
## Flat updates

Updating state with Zustand is simple! Call the provided `set` function with
the new state, and it will be shallowly merged with the existing state in the
store. **Note** See next section for nested state.

```tsx
import { create } from 'zustand'

type State = {
  firstName: string
  lastName: string
}

type Action = {
  updateFirstName: (firstName: State['firstName']) => void
  updateLastName: (lastName: State['lastName']) => void
}

// Create your store, which includes both state and (optionally) actions
const usePersonStore = create<State & Action>((set) => ({
  firstName: '',
  lastName: '',
  updateFirstName: (firstName) => set(() => ({ firstName: firstName })),
  updateLastName: (lastName) => set(() => ({ lastName: lastName })),
}))

// In consuming app
function App() {
  // "select" the needed state and actions, in this case, the firstName value
  // and the action updateFirstName
  const firstName = usePersonStore((state) => state.firstName)
  const updateFirstName = usePersonStore((state) => state.updateFirstName)

  return (
    <main>
      <label>
        First name
        <input
          // Update the "firstName" state
          onChange={(e) => updateFirstName(e.currentTarget.value)}
          value={firstName}
        />
      </label>

      <p>
        Hello, <strong>{firstName}!</strong>
      </p>
    </main>
  )
}
```

## Deeply nested object

If you have a deep state object like this:

```ts
type State = {
  deep: {
    nested: {
      obj: { count: number }
    }
  }
}
```

Updating nested state requires some effort to ensure the process is completed
immutably.

### Normal approach

Similar to React or Redux, the normal approach is to copy each level of the
state object. This is done with the spread operator `...`, and by manually
merging that in with the new state values. Like so:

```ts
  normalInc: () =>
    set((state) => ({
      deep: {
        ...state.deep,
        nested: {
          ...state.deep.nested,
          obj: {
            ...state.deep.nested.obj,
            count: state.deep.nested.obj.count + 1
          }
        }
      }
    })),
```

This is very long! Let's explore some alternatives that will make your life
easier.

### With Immer

Many people use [Immer](https://github.com/immerjs/immer) to update nested
values. Immer can be used anytime you need to update nested state such as in
React, Redux and of course, Zustand!

You can use Immer to shorten your state updates for deeply nested object. Let's
take a look at an example:

```ts
  immerInc: () =>
    set(produce((state: State) => { ++state.deep.nested.obj.count })),
```

What a reduction! Please take note of the [gotchas listed here](../integrations/immer-middleware.md).

### With optics-ts

There is another option with [optics-ts](https://github.com/akheron/optics-ts/):

```ts
  opticsInc: () =>
    set(O.modify(O.optic<State>().path("deep.nested.obj.count"))((c) => c + 1)),
```

Unlike Immer, optics-ts doesn't use proxies or mutation syntax.

### With Ramda

You can also use [Ramda](https://ramdajs.com/):

```ts
  ramdaInc: () =>
    set(R.over(R.lensPath(["deep", "nested", "obj", "count"]), (c) => c + 1)),
```

Both ramda and optics-ts also work with types.

### CodeSandbox Demo

https://codesandbox.io/s/zustand-normal-immer-optics-ramda-updating-ynn3o?file=/src/App.tsx


Updating state with Zustand is simple! Call the provided set function with
the new state, and it will be shallowly merged with the existing state in the
store. Note See next section for nested state.

Flat updates

If you have a deep state object like this:

Deeply nested object

Similar to React or Redux, the normal approach is to copy each level of the
state object. This is done with the spread operator ..., and by manually
merging that in with the new state values. Like so:

Normal approach

Many people use Immer to update nested
values. Immer can be used anytime you need to update nested state such as in
React, Redux and of course, Zustand!

With Immer

With optics-ts

With Ramda

https://codesandbox.io/s/zustand-normal-immer-optics-ramda-updating-ynn3o?file=/src/App.tsx

CodeSandbox Demo

Updating state


<div class="flex justify-center mb-4">
  <img src="https://github.com/pmndrs/zustand/raw/main/bear.jpg" />
</div>

A small, fast, and scalable bearbones state management solution.
Zustand has a comfy API based on hooks.
It isn't boilerplatey or opinionated,
but has enough convention to be explicit and flux-like.

Don't disregard it because it's cute, it has claws!
Lots of time was spent to deal with common pitfalls,
like the dreaded [zombie child problem],
[React concurrency], and [context loss]
between mixed renderers.
It may be the one state manager in the React space that gets all of these right.

You can try a live demo [here](https://codesandbox.io/s/dazzling-moon-itop4).

[zombie child problem]: https://react-redux.js.org/api/hooks#stale-props-and-zombie-children
[react concurrency]: https://github.com/bvaughn/rfcs/blob/useMutableSource/text/0000-use-mutable-source.md
[context loss]: https://github.com/facebook/react/issues/13332

## Installation

Zustand is available as a package on NPM for use:

```bash
# NPM
npm install zustand
# Or, use any package manager of your choice.
```

## First create a store

Your store is a hook!
You can put anything in it: primitives, objects, functions.
The `set` function _merges_ state.

```js
import { create } from 'zustand'

const useStore = create((set) => ({
  bears: 0,
  increasePopulation: () => set((state) => ({ bears: state.bears + 1 })),
  removeAllBears: () => set({ bears: 0 }),
  updateBears: (newBears) => set({ bears: newBears }),
}))
```

## Then bind your components, and that's it!

You can use the hook anywhere, without the need of providers.
Select your state and the consuming component
will re-render when that state changes.

```jsx
function BearCounter() {
  const bears = useStore((state) => state.bears)
  return <h1>{bears} around here...</h1>
}

function Controls() {
  const increasePopulation = useStore((state) => state.increasePopulation)
  return <button onClick={increasePopulation}>one up</button>
}
```


Zustand is available as a package on NPM for use:

Installation

Your store is a hook!
You can put anything in it: primitives, objects, functions.
The set function merges state.

First create a store

You can use the hook anywhere, without the need of providers.
Select your state and the consuming component
will re-render when that state changes.

Then bind your components, and that's it!

Introduction

How Zustand stacks up against similar libraries


Zustand is one of many state management libraries for React.
On this page we will discuss Zustand
in comparison to some of these libraries,
including Redux, Valtio, Jotai, and Recoil.

Each library has its own strengths and weaknesses,
and we will compare key differences and similarities between each.

## Redux

### State Model (vs Redux)

Conceptually, Zustand and Redux are quite similar,
both are based on an immutable state model.
However, Redux requires your app to be wrapped
in context providers; Zustand does not.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  increment: (qty: number) => void
  decrement: (qty: number) => void
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  increment: (qty: number) => set((state) => ({ count: state.count + qty })),
  decrement: (qty: number) => set((state) => ({ count: state.count - qty })),
}))
```

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  increment: (qty: number) => void
  decrement: (qty: number) => void
}

type Action = {
  type: keyof Actions
  qty: number
}

const countReducer = (state: State, action: Action) => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + action.qty }
    case 'decrement':
      return { count: state.count - action.qty }
    default:
      return state
  }
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  dispatch: (action: Action) => set((state) => countReducer(state, action)),
}))
```

**Redux**

```ts
import { createStore } from 'redux'
import { useSelector, useDispatch } from 'react-redux'

type State = {
  count: number
}

type Action = {
  type: 'increment' | 'decrement'
  qty: number
}

const countReducer = (state: State, action: Action) => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + action.qty }
    case 'decrement':
      return { count: state.count - action.qty }
    default:
      return state
  }
}

const countStore = createStore(countReducer)
```

```ts
import { createSlice, configureStore } from '@reduxjs/toolkit'

const countSlice = createSlice({
  name: 'count',
  initialState: { value: 0 },
  reducers: {
    incremented: (state, qty: number) => {
      // Redux Toolkit does not mutate the state, it uses the Immer library
      // behind scenes, allowing us to have something called "draft state".
      state.value += qty
    },
    decremented: (state, qty: number) => {
      state.value -= qty
    },
  },
})

const countStore = configureStore({ reducer: countSlice.reducer })
```

### Render Optimization (vs Redux)

When it comes to render optimizations within your app,
there are no major differences in approach between Zustand and Redux.
In both libraries it is recommended
that you manually apply render optimizations by using selectors.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  increment: (qty: number) => void
  decrement: (qty: number) => void
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  increment: (qty: number) => set((state) => ({ count: state.count + qty })),
  decrement: (qty: number) => set((state) => ({ count: state.count - qty })),
}))

const Component = () => {
  const count = useCountStore((state) => state.count)
  const increment = useCountStore((state) => state.increment)
  const decrement = useCountStore((state) => state.decrement)
  // ...
}
```

**Redux**

```ts
import { createStore } from 'redux'
import { useSelector, useDispatch } from 'react-redux'

type State = {
  count: number
}

type Action = {
  type: 'increment' | 'decrement'
  qty: number
}

const countReducer = (state: State, action: Action) => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + action.qty }
    case 'decrement':
      return { count: state.count - action.qty }
    default:
      return state
  }
}

const countStore = createStore(countReducer)

const Component = () => {
  const count = useSelector((state) => state.count)
  const dispatch = useDispatch()
  // ...
}
```

```ts
import { useSelector } from 'react-redux'
import type { TypedUseSelectorHook } from 'react-redux'
import { createSlice, configureStore } from '@reduxjs/toolkit'

const countSlice = createSlice({
  name: 'count',
  initialState: { value: 0 },
  reducers: {
    incremented: (state, qty: number) => {
      // Redux Toolkit does not mutate the state, it uses the Immer library
      // behind scenes, allowing us to have something called "draft state".
      state.value += qty
    },
    decremented: (state, qty: number) => {
      state.value -= qty
    },
  },
})

const countStore = configureStore({ reducer: countSlice.reducer })

const useAppSelector: TypedUseSelectorHook<typeof countStore.getState> =
  useSelector

const useAppDispatch: () => typeof countStore.dispatch = useDispatch

const Component = () => {
  const count = useAppSelector((state) => state.count.value)
  const dispatch = useAppDispatch()
  // ...
}
```

## Valtio

### State Model (vs Valtio)

Zustand and Valtio approach state management
in a fundamentally different way.
Zustand is based on the **immutable** state model,
while Valtio is based on the **mutable** state model.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  obj: { count: number }
}

const store = create<State>(() => ({ obj: { count: 0 } }))

store.setState((prev) => ({ obj: { count: prev.obj.count + 1 } }))
```

