| title | nav |
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Persist middleware |
15 |
The persist middleware enables you to store your Zustand state in a storage (e.g. localStorage, AsyncStorage, IndexedDB, etc...) thus persisting it's data.
Note that this middleware does support both synchronous storages (e.g. localStorage) and asynchronous storages (e.g. AsyncStorage), but using an asynchronous storage does come with a cost.
See Hydration and asynchronous storages for more details.
Quick example:
import create from 'zustand'
import { persist } from 'zustand/middleware'
export const useFishStore = create(
persist(
(set, get) => ({
fishes: 0,
addAFish: () => set({ fishes: get().fishes + 1 }),
}),
{
name: 'food-storage', // name of item in the storage (must be unique)
getStorage: () => sessionStorage, // (optional) by default the 'localStorage' is used
}
)
)See Options for more details.
This is the only required option. The given name is going to be the key used to store your Zustand state in the storage, so it must be unique.
Default:
() => localStorage
Enables you to use your own storage. Simply pass a function that returns the storage you want to use.
Example:
export const useBoundStore = create(
persist(
(set, get) => ({
// ...
}),
{
// ...
getStorage: () => AsyncStorage,
}
)
)The given storage must match the following interface:
interface Storage {
getItem: (name: string) => string | null | Promise<string | null>
setItem: (name: string, value: string) => void | Promise<void>
removeItem: (name: string) => void | Promise<void>
}Schema:
(state: Object) => string | Promise<string>
Default:
(state) => JSON.stringify(state)
Since the only way to store an object in a storage is via a string, you can use this option to give a custom function to serialize your state to a string.
For example, if you want to store your state in base64:
export const useBoundStore = create(
persist(
(set, get) => ({
// ...
}),
{
// ...
serialize: (state) => btoa(JSON.stringify(state)),
}
)
)Note that you would also need a custom deserialize function to make this work properly. See below.
Schema:
(str: string) => Object | Promise<Object>
Default:
(str) => JSON.parse(str)
If you pass a custom serialize function, you will most likely need to pass a custom deserialize function as well.
To continue the example above, you could deserialize the base64 value using the following:
export const useBoundStore = create(
persist(
(set, get) => ({
// ...
}),
{
// ...
deserialize: (str) => JSON.parse(atob(str)),
}
)
)Schema:
(state: Object) => Object
Default:
(state) => state
Enables you to omit some of the state's fields to be stored in the storage.
You could omit multiple fields using the following:
export const useBoundStore = create(
persist(
(set, get) => ({
foo: 0,
bar: 1,
}),
{
// ...
partialize: (state) =>
Object.fromEntries(
Object.entries(state).filter(([key]) => !['foo'].includes(key))
),
}
)
)Or you could allow only specific fields using the following:
export const useBoundStore = create(
persist(
(set, get) => ({
foo: 0,
bar: 1,
}),
{
// ...
partialize: (state) => ({ foo: state.foo }),
}
)
)Schema:
(state: Object) => ((state?: Object, error?: Error) => void) | void
This option enables you to pass a listener function that will be called when the storage is hydrated.
Example:
export const useBoundStore = create(
persist(
(set, get) => ({
// ...
}),
{
// ...
onRehydrateStorage: (state) => {
console.log('hydration starts')
// optional
return (state, error) => {
if (error) {
console.log('an error happened during hydration', error)
} else {
console.log('hydration finished')
}
}
},
}
)
)Schema:
number
Default:
0
If you want to introduce a breaking change in your storage (e.g. renaming a field), you can specify a new version number. By default, if the version in the storage does not match the version in the code, the stored value won't be used.
See the migrate option below for more details about handling breaking changes.
Schema:
(persistedState: Object, version: number) => Object | Promise<Object>
Default:
(persistedState) => persistedState
You can use this option to handle versions migration. The migrate function takes the persisted state and the version number as arguments. It must return a state that is compliant to the latest version (the version in the code).
For instance, if you want to rename a field, you can use the following:
export const useBoundStore = create(
persist(
(set, get) => ({
newField: 0, // let's say this field was named otherwise in version 0
}),
{
// ...
version: 1, // a migration will be triggered if the version in the storage mismatches this one
migrate: (persistedState, version) => {
if (version === 0) {
// if the stored value is in version 0, we rename the field to the new name
persistedState.newField = persistedState.oldField
delete persistedState.oldField
}
return persistedState
},
}
)
)Schema:
(persistedState: Object, currentState: Object) => Object
Default:
(persistedState, currentState) => ({ ...currentState, ...persistedState })
In some cases, you might want to use a custom merge function to merge the persisted value with the current state.
By default, the middleware does a shallow merge. The shallow merge might not be enough if you have partially persisted nested objects. For instance, if the storage contains the following:
{
foo: {
bar: 0,
}
}But your Zustand store contains:
{
foo: {
bar: 0,
baz: 1,
}
}The shallow merge will erase the baz field from the foo object.
One way to fix this would be to give a custom deep merge function:
export const useBoundStore = create(
persist(
(set, get) => ({
foo: {
bar: 0,
baz: 1,
},
}),
{
// ...
merge: (persistedState, currentState) =>
deepMerge(currentState, persistedState),
}
)
)Version: >=3.6.3
The persist api enables you to do numbers of interactions with the persist middleware from inside or outside a React component.
Schema:
() => Partial<PersistOptions>
This method can you get the options of the middleware.
For example, it can be used to obtain the storage name:
useBoundStore.persist.getOptions().nameSchema:
(newOptions: PersistOptions) => void
This method enables you to change the middleware options. Note that the new options will be merged with the current ones.
