value)
    } catch() {
      return value
    }
  }
})

Server-Side Rendering

The store has built-in SSR support. On the server, they will use empty objects instead of localStorage.

You can manually initialize stores with specific data:

if (isServer) {
  locale.set(user.locale)
}

Persistent Engines

You can switch localStorage to any other storage for all used stores.

import { setPersistentEngine } from '@nanostores/persistent'

let listeners = []
function onChange (key, newValue) {
  const event = { key, newValue }
  for (const i of listeners) i(event)
}

// Must implement storage[key] = value, storage[key], and delete storage[key]
const storage = new Proxy({}, {
  set(target, name, value) {
    target[name] = value
    onChange(name, value)
  },
  get(target, name) {
    return target[name]
  },
  deleteProperty(target, name) {
    delete target[name]
    onChange(name, undefined)
  }
})

// Must implement addEventListener and removeEventListener
const events = {
  addEventListener (key, callback) {
    listeners.push(callback)
  },
  removeEventListener (key, callback) {
    listeners = listeners.filter(i => i !== callback)
  },
  // window dispatches "storage" events for any key change
  // => One listener for all map keys is enough
  perKey: false
}

setPersistentEngine(storage, events)

You do not need to do anything for server-side rendering. We have build-in support.

You need to specify bodies of events.addEventListener and events.removeEventListener only for environments with browser tabs or another reasons for storage synchronization.

perKey makes PersistentMap add one listener for each of its keys in addition to the one for all keys. This is relevant when events for key changes are only dispatched for keys that were specifically subscribed too.

For TypeScript, we have PersistentListener and PersistentEvent types for events object.

import { PersistentListener, PersistentEvent } from '@nanostores/persistent'

const events = {
  addEventListener (key: string, callback: PersistentListener) {
    …
  },
  removeEventListener (key: string, callback: PersistentListener) {
    …
  }
}

function onChange () {
  const event: PersistentEvent = {
    key: 'locale' // Changed storage key
    newValue: 'ru'
  }
  …
}

Tests

There is a special API to replace localStorage to a fake storage engine with helpers to change key and get all values.

import {
  useTestStorageEngine,
  setTestStorageKey,
  cleanTestStorage,
  getTestStorage,
} from '@nanostores/persistent'

import { settings } from './storage.js'

beforeAll(() => {
  useTestStorageEngine()
})

afterEach(() => {
  cleanTestStorage()
})

it('listens for changes', () => {
  setTestStorageKey('settings:locale', 'ru')
  expect(settings.get()).toEqual({ locale: 'ru' })
})

it('changes storage', () => {
  settings.setKey('locale')
  expect(getTestStorage()).toEqual({ 'settings:locale': 'ru' })
})