**Valtio**

```ts
import { proxy } from 'valtio'

const state = proxy({ obj: { count: 0 } })

state.obj.count += 1
```

### Render Optimization (vs Valtio)

The other difference between Zustand and Valtio
is Valtio makes render optimizations through property access.
However, with Zustand, it is recommended that
you manually apply render optimizations by using selectors.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

const useCountStore = create<State>(() => ({
  count: 0,
}))

const Component = () => {
  const count = useCountStore((state) => state.count)
  // ...
}
```

**Valtio**

```ts
import { proxy, useSnapshot } from 'valtio'

const state = proxy({
  count: 0,
})

const Component = () => {
  const { count } = useSnapshot(state)
  // ...
}
```

## Jotai

### State Model (vs Jotai)

There are two major differences between Zustand and Jotai.
Firstly, Zustand is a single store,
while Jotai consists of primitive atoms
that can be composed together.
Secondly, a Zustand store is an external store,
making it more suitable when access outside of React is required.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  updateCount: (
    countCallback: (count: State['count']) => State['count'],
  ) => void
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  updateCount: (countCallback) =>
    set((state) => ({ count: countCallback(state.count) })),
}))
```

**Jotai**

```ts
import { atom } from 'jotai'

const countAtom = atom<number>(0)
```

### Render Optimization (vs Jotai)

Jotai achieves render optimizations through atom dependency.
However, with Zustand it is recommended that
you manually apply render optimizations by using selectors.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  updateCount: (
    countCallback: (count: State['count']) => State['count'],
  ) => void
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  updateCount: (countCallback) =>
    set((state) => ({ count: countCallback(state.count) })),
}))

const Component = () => {
  const count = useCountStore((state) => state.count)
  const updateCount = useCountStore((state) => state.updateCount)
  // ...
}
```

**Jotai**

```ts
import { atom, useAtom } from 'jotai'

const countAtom = atom<number>(0)

const Component = () => {
  const [count, updateCount] = useAtom(countAtom)
  // ...
}
```

## Recoil

### State Model (vs Recoil)

The difference between Zustand and Recoil
is similar to that between Zustand and Jotai.
Recoil depends on atom string keys
instead of atom object referential identities.
Additionally, Recoil needs to wrap your app in a context provider.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  setCount: (countCallback: (count: State['count']) => State['count']) => void
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  setCount: (countCallback) =>
    set((state) => ({ count: countCallback(state.count) })),
}))
```

**Recoil**

```ts
import { atom } from 'recoil'

const count = atom({
  key: 'count',
  default: 0,
})
```

### Render Optimization (vs Recoil)

Similar to previous optimization comparisons,
Recoil makes render optimizations through atom dependency.
Whereas with Zustand, it is recommended that
you manually apply render optimizations by using selectors.

**Zustand**

```ts
import { create } from 'zustand'

type State = {
  count: number
}

type Actions = {
  setCount: (countCallback: (count: State['count']) => State['count']) => void
}

const useCountStore = create<State & Actions>((set) => ({
  count: 0,
  setCount: (countCallback) =>
    set((state) => ({ count: countCallback(state.count) })),
}))

const Component = () => {
  const count = useCountStore((state) => state.count)
  const setCount = useCountStore((state) => state.setCount)
  // ...
}
```

**Recoil**

```ts
import { atom, useRecoilState } from 'recoil'

const countAtom = atom({
  key: 'count',
  default: 0,
})

const Component = () => {
  const [count, setCount] = useRecoilState(countAtom)
  // ...
}
```

## Npm Downloads Trend

- [Npm Downloads Trend of State Management Libraries for React](https://npm-compare.com/@reduxjs/toolkit,zustand,recoil,jotai,valtio/#timeRange=THREE_YEARS)


Redux

Conceptually, Zustand and Redux are quite similar,
both are based on an immutable state model.
However, Redux requires your app to be wrapped
in context providers; Zustand does not.

State Model (vs Redux)

When it comes to render optimizations within your app,
there are no major differences in approach between Zustand and Redux.
In both libraries it is recommended
that you manually apply render optimizations by using selectors.

Render Optimization (vs Redux)

Valtio

Zustand and Valtio approach state management
in a fundamentally different way.
Zustand is based on the immutable state model,
while Valtio is based on the mutable state model.

State Model (vs Valtio)

The other difference between Zustand and Valtio
is Valtio makes render optimizations through property access.
However, with Zustand, it is recommended that
you manually apply render optimizations by using selectors.

Render Optimization (vs Valtio)

Jotai

There are two major differences between Zustand and Jotai.
Firstly, Zustand is a single store,
while Jotai consists of primitive atoms
that can be composed together.
Secondly, a Zustand store is an external store,
making it more suitable when access outside of React is required.

State Model (vs Jotai)

Jotai achieves render optimizations through atom dependency.
However, with Zustand it is recommended that
you manually apply render optimizations by using selectors.

Render Optimization (vs Jotai)

Recoil

The difference between Zustand and Recoil
is similar to that between Zustand and Jotai.
Recoil depends on atom string keys
instead of atom object referential identities.
Additionally, Recoil needs to wrap your app in a context provider.

State Model (vs Recoil)

Similar to previous optimization comparisons,
Recoil makes render optimizations through atom dependency.
Whereas with Zustand, it is recommended that
you manually apply render optimizations by using selectors.

Render Optimization (vs Recoil)

Npm Downloads Trend

Comparison


Like with React's `useState`, we need to update state immutably.

Here's a typical example:

```jsx
import { create } from 'zustand'

const useCountStore = create((set) => ({
  count: 0,
  inc: () => set((state) => ({ count: state.count + 1 })),
}))
```

The `set` function is to update state in the store.
Because the state is immutable, it should have been like this:

```js
set((state) => ({ ...state, count: state.count + 1 }))
```

However, as this is a common pattern, `set` actually merges state, and
we can skip the `...state` part:

```js
set((state) => ({ count: state.count + 1 }))
```

## Nested objects

The `set` function merges state at only one level.
If you have a nested object, you need to merge them explicitly. You will use the spread operator pattern like so:

```jsx
import { create } from 'zustand'

const useCountStore = create((set) => ({
  nested: { count: 0 },
  inc: () =>
    set((state) => ({
      nested: { ...state.nested, count: state.nested.count + 1 },
    })),
}))
```

For complex use cases, consider using some libraries that help with immutable updates.
You can refer to [Updating nested state object values](./updating-state.md#deeply-nested-object).

## Replace flag

To disable the merging behavior, you can specify a `replace` boolean value for `set` like so:

```js
set((state) => newState, true)
```


The set function merges state at only one level.
If you have a nested object, you need to merge them explicitly. You will use the spread operator pattern like so:

Nested objects

To disable the merging behavior, you can specify a replace boolean value for set like so:

Replace flag

Immutable state and merging


Although Zustand is an unopinionated library, we do recommend a few patterns.
These are inspired by practices originally found in [Flux](https://github.com/facebookarchive/flux),
and more recently [Redux](https://redux.js.org/understanding/thinking-in-redux/three-principles),
so if you are coming from another library, you should feel right at home.

However, Zustand does differ in some fundamental ways,
so some terminology may not perfectly align to other libraries.

## Recommended patterns

### Single store

Your applications global state should be located in a single Zustand store.

If you have a large application, Zustand supports [splitting the store into slices](./slices-pattern.md).

### Use `set` / `setState` to update the store

Always use `set` (or `setState`) to perform updates to your store.
`set` (and `setState`) ensures the described update is correctly merged and listeners are appropriately notified.

### Colocate store actions

In Zustand, state can be updated without the use of dispatched actions and reducers found in other Flux libraries.
These store actions can be added directly to the store as shown below.

Optionally, by using `setState` they can be [located external to the store](./practice-with-no-store-actions.md)

```js
const useBoundStore = create((set) => ({
  storeSliceA: ...,
  storeSliceB: ...,
  storeSliceC: ...,
  updateX: () => set(...),
  updateY: () => set(...),
}))
```

## Redux-like patterns

If you can't live without Redux-like reducers, you can define a `dispatch` function on the root level of the store:

```typescript
const types = { increase: 'INCREASE', decrease: 'DECREASE' }

const reducer = (state, { type, by = 1 }) => {
  switch (type) {
    case types.increase:
      return { grumpiness: state.grumpiness + by }
    case types.decrease:
      return { grumpiness: state.grumpiness - by }
  }
}

const useGrumpyStore = create((set) => ({
  grumpiness: 0,
  dispatch: (args) => set((state) => reducer(state, args)),
}))

const dispatch = useGrumpyStore((state) => state.dispatch)
dispatch({ type: types.increase, by: 2 })
```

You could also use our redux-middleware. It wires up your main reducer, sets initial state, and adds a dispatch function to the state itself and the vanilla api.

```typescript
import { redux } from 'zustand/middleware'

const useReduxStore = create(redux(reducer, initialState))
```

Another way to update the store could be through functions wrapping the state functions. These could also handle side-effects of actions. For example, with HTTP-calls. To use Zustand in a non-reactive way, see [the readme](https://github.com/pmndrs/zustand#readingwriting-state-and-reacting-to-changes-outside-of-components).


Recommended patterns

Your applications global state should be located in a single Zustand store.

Single store

Always use set (or setState) to perform updates to your store.
set (and setState) ensures the described update is correctly merged and listeners are appropriately notified.

Use set / setState to update the store

In Zustand, state can be updated without the use of dispatched actions and reducers found in other Flux libraries.
These store actions can be added directly to the store as shown below.