For instance, this can be used to change the storage name:
useBoundStore.persist.setOptions({
name: 'new-name',
})Or even to change the storage engine:
useBoundStore.persist.setOptions({
getStorage: () => sessionStorage,
})Schema:
() => void
This can be used to fully clear the persisted value in the storage.
useBoundStore.persist.clearStorage()Schema:
() => Promise<void>
In some cases, you might want to trigger a rehydration manually.
This can be done by calling the rehydrate method.
await useBoundStore.persist.rehydrate()Schema:
() => boolean
This is a non-reactive getter to know if the storage has been hydrated (note that this does update when calling useBoundStore.persist.rehydrate()).
useBoundStore.persist.hasHydrated()Schema:
(listener: (state) => void) => () => void
The given listener will be called when the hydration process starts.
const unsub = useBoundStore.persist.onHydrate((state) => {
console.log('hydration starts')
})
// later on...
unsub()Schema:
(listener: (state) => void) => () => void
The given listener will be called when the hydration process ends.
const unsub = useBoundStore.persist.onFinishHydration((state) => {
console.log('hydration finished')
})
// later on...
unsub()To explain what's the "cost" of asynchronous storages, you need to understand what's hydration. In a nutshell, hydration is the process of retrieving the persisted state from the storage and merging it with the current state.
The persist middleware does two kinds of hydration: synchronous and asynchronous.
If the given storage is synchronous (e.g. localStorage), hydration will be done synchronously. On the other hand, if the given storage is asynchronous (e.g. AsyncStorage), hydration will be done ... 🥁 asynchronously.
But what's the catch? With synchronous hydration, the Zustand store will have been hydrated at its creation. While with asynchronous hydration, the Zustand store will be hydrated later on, in a microtask.
Why does it matter? Asynchronous hydration can cause some unexpected behaviors. For instance, if you use Zustand in a React app, the store will not be hydrated at the initial render. In cases where you app depends on the persisted value at page load, you might want to wait until the store has been hydrated before showing anything (e.g. your app might think the user is not logged in because it's the default, while in reality the store has not been hydrated yet).
If your app does depends on the persisted state at page load, see How can I check if my store has been hydrated? in the Q/A.
There's a fiew different ways to do this.
You can use the onRehydrateStorage option to update a field in the store:
const useBoundStore = create(
persist(
(set, get) => ({
// ...
_hasHydrated: false,
setHasHydrated: (state) => {
set({
_hasHydrated: state
});
}
}),
{
// ...
onRehydrateStorage: () => (state) => {
state.setHasHydrated(true)
}
}
)
);
export default function App() {
const hasHydrated = useBoundStore(state => state._hasHydrated);
if (!hasHydrated) {
return <p>Loading...</p>
}
return (
// ...
);
}You can also create a custom useHydration hook:
const useBoundStore = create(persist(...))
const useHydration = () => {
const [hydrated, setHydrated] = useState(useBoundStore.persist.hasHydrated)
useEffect(() => {
const unsubHydrate = useBoundStore.persist.onHydrate(() => setHydrated(false)) // Note: this is just in case you want to take into account manual rehydrations. You can remove this if you don't need it/don't want it.
const unsubFinishHydration = useBoundStore.persist.onFinishHydration(() => setHydrated(true))
setHydrated(useBoundStore.persist.hasHydrated())
return () => {
unsubHydrate()
unsubFinishHydration()
}
}, [])
return hydrated
}If the storage you want to use does not match the expected API, you can create your own storage:
import create from 'zustand'
import { persist, StateStorage } from 'zustand/middleware'
import { get, set, del } from 'idb-keyval' // can use anything: IndexedDB, Ionic Storage, etc.
// Custom storage object
const storage: StateStorage = {
getItem: async (name: string): Promise<string | null> => {
console.log(name, 'has been retrieved')
return (await get(name)) || null
},
setItem: async (name: string, value: string): Promise<void> => {
console.log(name, 'with value', value, 'has been saved')
await set(name, value)
},
removeItem: async (name: string): Promise<void> => {
console.log(name, 'has been deleted')
await del(name)
},
}
export const useBoundStore = create(
persist(
(set, get) => ({
fishes: 0,
addAFish: () => set({ fishes: get().fishes + 1 }),
}),
{
name: 'food-storage', // unique name
getStorage: () => storage,
}
)
)You can use the persist api to create your own implementation, similar to what we see below
type StoreWithPersist = Mutate<StoreApi<State>, [["zustand/persist", unknown]]>
export const withStorageDOMEvents = (store: StoreWithPersist) => {
const storageEventCallback = (e: StorageEvent) => {
if (e.key === store.persist.getOptions().name && e.newValue) {
store.persist.rehydrate()
}
}
window.addEventListener('storage', storageEventCallback)
return () => {
window.removeEventListener('storage', storageEventCallback)
}
}
const useBoundStore = create(persist(...))
withStorageDOMEvents(useBoundStore)Basic typescript usage doesn't require anything special except for writing create<State>()(...) instead of create(...).
import create from 'zustand'
import { persist } from 'zustand/middleware'
interface MyState {
fishes: number
addAFish: () => void
}
export const useFishStore = create<MyState>()(
persist(
(set, get) => ({
fishes: 0,
addAFish: () => set({ fishes: get().fishes + 1 }),
}),
{
name: 'food-storage', // name of item in the storage (must be unique)
getStorage: () => sessionStorage, // (optional) by default the 'localStorage' is used
partialize: (state) => ({ fishes: state.fishes }),
}
)
)