useSyncedLocalStorage (React)
Save state to localStorage and sync it between hook instances, tabs and windows!
It also has an almost identical interface to useState
.
This is an extended / advanced version of
useLocalStorage
that adds the following
features:
- SSR / SSG support (defaults to
initialValue
on the server, and on initial render to prevent react's hydration warnings) - Synchronisation of state for
key
acrossuseSyncedLocalStorage
instances in the same / different tabs and between windows of the same app.
import type { Dispatch, SetStateAction } from 'react'import { useEffect, useRef, useState } from 'react'import mitt from 'mitt'const em = mitt<Record<string, any>>()// Store which keys are tracked so we don't do unnecessary work for other uses of localStorageconst trackedKeys: Record<string, number> = {}// This block relies on window, so to make sure it only runs on the client// we need to icheck if `window` is definedif (typeof window !== 'undefined') { // We define this globally since it simplifies the already extremely // complicated hook a tiny bit, and we can handle it all in one event handler window.addEventListener('storage', event => { if ( // the `storage` event also fires for `sessonStorage`, and we don't care about that for this hook event.storageArea === localStorage && // Intentionally using `!=` instead of `!==` since it checks `null` and `undefined` event.key != null && trackedKeys[event.key] ) { let parsed try { parsed = JSON.parse(event.newValue ?? '') as unknown } catch { parsed = null } em.emit(event.key, event.newValue == null ? null : parsed) } }) // Same as above - we can handle setting localStorage all in one spot. em.on('*', (key, data) => { localStorage.setItem(key, JSON.stringify(data)) })}type JsonValue = NonNullJsonValue | nulltype NonNullJsonValue = | string | number | boolean | { [key: string]: JsonValue } | JsonValue[]export const useSyncedLocalStorage = < T extends NonNullJsonValue = NonNullJsonValue>( key: string, initialValue: T): [T, Dispatch<SetStateAction<T>>] => { const [state, setState] = useState<T>(initialValue) const initialised = useRef(false) const shouldSync = useRef(false) const emitting = useRef(false) const initialValueRef = useRef(initialValue) useEffect(() => { initialValueRef.current = initialValue }, [initialValue]) useEffect(() => { trackedKeys[key] = (trackedKeys[key] ?? 0) + 1 return () => { trackedKeys[key]-- } }, [key]) useEffect(() => { if (initialised.current) return initialised.current = true const cached = localStorage.getItem(key) if (!cached) { setState(initialValueRef.current) return } try { setState(JSON.parse(cached) as T) } catch (err) { setState(initialValueRef.current) } }, [key]) useEffect(() => { const handler = (data: T | null) => { // If this hook is the one that sent the message, just ignore it if (!emitting.current) { shouldSync.current = false setState(data ?? initialValueRef.current) } } em.on(key, handler) return () => { em.off(key, handler) } }, [key]) useEffect(() => { // Prevents this hook from re-sending an update if (!shouldSync.current) { shouldSync.current = true return } // Prevents this hook from setting itself again emitting.current = true em.emit(key, state) emitting.current = false }, [state, key]) return [state, setState]}
Warning
-
The type of the
state
that you use this hook with cannot be nullable (i.e. cannot benull
orundefined
) - and must be something that can be serialised as JSON (you can't used things likeSet
orMap
). This is enforced in the type signature of the hook so if you're using TypeScript, issues should be caught at compile-time. If you do want the value to be nullable, I recommend wrapping your state in an object (e.g.useSyncedLocalStorage<{ user: User | null }>('user', { user: null })
), or you can modify the hook to suit your needs. -
Unfortunately we have 4 refs of which none are used for DOM elements, and I'm not sure of a better way to get around the issues they solve without adding extra state