Colocate store actions

If you can't live without Redux-like reducers, you can define a dispatch function on the root level of the store:

Redux-like patterns

Flux inspired practice


We recommend using selectors when using either the properties or actions from the store. You can access values from the store like so:

```typescript
const bears = useBearStore((state) => state.bears)
```

However, writing these could be tedious. If that is the case for you, you can auto-generate your selectors.

## create the following function: `createSelectors`

```typescript
import { StoreApi, UseBoundStore } from 'zustand'

type WithSelectors<S> = S extends { getState: () => infer T }
  ? S & { use: { [K in keyof T]: () => T[K] } }
  : never

const createSelectors = <S extends UseBoundStore<StoreApi<object>>>(
  _store: S,
) => {
  let store = _store as WithSelectors<typeof _store>
  store.use = {}
  for (let k of Object.keys(store.getState())) {
    ;(store.use as any)[k] = () => store((s) => s[k as keyof typeof s])
  }

  return store
}
```

If you have a store like this:

```typescript
interface BearState {
  bears: number
  increase: (by: number) => void
  increment: () => void
}

const useBearStoreBase = create<BearState>()((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
  increment: () => set((state) => ({ bears: state.bears + 1 })),
}))
```

Apply that function to your store:

```typescript
const useBearStore = createSelectors(useBearStoreBase)
```

Now the selectors are auto generated and you can access them directly:

```typescript
// get the property
const bears = useBearStore.use.bears()

// get the action
const increment = useBearStore.use.increment()
```

## Vanilla Store

If you are using a vanilla store, use the following `createSelectors` function:

```typescript
import { StoreApi, useStore } from 'zustand'

type WithSelectors<S> = S extends { getState: () => infer T }
  ? S & { use: { [K in keyof T]: () => T[K] } }
  : never

const createSelectors = <S extends StoreApi<object>>(_store: S) => {
  const store = _store as WithSelectors<typeof _store>
  store.use = {}
  for (const k of Object.keys(store.getState())) {
    ;(store.use as any)[k] = () =>
      useStore(_store, (s) => s[k as keyof typeof s])
  }

  return store
}
```

The usage is the same as a React store. If you have a store like this:

```typescript
import { createStore } from 'zustand'

interface BearState {
  bears: number
  increase: (by: number) => void
  increment: () => void
}

const store = createStore<BearState>((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
  increment: () => set((state) => ({ bears: state.bears + 1 })),
}))
```

Apply that function to your store:

```typescript
const useBearStore = createSelectors(store)
```

Now the selectors are auto generated and you can access them directly:

```typescript
// get the property
const bears = useBearStore.use.bears()

// get the action
const increment = useBearStore.use.increment()
```

## Live Demo

For a working example of this, see the [Code Sandbox](https://codesandbox.io/s/zustand-auto-generate-selectors-forked-rl8v5e?file=/src/selectors.ts).

## Third-party Libraries

- [auto-zustand-selectors-hook](https://github.com/Albert-Gao/auto-zustand-selectors-hook)
- [react-hooks-global-state](https://github.com/dai-shi/react-hooks-global-state)
- [zustood](https://github.com/udecode/zustood)
- [@davstack/store](https://www.npmjs.com/package/@davstack/store)


create the following function: createSelectors

If you are using a vanilla store, use the following createSelectors function:

Vanilla Store

For a working example of this, see the Code Sandbox.

Live Demo

Third-party Libraries

Auto Generating Selectors

Practice with no store actions


## Basic usage

The difference when using TypeScript is that instead of writing `create(...)`, you have to write `create<T>()(...)` (notice the extra parentheses `()` too along with the type parameter) where `T` is the type of the state to annotate it. For example:

```ts
import { create } from 'zustand'

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useBearStore = create<BearState>()((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}))
```

<details>
  <summary>Why can't we simply infer the type from the initial state?</summary>

  <br/>

**TLDR**: Because state generic `T` is invariant.

Consider this minimal version `create`:

```ts
declare const create: <T>(f: (get: () => T) => T) => T

const x = create((get) => ({
  foo: 0,
  bar: () => get(),
}))
// `x` is inferred as `unknown` instead of
// interface X {
//   foo: number,
//   bar: () => X
// }
```

Here, if you look at the type of `f` in `create`, i.e. `(get: () => T) => T`, it "gives" `T` via return (making it covariant), but it also "takes" `T` via `get` (making it contravariant). "So where does `T` come from?" TypeScript wonders. It's like that chicken or egg problem. At the end TypeScript, gives up and infers `T` as `unknown`.

So, as long as the generic to be inferred is invariant (i.e. both covariant and contravariant), TypeScript will be unable to infer it. Another simple example would be this:

```ts
const createFoo = {} as <T>(f: (t: T) => T) => T
const x = createFoo((_) => 'hello')
```

Here again, `x` is `unknown` instead of `string`.

  <details>
    <summary>More about the inference (just for the people curious and interested in TypeScript)</summary>

In some sense this inference failure is not a problem because a value of type `<T>(f: (t: T) => T) => T` cannot be written. That is to say you can't write the real runtime implementation of `createFoo`. Let's try it:

```js
const createFoo = (f) => f(/* ? */)
```

`createFoo` needs to return the return value of `f`. And to do that we first have to call `f`. And to call it we have to pass a value of type `T`. And to pass a value of type `T` we first have to produce it. But how can we produce a value of type `T` when we don't even know what `T` is? The only way to produce a value of type `T` is to call `f`, but then to call `f` itself we need a value of type `T`. So you see it's impossible to actually write `createFoo`.

So what we're saying is, the inference failure in case of `createFoo` is not really a problem because it's impossible to implement `createFoo`. But what about the inference failure in case of `create`? That also is not really a problem because it's impossible to implement `create` too. Wait a minute, if it's impossible to implement `create` then how does Zustand implement it? The answer is, it doesn't.

Zustand lies that it implemented `create`'s type, it implemented only the most part of it. Here's a simple proof by showing unsoundness. Consider the following code:

```ts
import { create } from 'zustand'

const useBoundStore = create<{ foo: number }>()((_, get) => ({
  foo: get().foo,
}))
```

This code compiles. But if we run it, we'll get an exception: "Uncaught TypeError: Cannot read properties of undefined (reading 'foo')". This is because `get` would return `undefined` before the initial state is created (hence you shouldn't call `get` when creating the initial state). The types promise that `get` will never return `undefined` but it does initially, which means Zustand failed to implement it.

And of course Zustand failed because it's impossible to implement `create` the way types promise (in the same way it's impossible to implement `createFoo`). In other words we don't have a type to express the actual `create` we have implemented. We can't type `get` as `() => T | undefined` because it would cause inconveince and it still won't be correct as `get` is indeed `() => T` eventually, just if called synchronously it would be `() => undefined`. What we need is some kind of TypeScript feature that allows us to type `get` as `(() => T) & WhenSync<() => undefined>`, which of course is extremly far-fetched.

So we have two problems: lack of inference and unsoundness. Lack of inference can be solved if TypeScript can improves its inference for invariants. And unsoundness can be solved if TypeScript introduces something like `WhenSync`. To work around lack of inference we manually annotate the state type. And we can't work around unsoundness, but it's not a big deal because it's not much, calling `get` synchronously anyway doesn't make sense.

</details>

</details>

<details>
  <summary>Why the currying `()(...)`?</summary>

  <br/>

**TLDR**: It is a workaround for [microsoft/TypeScript#10571](https://github.com/microsoft/TypeScript/issues/10571).

Imagine you have a scenario like this:

```ts
declare const withError: <T, E>(
  p: Promise<T>,
) => Promise<[error: undefined, value: T] | [error: E, value: undefined]>
declare const doSomething: () => Promise<string>

const main = async () => {
  let [error, value] = await withError(doSomething())
}
```

Here, `T` is inferred to be a `string` and `E` is inferred to be `unknown`. You might want to annotate `E` as `Foo`, because you are certain of the shape of error `doSomething()` would throw. However, you can't do that. You can either pass all generics or none. Along with annotating `E` as `Foo`, you will also have to annotate `T` as `string` even though it gets inferred anyway. The solution is to make a curried version of `withError` that does nothing at runtime. Its purpose is to just allow you annotate `E`.

```ts
declare const withError: {
  <E>(): <T>(
    p: Promise<T>,
  ) => Promise<[error: undefined, value: T] | [error: E, value: undefined]>
  <T, E>(
    p: Promise<T>,
  ): Promise<[error: undefined, value: T] | [error: E, value: undefined]>
}
declare const doSomething: () => Promise<string>
interface Foo {
  bar: string
}

const main = async () => {
  let [error, value] = await withError<Foo>()(doSomething())
}
```

This way, `T` gets inferred and you get to annotate `E`. Zustand has the same use case when we want to annotate the state (the first type parameter) but allow other parameters to get inferred.

</details>

Alternatively, you can also use `combine`, which infers the state so that you do not need to type it.

```ts
import { create } from 'zustand'
import { combine } from 'zustand/middleware'

const useBearStore = create(
  combine({ bears: 0 }, (set) => ({
    increase: (by: number) => set((state) => ({ bears: state.bears + by })),
  })),
)
```

<details>
  <summary>Be a little careful</summary>

  <br/>

We achieve the inference by lying a little in the types of `set`, `get`, and `store` that you receive as parameters. The lie is that they're typed as if the state is the first parameter, when in fact the state is the shallow-merge (`{ ...a, ...b }`) of both first parameter and the second parameter's return. For example, `get` from the second parameter has type `() => { bears: number }` and that is a lie as it should be `() => { bears: number, increase: (by: number) => void }`. And `useBearStore` still has the correct type; for example, `useBearStore.getState` is typed as `() => { bears: number, increase: (by: number) => void }`.

It isn't really a lie because `{ bears: number }` is still a subtype of `{ bears: number, increase: (by: number) => void }`. Therefore, there will be no problem in most cases. You should just be careful while using replace. For example, `set({ bears: 0 }, true)` would compile but will be unsound as it will delete the `increase` function. Another instance where you should be careful is if you use `Object.keys`. `Object.keys(get())` will return `["bears", "increase"]` and not `["bears"]`. The return type of `get` can make you fall for these mistakes.

`combine` trades off a little type-safety for the convenience of not having to write a type for state. Hence, you should use `combine` accordingly. It is fine in most cases and you can use it conveniently.

</details>

Note that we don't use the curried version when using `combine` because `combine` "creates" the state. When using a middleware that creates the state, it isn't necessary to use the curried version because the state now can be inferred. Another middleware that creates state is `redux`. So when using `combine`, `redux`, or any other custom middleware that creates the state, we don't recommend using the curried version.

## Using middlewares

You do not have to do anything special to use middlewares in TypeScript.

```ts
import { create } from 'zustand'
import { devtools, persist } from 'zustand/middleware'

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useBearStore = create<BearState>()(
  devtools(
    persist(
      (set) => ({
        bears: 0,
        increase: (by) => set((state) => ({ bears: state.bears + by })),
      }),
      { name: 'bearStore' },
    ),
  ),
)
```

Just make sure you are using them immediately inside `create` so as to make the contextual inference work. Doing something even remotely fancy like the following `myMiddlewares` would require more advanced types.

```ts
import { create } from 'zustand'
import { devtools, persist } from 'zustand/middleware'

const myMiddlewares = (f) => devtools(persist(f, { name: 'bearStore' }))

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useBearStore = create<BearState>()(
  myMiddlewares((set) => ({
    bears: 0,
    increase: (by) => set((state) => ({ bears: state.bears + by })),
  })),
)
```

Also, we recommend using `devtools` middleware as last as possible. For example, when you use it with `immer` as a middleware, it should be `immer(devtools(...))` and not `devtools(immer(...))`. This is because`devtools` mutates the `setState` and adds a type parameter on it, which could get lost if other middlewares (like `immer`) also mutate `setState` before `devtools`. Hence using `devtools` at the end makes sure that no middlewares mutate `setState` before it.

## Authoring middlewares and advanced usage

Imagine you had to write this hypothetical middleware.

```ts
import { create } from 'zustand'

const foo = (f, bar) => (set, get, store) => {
  store.foo = bar
  return f(set, get, store)
}

const useBearStore = create(foo(() => ({ bears: 0 }), 'hello'))
console.log(useBearStore.foo.toUpperCase())
```

Zustand middlewares can mutate the store. But how could we possibly encode the mutation on the type-level? That is to say how could do we type `foo` so that this code compiles?

For a usual statically typed language, this is impossible. But thanks to TypeScript, Zustand has something called a "higher-kinded mutator" that makes this possible. If you are dealing with complex type problems, like typing a middleware or using the `StateCreator` type, you will have to understand this implementation detail. For this, you can [check out #710](https://github.com/pmndrs/zustand/issues/710).

If you are eager to know what the answer is to this particular problem then you can [see it here](#middleware-that-changes-the-store-type).

## Common recipes

### Middleware that doesn't change the store type

```ts
import { create, State, StateCreator, StoreMutatorIdentifier } from 'zustand'

type Logger = <
  T extends State,
  Mps extends [StoreMutatorIdentifier, unknown][] = [],
  Mcs extends [StoreMutatorIdentifier, unknown][] = [],
>(
  f: StateCreator<T, Mps, Mcs>,
  name?: string,
) => StateCreator<T, Mps, Mcs>

type LoggerImpl = <T extends State>(
  f: StateCreator<T, [], []>,
  name?: string,
) => StateCreator<T, [], []>

const loggerImpl: LoggerImpl = (f, name) => (set, get, store) => {
  type T = ReturnType<typeof f>
  const loggedSet: typeof set = (...a) => {
    set(...a)
    console.log(...(name ? [`${name}:`] : []), get())
  }
  const setState = store.setState
  store.setState = (...a) => {
    setState(...a)
    console.log(...(name ? [`${name}:`] : []), store.getState())
  }

  return f(loggedSet, get, store)
}

export const logger = loggerImpl as unknown as Logger

// ---

const useBearStore = create<BearState>()(
  logger(
    (set) => ({
      bears: 0,
      increase: (by) => set((state) => ({ bears: state.bears + by })),
    }),
    'bear-store',
  ),
)
```

### Middleware that changes the store type

```ts
import {
  create,
  State,
  StateCreator,
  StoreMutatorIdentifier,
  Mutate,
  StoreApi,
} from 'zustand'

type Foo = <
  T extends State,
  A,
  Mps extends [StoreMutatorIdentifier, unknown][] = [],
  Mcs extends [StoreMutatorIdentifier, unknown][] = [],
>(
  f: StateCreator<T, [...Mps, ['foo', A]], Mcs>,
  bar: A,
) => StateCreator<T, Mps, [['foo', A], ...Mcs]>

declare module 'zustand' {
  interface StoreMutators<S, A> {
    foo: Write<Cast<S, object>, { foo: A }>
  }
}

type FooImpl = <T extends State, A>(
  f: StateCreator<T, [], []>,
  bar: A,
) => StateCreator<T, [], []>

const fooImpl: FooImpl = (f, bar) => (set, get, _store) => {
  type T = ReturnType<typeof f>
  type A = typeof bar

  const store = _store as Mutate<StoreApi<T>, [['foo', A]]>
  store.foo = bar
  return f(set, get, _store)
}

export const foo = fooImpl as unknown as Foo

type Write<T extends object, U extends object> = Omit<T, keyof U> & U

type Cast<T, U> = T extends U ? T : U

// ---

const useBearStore = create(foo(() => ({ bears: 0 }), 'hello'))
console.log(useBearStore.foo.toUpperCase())
```

### `create` without curried workaround

The recommended way to use `create` is using the curried workaround like so: `create<T>()(...)`. This is because it enables you to infer the store type. But if for some reason you do not want to use the workaround, you can pass the type parameters like the following. Note that in some cases, this acts as an assertion instead of annotation, so we don't recommend it.

```ts
import { create } from "zustand"

interface BearState {
  bears: number
  increase: (by: number) => void
}

const useBearStore = create<
  BearState,
  [
    ['zustand/persist', BearState],
    ['zustand/devtools', never]
  ]
>(devtools(persist((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}), { name: 'bearStore' }))
```

### Slices pattern

```ts
import { create, StateCreator } from 'zustand'

interface BearSlice {
  bears: number
  addBear: () => void
  eatFish: () => void
}

interface FishSlice {
  fishes: number
  addFish: () => void
}

interface SharedSlice {
  addBoth: () => void
  getBoth: () => void
}

const createBearSlice: StateCreator<
  BearSlice & FishSlice,
  [],
  [],
  BearSlice
> = (set) => ({
  bears: 0,
  addBear: () => set((state) => ({ bears: state.bears + 1 })),
  eatFish: () => set((state) => ({ fishes: state.fishes - 1 })),
})

const createFishSlice: StateCreator<
  BearSlice & FishSlice,
  [],
  [],
  FishSlice
> = (set) => ({
  fishes: 0,
  addFish: () => set((state) => ({ fishes: state.fishes + 1 })),
})

const createSharedSlice: StateCreator<
  BearSlice & FishSlice,
  [],
  [],
  SharedSlice
> = (set, get) => ({
  addBoth: () => {
    // you can reuse previous methods
    get().addBear()
    get().addFish()
    // or do them from scratch
    // set((state) => ({ bears: state.bears + 1, fishes: state.fishes + 1 })
  },
  getBoth: () => get().bears + get().fishes,
})

const useBoundStore = create<BearSlice & FishSlice & SharedSlice>()((...a) => ({
  ...createBearSlice(...a),
  ...createFishSlice(...a),
  ...createSharedSlice(...a),
}))
```

A detailed explanation on the slices pattern can be found [here](./slices-pattern.md).

If you have some middlewares then replace `StateCreator<MyState, [], [], MySlice>` with `StateCreator<MyState, Mutators, [], MySlice>`. For example, if you are using `devtools` then it will be `StateCreator<MyState, [["zustand/devtools", never]], [], MySlice>`. See the ["Middlewares and their mutators reference"](#middlewares-and-their-mutators-reference) section for a list of all mutators.

### Bounded `useStore` hook for vanilla stores

```ts
import { useStore } from 'zustand'
import { createStore } from 'zustand/vanilla'

interface BearState {
  bears: number
  increase: (by: number) => void
}

const bearStore = createStore<BearState>()((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}))

function useBearStore(): BearState
function useBearStore<T>(selector: (state: BearState) => T): T
function useBearStore<T>(selector?: (state: BearState) => T) {
  return useStore(bearStore, selector!)
}
```

You can also make an abstract `createBoundedUseStore` function if you need to create bounded `useStore` hooks often and want to DRY things up...

```ts
import { useStore, StoreApi } from 'zustand'
import { createStore } from 'zustand/vanilla'

interface BearState {
  bears: number
  increase: (by: number) => void
}

const bearStore = createStore<BearState>()((set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
}))

const createBoundedUseStore = ((store) => (selector) => useStore(store)) as <
  S extends StoreApi<unknown>,
>(
  store: S,
) => {
  (): ExtractState<S>
  <T>(selector: (state: ExtractState<S>) => T): T
}

type ExtractState<S> = S extends { getState: () => infer X } ? X : never

const useBearStore = createBoundedUseStore(bearStore)
```

## Middlewares and their mutators reference

- `devtools` — `["zustand/devtools", never]`
- `persist` — `["zustand/persist", YourPersistedState]`<br/>
  `YourPersistedState` is the type of state you are going to persist, ie the return type of `options.partialize`, if you're not passing `partialize` options the `YourPersistedState` becomes `Partial<YourState>`. Also [sometimes](https://github.com/pmndrs/zustand/issues/980#issuecomment-1162289836) passing actual `PersistedState` won't work. In those cases, try passing `unknown`.
- `immer` — `["zustand/immer", never]`
- `subscribeWithSelector` — `["zustand/subscribeWithSelector", never]`
- `redux` — `["zustand/redux", YourAction]`
- `combine` — no mutator as `combine` does not mutate the store


The difference when using TypeScript is that instead of writing create(...), you have to write create<T>()(...) (notice the extra parentheses () too along with the type parameter) where T is the type of the state to annotate it. For example:

Basic usage

You do not have to do anything special to use middlewares in TypeScript.

Using middlewares

Imagine you had to write this hypothetical middleware.

Authoring middlewares and advanced usage

Common recipes

Middleware that doesn't change the store type

Middleware that changes the store type

The recommended way to use create is using the curried workaround like so: create<T>()(...). This is because it enables you to infer the store type. But if for some reason you do not want to use the workaround, you can pass the type parameters like the following. Note that in some cases, this acts as an assertion instead of annotation, so we don't recommend it.

create without curried workaround

Slices pattern

Bounded useStore hook for vanilla stores

Middlewares and their mutators reference

TypeScript Guide


## Setting Up a Test Environment

### Test Runners

Usually, your test runner needs to be configured to run JavaScript/TypeScript syntax. If you're
going to be testing UI components, you will likely need to configure the test runner to use JSDOM
to provide a mock DOM environment.

See these resources for test runner configuration instructions:

- **Jest**
  - [Jest: Getting Started](https://jestjs.io/docs/getting-started)
  - [Jest: Configuration - Test Environment](https://jestjs.io/docs/configuration#testenvironment-string)
- **Vitest**
  - [Vitest: Getting Started](https://vitest.dev/guide)
  - [Vitest: Configuration - Test Environment](https://vitest.dev/config/#environment)

### UI and Network Testing Tools

**We recommend using [React Testing Library (RTL)](https://testing-library.com/docs/react-testing-library/intro)
to test out React components that connect to Zustand**. RTL is a simple and complete React DOM
testing utility that encourages good testing practices. It uses ReactDOM's `render` function and
`act` from `react-dom/tests-utils`. Futhermore, [Native Testing Library (RNTL)](https://testing-library.com/docs/react-native-testing-library/intro)
is the alternative to RTL to test out React Native components. The [Testing Library](https://testing-library.com/)
family of tools also includes adapters for many other popular frameworks.

We also recommend using [Mock Service Worker (MSW)](https://mswjs.io/) to mock network requests, as
this means your application logic does not need to be changed or mocked when writing tests.

- **React Testing Library (DOM)**
  - [DOM Testing Library: Setup](https://testing-library.com/docs/dom-testing-library/setup)
  - [React Testing Library: Setup](https://testing-library.com/docs/react-testing-library/setup)
  - [Testing Library Jest-DOM Matchers](https://testing-library.com/docs/ecosystem-jest-dom)
- **Native Testing Library (React Native)**
  - [Native Testing Library: Setup](https://testing-library.com/docs/react-native-testing-library/setup)
- **User Event Testing Library (DOM)**
  - [User Event Testing Library: Setup](https://testing-library.com/docs/user-event/setup)
- **TypeScript for Jest**
  - [TypeScript for Jest: Setup](https://kulshekhar.github.io/ts-jest/docs/getting-started/installation)
- **TypeScript for Node**
  - [TypeScript for Node: Setup](https://typestrong.org/ts-node/docs/installation)
- **Mock Service Worker**
  - [MSW: Installation](https://mswjs.io/docs/getting-started/install)
  - [MSW: Setting up mock requests](https://mswjs.io/docs/getting-started/mocks/rest-api)
  - [MSW: Mock server configuration for Node](https://mswjs.io/docs/getting-started/integrate/node)

## Setting Up Zustand for testing

> **Note**: Since Jest and Vitest have slight differences, like Vitest using **ES modules** and Jest using
> **CommonJS modules**, you need to keep that in mind if you are using Vitest instead of Jest.

The mock provided below will enable the relevant test runner to reset the zustand stores after each test.

> **Warning:** In the following examples we are only supporting the curried version of create `(create()(...))`,
> since it is supported on both JavaScript and TypeScript. If you are using the uncurried version of
> `create(...)` you will need to update your hooks to use the curried version, or update the mocks in order
> to support the uncurried version.

### Jest

In the next steps we are going to setup our Jest environment in order to mock Zustand.

```ts
// __mocks__/zustand.ts
import * as zustand from 'zustand'
import { act } from '@testing-library/react'

const { create: actualCreate, createStore: actualCreateStore } =
  jest.requireActual<typeof zustand>('zustand')

// a variable to hold reset functions for all stores declared in the app
export const storeResetFns = new Set<() => void>()

const createUncurried = <T>(stateCreator: zustand.StateCreator<T>) => {
  const store = actualCreate(stateCreator)
  const initialState = store.getInitialState()
  storeResetFns.add(() => {
    store.setState(initialState, true)
  })
  return store
}

// when creating a store, we get its initial state, create a reset function and add it in the set
export const create = (<T>(stateCreator: zustand.StateCreator<T>) => {
  console.log('zustand create mock')

  // to support curried version of create
  return typeof stateCreator === 'function'
    ? createUncurried(stateCreator)
    : createUncurried
}) as typeof zustand.create

const createStoreUncurried = <T>(stateCreator: zustand.StateCreator<T>) => {
  const store = actualCreateStore(stateCreator)
  const initialState = store.getInitialState()
  storeResetFns.add(() => {
    store.setState(initialState, true)
  })
  return store
}

// when creating a store, we get its initial state, create a reset function and add it in the set
export const createStore = (<T>(stateCreator: zustand.StateCreator<T>) => {
  console.log('zustand createStore mock')

  // to support curried version of createStore
  return typeof stateCreator === 'function'
    ? createStoreUncurried(stateCreator)
    : createStoreUncurried
}) as typeof zustand.createStore

// reset all stores after each test run
afterEach(() => {
  act(() => {
    storeResetFns.forEach((resetFn) => {
      resetFn()
    })
  })
})
```

```ts
// src/setup-jest.ts
import '@testing-library/jest-dom'
```

```ts
// jest.config.ts
import type { JestConfigWithTsJest } from 'ts-jest'

const config: JestConfigWithTsJest = {
  preset: 'ts-jest',
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['./src/setup-jest.ts'],
}

export default config
```

> **Note**: to use TypeScript we need to install two packages `ts-jest` and `ts-node`.

### Vitest

In the next steps we are going to setup our Vitest environment in order to mock Zustand.

> **Warning:** In Vitest you can change the [root](https://vitest.dev/config/#root).
> Due to that, you need make sure that you are creating your `__mocks__` directory in the right place.
> Let's say that you change the **root** to `./src`, that means you need to create a `__mocks__`
> directory under `./src`. The end result would be `./src/__mocks__`, rather than `./__mocks__`.
> Creating `__mocks__` directory in the wrong place can lead to issues when using Vitest.

```ts
// __mocks__/zustand.ts
import * as zustand from 'zustand'
import { act } from '@testing-library/react'

const { create: actualCreate, createStore: actualCreateStore } =
  await vi.importActual<typeof zustand>('zustand')

// a variable to hold reset functions for all stores declared in the app
export const storeResetFns = new Set<() => void>()

const createUncurried = <T>(stateCreator: zustand.StateCreator<T>) => {
  const store = actualCreate(stateCreator)
  const initialState = store.getInitialState()
  storeResetFns.add(() => {
    store.setState(initialState, true)
  })
  return store
}

// when creating a store, we get its initial state, create a reset function and add it in the set
export const create = (<T>(stateCreator: zustand.StateCreator<T>) => {
  console.log('zustand create mock')

  // to support curried version of create
  return typeof stateCreator === 'function'
    ? createUncurried(stateCreator)
    : createUncurried
}) as typeof zustand.create

const createStoreUncurried = <T>(stateCreator: zustand.StateCreator<T>) => {
  const store = actualCreateStore(stateCreator)
  const initialState = store.getInitialState()
  storeResetFns.add(() => {
    store.setState(initialState, true)
  })
  return store
}

// when creating a store, we get its initial state, create a reset function and add it in the set
export const createStore = (<T>(stateCreator: zustand.StateCreator<T>) => {
  console.log('zustand createStore mock')

  // to support curried version of createStore
  return typeof stateCreator === 'function'
    ? createStoreUncurried(stateCreator)
    : createStoreUncurried
}) as typeof zustand.createStore

// reset all stores after each test run
afterEach(() => {
  act(() => {
    storeResetFns.forEach((resetFn) => {
      resetFn()
    })
  })
})
```

> **Note**: without [globals configuration](https://vitest.dev/config/#globals) enabled, we need
> to add `import { afterEach, vi } from 'vitest'` at the top.

```ts
// __mocks__/vitest-env.d.ts
/// <reference types="vite/client" />
/// <reference types="vitest/globals" />
```

> **Note**: without [globals configuration](https://vitest.dev/config/#globals) enabled, we do
> need to remove `/// <reference types="vitest/globals" />`.

```ts
// src/setup-vitest.ts
import '@testing-library/jest-dom'

vi.mock('zustand') // to make it works like Jest (auto-mocking)
```

> **Note**: without [globals configuration](https://vitest.dev/config/#globals) enabled, we need
> to add `import { vi } from 'vitest'` at the top.

```ts
// vitest.config.ts
import { defineConfig, mergeConfig } from 'vitest/config'
import viteConfig from './vite.config'

export default mergeConfig(
  viteConfig,
  defineConfig({
    test: {
      globals: true,
      environment: 'jsdom',
      setupFiles: ['./setup-vitest.ts'],
    },
  }),
)
```

### Testing components

In the next examples we are going to use `useCounterStore`

> **Note**: all of these examples are written using TypeScript.

```ts
// stores/counter-store-creator.ts
import { type StateCreator } from 'zustand'

export type CounterStore = {
  count: number
  inc: () => void
}

export const counterStoreCreator: StateCreator<CounterStore> = (set) => ({
  count: 1,
  inc: () => set((state) => ({ count: state.count + 1 })),
})
```

```ts
// stores/user-counter-store.ts
import { create } from 'zustand'

import { type CounterStore, counterStoreCreator } from './counter-store-creator'

export const useCounterStore = create<CounterStore>()(counterStoreCreator)
```

```tsx
// stores/use-counter-store-context.tsx
import {
  type PropsWithChildren,
  createContext,
  useContext,
  useRef,
} from 'react'
import { type StoreApi, createStore } from 'zustand'
import { useStoreWithEqualityFn } from 'zustand/traditional'
import { shallow } from 'zustand/shallow'

import { type CounterStore, counterStoreCreator } from './counter-store-creator'

export const createCounterStore = () => {
  return createStore<CounterStore>(counterStoreCreator)
}

export const CounterStoreContext = createContext<StoreApi<CounterStore>>(
  null as never,
)

export type CounterStoreProviderProps = PropsWithChildren

export const CounterStoreProvider = ({
  children,
}: CounterStoreProviderProps) => {
  const counterStoreRef = useRef(createCounterStore())

  return (
    <CounterStoreContext.Provider value={counterStoreRef.current}>
      {children}
    </CounterStoreContext.Provider>
  )
}

export type UseCounterStoreContextSelector<T> = (store: CounterStore) => T

export const useCounterStoreContext = <T,>(
  selector: UseCounterStoreContextSelector<T>,
): T => {
  const counterStoreContext = useContext(CounterStoreContext)

  if (counterStoreContext === undefined) {
    throw new Error(
      'useCounterStoreContext must be used within CounterStoreProvider',
    )
  }

  return useStoreWithEqualityFn(counterStoreContext, selector, shallow)
}
```

```tsx
// components/counter/counter.tsx
import { useCounterStore } from '../../stores/use-counter-store'

export function Counter() {
  const { count, inc } = useCounterStore()

  return (
    <div>
      <h2>Counter Store</h2>
      <h4>{count}</h4>
      <button onClick={inc}>One Up</button>
    </div>
  )
}
```

```ts
// components/counter/index.ts
export * from './counter'
```

```tsx
// components/counter/counter.test.tsx
import { act, render, screen } from '@testing-library/react'
import userEvent from '@testing-library/user-event'

import { Counter } from './counter'

describe('Counter', () => {
  test('should render with initial state of 1', async () => {
    renderCounter()

    expect(await screen.findByText(/^1$/)).toBeInTheDocument()
    expect(
      await screen.findByRole('button', { name: /one up/i }),
    ).toBeInTheDocument()
  })

  test('should increase count by clicking a button', async () => {
    const user = userEvent.setup()

    renderCounter()

    expect(await screen.findByText(/^1$/)).toBeInTheDocument()

    await act(async () => {
      await user.click(await screen.findByRole('button', { name: /one up/i }))
    })

    expect(await screen.findByText(/^2$/)).toBeInTheDocument()
  })
})

const renderCounter = () => {
  return render(<Counter />)
}
```

```tsx
// components/counter-with-context/counter-with-context.tsx
import {
  CounterStoreProvider,
  useCounterStoreContext,
} from '../../stores/use-counter-store-context'

const Counter = () => {
  const { count, inc } = useCounterStoreContext((state) => state)

  return (
    <div>
      <h2>Counter Store Context</h2>
      <h4>{count}</h4>
      <button onClick={inc}>One Up</button>
    </div>
  )
}

export const CounterWithContext = () => {
  return (
    <CounterStoreProvider>
      <Counter />
    </CounterStoreProvider>
  )
}
```

```tsx
// components/counter-with-context/index.ts
export * from './counter-with-context'
```

```tsx
// components/counter-with-context/counter-with-context.test.tsx
import { act, render, screen } from '@testing-library/react'
import userEvent from '@testing-library/user-event'

import { CounterWithContext } from './counter-with-context'

describe('CounterWithContext', () => {
  test('should render with initial state of 1', async () => {
    renderCounterWithContext()

    expect(await screen.findByText(/^1$/)).toBeInTheDocument()
    expect(
      await screen.findByRole('button', { name: /one up/i }),
    ).toBeInTheDocument()
  })

  test('should increase count by clicking a button', async () => {
    const user = userEvent.setup()

    renderCounterWithContext()

    expect(await screen.findByText(/^1$/)).toBeInTheDocument()

    await act(async () => {
      await user.click(await screen.findByRole('button', { name: /one up/i }))
    })

    expect(await screen.findByText(/^2$/)).toBeInTheDocument()
  })
})

const renderCounterWithContext = () => {
  return render(<CounterWithContext />)
}
```

> **Note**: without [globals configuration](https://vitest.dev/config/#globals) enabled, we need
> to add `import { describe, test, expect } from 'vitest'` at the top of each test file.

**CodeSandbox Demos**

- Jest Demo: https://codesandbox.io/p/sandbox/zustand-jest-demo-twkypn
- Vitest Demo: https://codesandbox.io/p/sandbox/zustand-vitest-demo-ph5gnj

## References

- **React Testing Library**: [React Testing Library (RTL)](https://testing-library.com/docs/react-testing-library/intro)
  is a very lightweight solution for testing React components. It provides utility functions on top
  of `react-dom` and `react-dom/test-utils`, in a way that encourages better testing practices. Its
  primary guiding principle is: "The more your tests resemble the way your software is used, the
  more confidence they can give you."
- **Native Testing Library**: [Native Testing Library (RNTL)](https://testing-library.com/docs/react-native-testing-library/intro)
  is a very lightweight solution for testing React Native components, similarly to RTL, but its
  functions are built on top of `react-test-renderer`.
- **Testing Implementation Details**: Blog post by Kent C. Dodds on why he recommends to avoid
  [testing implementation details](https://kentcdodds.com/blog/testing-implementation-details).


Setting Up a Test Environment

Usually, your test runner needs to be configured to run JavaScript/TypeScript syntax. If you're
going to be testing UI components, you will likely need to configure the test runner to use JSDOM
to provide a mock DOM environment.

Test Runners

We recommend using React Testing Library (RTL)
to test out React components that connect to Zustand. RTL is a simple and complete React DOM
testing utility that encourages good testing practices. It uses ReactDOM's render function and
act from react-dom/tests-utils. Futhermore, Native Testing Library (RNTL)
is the alternative to RTL to test out React Native components. The Testing Library
family of tools also includes adapters for many other popular frameworks.

UI and Network Testing Tools

Setting Up Zustand for testing

In the next steps we are going to setup our Jest environment in order to mock Zustand.

Jest

In the next steps we are going to setup our Vitest environment in order to mock Zustand.

Vitest

In the next examples we are going to use useCounterStore

Testing components

References

Testing

Calling actions outside a React event handler in pre React 18

Map and Set Usage


## Connect State with URL Hash

If you want to connect state of a store to URL hash, you can create your own hash storage.

```ts
import { create } from 'zustand'
import { persist, StateStorage, createJSONStorage } from 'zustand/middleware'

const hashStorage: StateStorage = {
  getItem: (key): string => {
    const searchParams = new URLSearchParams(location.hash.slice(1))
    const storedValue = searchParams.get(key) ?? ''
    return JSON.parse(storedValue)
  },
  setItem: (key, newValue): void => {
    const searchParams = new URLSearchParams(location.hash.slice(1))
    searchParams.set(key, JSON.stringify(newValue))
    location.hash = searchParams.toString()
  },
  removeItem: (key): void => {
    const searchParams = new URLSearchParams(location.hash.slice(1))
    searchParams.delete(key)
    location.hash = searchParams.toString()
  },
}

export const useBoundStore = create(
  persist(
    (set, get) => ({
      fishes: 0,
      addAFish: () => set({ fishes: get().fishes + 1 }),
    }),
    {
      name: 'food-storage', // unique name
      storage: createJSONStorage(() => hashStorage),
    },
  ),
)
```

### CodeSandbox Demo

https://codesandbox.io/s/zustand-state-with-url-hash-demo-f29b88?file=/src/store/index.ts

## Persist and Connect State with URL Parameters (Example: URL Query Parameters)

There are times when you want to conditionally connect the state to the URL.
This example depicts usage of the URL query parameters
while keeping it synced with another persistence implementation, like `localstorage`.

If you want the URL params to always populate, the conditional check on `getUrlSearch()` can be removed.

The implementation below will update the URL in place, without refresh, as the relevant states change.

```ts
import { create } from 'zustand'
import { persist, StateStorage, createJSONStorage } from 'zustand/middleware'

const getUrlSearch = () => {
  return window.location.search.slice(1)
}

const persistentStorage: StateStorage = {
  getItem: (key): string => {
    // Check URL first
    if (getUrlSearch()) {
      const searchParams = new URLSearchParams(getUrlSearch())
      const storedValue = searchParams.get(key)
      return JSON.parse(storedValue as string)
    } else {
      // Otherwise, we should load from localstorage or alternative storage
      return JSON.parse(localStorage.getItem(key) as string)
    }
  },
  setItem: (key, newValue): void => {
    // Check if query params exist at all, can remove check if always want to set URL
    if (getUrlSearch()) {
      const searchParams = new URLSearchParams(getUrlSearch())
      searchParams.set(key, JSON.stringify(newValue))
      window.history.replaceState(null, '', `?${searchParams.toString()}`)
    }

    localStorage.setItem(key, JSON.stringify(newValue))
  },
  removeItem: (key): void => {
    const searchParams = new URLSearchParams(getUrlSearch())
    searchParams.delete(key)
    window.location.search = searchParams.toString()
  },
}

type LocalAndUrlStore = {
  typesOfFish: string[]
  addTypeOfFish: (fishType: string) => void
  numberOfBears: number
  setNumberOfBears: (newNumber: number) => void
}

const storageOptions = {
  name: 'fishAndBearsStore',
  storage: createJSONStorage<LocalAndUrlStore>(() => persistentStorage),
}

const useLocalAndUrlStore = create(
  persist<LocalAndUrlStore>(
    (set) => ({
      typesOfFish: [],
      addTypeOfFish: (fishType) =>
        set((state) => ({ typesOfFish: [...state.typesOfFish, fishType] })),

      numberOfBears: 0,
      setNumberOfBears: (numberOfBears) => set(() => ({ numberOfBears })),
    }),
    storageOptions,
  ),
)

export default useLocalAndUrlStore
```

When generating the URL from a component, you can call buildShareableUrl:

```ts
const buildURLSuffix = (params, version = 0) => {
  const searchParams = new URLSearchParams()

  const zustandStoreParams = {
    state: {
      typesOfFish: params.typesOfFish,
      numberOfBears: params.numberOfBears,
    },
    version: version, // version is here because that is included with how Zustand sets the state
  }

  // The URL param key should match the name of the store, as specified as in storageOptions above
  searchParams.set('fishAndBearsStore', JSON.stringify(zustandStoreParams))
  return searchParams.toString()
}

export const buildShareableUrl = (params, version) => {
  return `${window.location.origin}?${buildURLSuffix(params, version)}`
}
```

The generated URL would look like (here without any encoding, for readability):

`https://localhost/search?fishAndBearsStore={"state":{"typesOfFish":["tilapia","salmon"],"numberOfBears":15},"version":0}}`


If you want to connect state of a store to URL hash, you can create your own hash storage.

Connect State with URL Hash

https://codesandbox.io/s/zustand-state-with-url-hash-demo-f29b88?file=/src/store/index.ts

There are times when you want to conditionally connect the state to the URL.
This example depicts usage of the URL query parameters
while keeping it synced with another persistence implementation, like localstorage.

Persist and Connect State with URL Parameters (Example: URL Query Parameters)

Connect to state with URL


The following pattern can be used to reset the state to its initial value.

```ts
import { create } from 'zustand'

// define types for state values and actions separately
type State = {
  salmon: number
  tuna: number
}

type Actions = {
  addSalmon: (qty: number) => void
  addTuna: (qty: number) => void
  reset: () => void
}

// define the initial state
const initialState: State = {
  salmon: 0,
  tuna: 0,
}

// create store
const useSlice = create<State & Actions>()((set, get) => ({
  ...initialState,
  addSalmon: (qty: number) => {
    set({ salmon: get().salmon + qty })
  },
  addTuna: (qty: number) => {
    set({ tuna: get().tuna + qty })
  },
  reset: () => {
    set(initialState)
  },
}))
```

Resetting multiple stores at once

```ts
import { create as _create } from 'zustand'
import type { StateCreator } from 'zustand'

const storeResetFns = new Set<() => void>()

const resetAllStores = () => {
  storeResetFns.forEach((resetFn) => {
    resetFn()
  })
}

export const create = (<T extends unknown>() => {
  return (stateCreator: StateCreator<T>) => {
    const store = _create(stateCreator)
    const initialState = store.getState()
    storeResetFns.add(() => {
      store.setState(initialState, true)
    })
    return store
  }
}) as typeof _create
```

Resetting bound store using Slices pattern

```ts
import create from 'zustand'
import type { StateCreator } from 'zustand'

const sliceResetFns = new Set<() => void>()

export const resetAllSlices = () => {
  sliceResetFns.forEach((resetFn) => {
    resetFn()
  })
}

const initialBearState = { bears: 0 }

interface BearSlice {
  bears: number
  addBear: () => void
  eatFish: () => void
}

const createBearSlice: StateCreator<
  BearSlice & FishSlice,
  [],
  [],
  BearSlice
> = (set) => {
  sliceResetFns.add(() => set(initialBearState))
  return {
    ...initialBearState,
    addBear: () => set((state) => ({ bears: state.bears + 1 })),
    eatFish: () => set((state) => ({ fishes: state.fishes - 1 })),
  }
}

const initialStateFish = { fishes: 0 }

interface FishSlice {
  fishes: number
  addFish: () => void
}

const createFishSlice: StateCreator<
  BearSlice & FishSlice,
  [],
  [],
  FishSlice
> = (set) => {
  sliceResetFns.add(() => set(initialStateFish))
  return {
    ...initialStateFish,
    addFish: () => set((state) => ({ fishes: state.fishes + 1 })),
  }
}

const useBoundStore = create<BearSlice & FishSlice>()((...a) => ({
  ...createBearSlice(...a),
  ...createFishSlice(...a),
}))

export default useBoundStore
```

## CodeSandbox Demo

- Basic: https://codesandbox.io/s/zustand-how-to-reset-state-basic-demo-rrqyon
- Advanced: https://codesandbox.io/s/zustand-how-to-reset-state-advanced-demo-gtu0qe
- Immer: https://codesandbox.io/s/how-to-reset-state-advance-immer-demo-nyet3f


How to reset state


In cases where [dependency injection](https://en.wikipedia.org/wiki/Dependency_injection) is needed, such as when a store should be initialized with props from a component, the recommended approach is to use a vanilla store with React.context.

## Store creator with `createStore`

```ts
import { createStore } from 'zustand'

interface BearProps {
  bears: number
}

interface BearState extends BearProps {
  addBear: () => void
}

type BearStore = ReturnType<typeof createBearStore>

const createBearStore = (initProps?: Partial<BearProps>) => {
  const DEFAULT_PROPS: BearProps = {
    bears: 0,
  }
  return createStore<BearState>()((set) => ({
    ...DEFAULT_PROPS,
    ...initProps,
    addBear: () => set((state) => ({ bears: ++state.bears })),
  }))
}
```

## Creating a context with `React.createContext`

```ts
import { createContext } from 'react'

export const BearContext = createContext<BearStore | null>(null)
```

## Basic component usage

```tsx
// Provider implementation
import { useRef } from 'react'

function App() {
  const store = useRef(createBearStore()).current
  return (
    <BearContext.Provider value={store}>
      <BasicConsumer />
    </BearContext.Provider>
  )
}
```

```tsx
// Consumer component
import { useContext } from 'react'
import { useStore } from 'zustand'

function BasicConsumer() {
  const store = useContext(BearContext)
  if (!store) throw new Error('Missing BearContext.Provider in the tree')
  const bears = useStore(store, (s) => s.bears)
  const addBear = useStore(store, (s) => s.addBear)
  return (
    <>
      <div>{bears} Bears.</div>
      <button onClick={addBear}>Add bear</button>
    </>
  )
}
```

## Common patterns

### Wrapping the context provider

```tsx
// Provider wrapper
import { useRef } from 'react'

type BearProviderProps = React.PropsWithChildren<BearProps>

function BearProvider({ children, ...props }: BearProviderProps) {
  const storeRef = useRef<BearStore>()
  if (!storeRef.current) {
    storeRef.current = createBearStore(props)
  }
  return (
    <BearContext.Provider value={storeRef.current}>
      {children}
    </BearContext.Provider>
  )
}
```

### Extracting context logic into a custom hook

```tsx
// Mimic the hook returned by `create`
import { useContext } from 'react'
import { useStore } from 'zustand'

function useBearContext<T>(selector: (state: BearState) => T): T {
  const store = useContext(BearContext)
  if (!store) throw new Error('Missing BearContext.Provider in the tree')
  return useStore(store, selector)
}
```

```tsx
// Consumer usage of the custom hook
function CommonConsumer() {
  const bears = useBearContext((s) => s.bears)
  const addBear = useBearContext((s) => s.addBear)
  return (
    <>
      <div>{bears} Bears.</div>
      <button onClick={addBear}>Add bear</button>
    </>
  )
}
```

### Optionally allow using a custom equality function

```tsx
// Allow custom equality function by using useStoreWithEqualityFn instead of useStore
import { useContext } from 'react'
import { useStoreWithEqualityFn } from 'zustand/traditional'

function useBearContext<T>(
  selector: (state: BearState) => T,
  equalityFn?: (left: T, right: T) => boolean,
): T {
  const store = useContext(BearContext)
  if (!store) throw new Error('Missing BearContext.Provider in the tree')
  return useStoreWithEqualityFn(store, selector, equalityFn)
}
```

### Complete example

```tsx
// Provider wrapper & custom hook consumer
function App2() {
  return (
    <BearProvider bears={2}>
      <HookConsumer />
    </BearProvider>
  )
}
```


Store creator with createStore

Creating a context with React.createContext

Basic component usage

Common patterns

Wrapping the context provider

Extracting context logic into a custom hook

Optionally allow using a custom equality function

Complete example

Initialize state with props


## Slicing the store into smaller stores

Your store can become bigger and bigger and tougher to maintain as you add more features.

You can divide your main store into smaller individual stores to achieve modularity. This is simple to accomplish in Zustand!

The first individual store:

```js
export const createFishSlice = (set) => ({
  fishes: 0,
  addFish: () => set((state) => ({ fishes: state.fishes + 1 })),
})
```

Another individual store:

```js
export const createBearSlice = (set) => ({
  bears: 0,
  addBear: () => set((state) => ({ bears: state.bears + 1 })),
  eatFish: () => set((state) => ({ fishes: state.fishes - 1 })),
})
```

You can now combine both the stores into **one bounded store**:

```js
import { create } from 'zustand'
import { createBearSlice } from './bearSlice'
import { createFishSlice } from './fishSlice'

export const useBoundStore = create((...a) => ({
  ...createBearSlice(...a),
  ...createFishSlice(...a),
}))
```

### Usage in a React component

```jsx
import { useBoundStore } from './stores/useBoundStore'

function App() {
  const bears = useBoundStore((state) => state.bears)
  const fishes = useBoundStore((state) => state.fishes)
  const addBear = useBoundStore((state) => state.addBear)
  return (
    <div>
      <h2>Number of bears: {bears}</h2>
      <h2>Number of fishes: {fishes}</h2>
      <button onClick={() => addBear()}>Add a bear</button>
    </div>
  )
}

export default App
```

### Updating multiple stores

You can update multiple stores, at the same time, in a single function.

```js
export const createBearFishSlice = (set, get) => ({
  addBearAndFish: () => {
    get().addBear()
    get().addFish()
  },
})
```

Combining all the stores together is the same as before.

```js
import { create } from 'zustand'
import { createBearSlice } from './bearSlice'
import { createFishSlice } from './fishSlice'
import { createBearFishSlice } from './createBearFishSlice'

export const useBoundStore = create((...a) => ({
  ...createBearSlice(...a),
  ...createFishSlice(...a),
  ...createBearFishSlice(...a),
}))
```

## Adding middlewares

Adding middlewares to a combined store is the same as with other normal stores.

Adding `persist` middleware to our `useBoundStore`:

```js
import { create } from 'zustand'
import { createBearSlice } from './bearSlice'
import { createFishSlice } from './fishSlice'
import { persist } from 'zustand/middleware'

export const useBoundStore = create(
  persist(
    (...a) => ({
      ...createBearSlice(...a),
      ...createFishSlice(...a),
    }),
    { name: 'bound-store' },
  ),
)
```

Please keep in mind you should only apply middlewares in the combined store. Applying them inside individual slices can lead to unexpected issues.

## Usage with TypeScript

A detailed guide on how to use the slice pattern in Zustand with TypeScript can be found [here](./typescript.md#slices-pattern).


Your store can become bigger and bigger and tougher to maintain as you add more features.

Slicing the store into smaller stores

Usage in a React component

You can update multiple stores, at the same time, in a single function.

Updating multiple stores

Adding middlewares to a combined store is the same as with other normal stores.

Adding middlewares

A detailed guide on how to use the slice pattern in Zustand with TypeScript can be found here.

Usage with TypeScript

Slices Pattern


When you need to subscribe to a computed state from a store, the recommended way is to
use a selector.

The computed selector will cause a rererender if the output has changed according to [Object.is](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is?retiredLocale=it).

In this case you might want to use `useShallow` to avoid a rerender if the computed value is always shallow
equal the previous one.

## Example

We have a store that associates to each bear a meal and we want to render their names.

```js
import { create } from 'zustand'

const useMeals = create(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  littleBear: 'A little, small, wee pot',
}))

export const BearNames = () => {
  const names = useMeals((state) => Object.keys(state))

  return <div>{names.join(', ')}</div>
}
```

Now papa bear wants a pizza instead:

```js
useMeals.setState({
  papaBear: 'a large pizza',
})
```

This change causes `BearNames` rerenders even though the actual output of `names` has not changed according to shallow equal.

We can fix that using `useShallow`!

```js
import { create } from 'zustand'
import { useShallow } from 'zustand/react/shallow'

const useMeals = create(() => ({
  papaBear: 'large porridge-pot',
  mamaBear: 'middle-size porridge pot',
  littleBear: 'A little, small, wee pot',
}))

export const BearNames = () => {
  const names = useMeals(useShallow((state) => Object.keys(state)))

  return <div>{names.join(', ')}</div>
}
```

Now they can all order other meals without causing unnecessary rerenders of our `BearNames` component.


We have a store that associates to each bear a meal and we want to render their names.

Example

Prevent rerenders with useShallow


The [Immer](https://github.com/immerjs/immer) middleware enables you
to use immutable state in a more convenient way.
Also, with Immer, you can simplify handling
immutable data structures in Zustand.

## Installation

In order to use the Immer middleware in Zustand,
you will need to install Immer as a direct dependency.

```bash
npm install immer
```

## Usage

(Notice the extra parentheses after the type parameter as mentioned in the [Typescript Guide](../guides/typescript.md)).

Updating simple states

```ts
import { create } from 'zustand'
import { immer } from 'zustand/middleware/immer'

type State = {
  count: number
}

type Actions = {
  increment: (qty: number) => void
  decrement: (qty: number) => void
}

export const useCountStore = create<State & Actions>()(
  immer((set) => ({
    count: 0,
    increment: (qty: number) =>
      set((state) => {
        state.count += qty
      }),
    decrement: (qty: number) =>
      set((state) => {
        state.count -= qty
      }),
  })),
)
```

Updating complex states

```ts
import { create } from 'zustand'
import { immer } from 'zustand/middleware/immer'

interface Todo {
  id: string
  title: string
  done: boolean
}

type State = {
  todos: Record<string, Todo>
}

type Actions = {
  toggleTodo: (todoId: string) => void
}

export const useTodoStore = create<State & Actions>()(
  immer((set) => ({
    todos: {
      '82471c5f-4207-4b1d-abcb-b98547e01a3e': {
        id: '82471c5f-4207-4b1d-abcb-b98547e01a3e',
        title: 'Learn Zustand',
        done: false,
      },
      '354ee16c-bfdd-44d3-afa9-e93679bda367': {
        id: '354ee16c-bfdd-44d3-afa9-e93679bda367',
        title: 'Learn Jotai',
        done: false,
      },
      '771c85c5-46ea-4a11-8fed-36cc2c7be344': {
        id: '771c85c5-46ea-4a11-8fed-36cc2c7be344',
        title: 'Learn Valtio',
        done: false,
      },
      '363a4bac-083f-47f7-a0a2-aeeee153a99c': {
        id: '363a4bac-083f-47f7-a0a2-aeeee153a99c',
        title: 'Learn Signals',
        done: false,
      },
    },
    toggleTodo: (todoId: string) =>
      set((state) => {
        state.todos[todoId].done = !state.todos[todoId].done
      }),
  })),
)
```

## Gotchas

In this section you will find some things
that you need to keep in mind when using Zustand with Immer.

### My subscriptions aren't being called

If you are using Immer,
make sure you are actually following
[the rules of Immer](https://immerjs.github.io/immer/pitfalls).

For example, you have to add `[immerable] = true` for
[class objects](https://immerjs.github.io/immer/complex-objects) to work.
If you don't do this, Immer will still mutate the object,
but not as a proxy, so it will also update the current state.
Zustand checks if the state has actually changed,
so since both the current state and the next state are
equal (if you don't do it correctly),
Zustand will skip calling the subscriptions.

## CodeSandbox Demo

- [Basic](https://codesandbox.io/p/sandbox/zustand-updating-draft-states-basic-demo-forked-96mkdw),
- [Advanced](https://codesandbox.io/p/sandbox/zustand-updating-draft-states-advanced-demo-forked-phkzzg).


In order to use the Immer middleware in Zustand,
you will need to install Immer as a direct dependency.

(Notice the extra parentheses after the type parameter as mentioned in the Typescript Guide).

Usage

In this section you will find some things
that you need to keep in mind when using Zustand with Immer.

Gotchas

If you are using Immer,
make sure you are actually following
the rules of Immer.

My subscriptions aren't being called

Immer middleware


The Persist middleware enables you to store
your Zustand state in a storage
(e.g., `localStorage`, `AsyncStorage`, `IndexedDB`, etc.),
thus persisting its data.

Note that this middleware supports both
synchronous storages, like `localStorage`,
and asynchronous storages, like `AsyncStorage`,
but using an asynchronous storage does come with a cost.
See [Hydration and asynchronous storages](#hydration-and-asynchronous-storages)
for more details.

## Simple example

```ts
import { create } from 'zustand'
import { persist, createJSONStorage } from 'zustand/middleware'

export const useBearStore = create(
  persist(
    (set, get) => ({
      bears: 0,
      addABear: () => set({ bears: get().bears + 1 }),
    }),
    {
      name: 'food-storage', // name of the item in the storage (must be unique)
      storage: createJSONStorage(() => sessionStorage), // (optional) by default, 'localStorage' is used
    },
  ),
)
```

## Options

### `name`

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.

### `storage`

> Type: `() => StateStorage`

The `StateStorage` can be imported with:

```ts
import { StateStorage } from 'zustand/middleware'
```

> Default: `createJSONStorage(() => localStorage)`

Enables you to use your own storage. Simply pass a function that returns the storage you want to use. It's recommended to use the [`createJSONStorage`](#createjsonstorage) helper function to create a `storage` object that is compliant with the `StateStorage` interface.

Example:

```ts
import { persist, createJSONStorage } from 'zustand/middleware'

export const useBoundStore = create(
  persist(
    (set, get) => ({
      // ...
    }),
    {
      // ...
      storage: createJSONStorage(() => AsyncStorage),
    },
  ),
)
```

### `partialize`

> Type: `(state: Object) => Object`

> Default: `(state) => state`

Enables you to pick some of the state's fields to be stored in the storage.

You could omit multiple fields using the following:

```ts
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:

```ts
export const useBoundStore = create(
  persist(
    (set, get) => ({
      foo: 0,
      bar: 1,
    }),
    {
      // ...
      partialize: (state) => ({ foo: state.foo }),
    },
  ),
)
```

### `onRehydrateStorage`

> Type: `(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:

```ts
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')
          }
        }
      },
    },
  ),
)
```

### `version`

> Type: `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.
You can use the [migrate](#migrate) function (see below)
to handle breaking changes in order to persist previously stored data.

### `migrate`

> Type: `(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:

```ts
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
      },
    },
  ),
)
```

### `merge`

> Type: `(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:

```ts
{
  foo: {
    bar: 0,
  }
}
```

But your Zustand store contains:

```ts
{
  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:

```ts
export const useBoundStore = create(
  persist(
    (set, get) => ({
      foo: {
        bar: 0,
        baz: 1,
      },
    }),
    {
      // ...
      merge: (persistedState, currentState) =>
        deepMerge(currentState, persistedState),
    },
  ),
)
```

### `skipHydration`

> Type: `boolean | undefined`

> Default: `undefined`

By default the store will be hydrated on initialization.

In some applications you may need to control when the first hydration occurs.
For example, in server-rendered apps.

If you set `skipHydration`, the initial call for hydration isn't called,
and it is left up to you to manually call `rehydrate()`.

```ts
export const useBoundStore = create(
  persist(
    () => ({
      count: 0,
      // ...
    }),
    {
      // ...
      skipHydration: true,
    },
  ),
)
```

```tsx
import { useBoundStore } from './path-to-store';

export function StoreConsumer() {
  // hydrate persisted store after on mount
  useEffect(() => {
    useBoundStore.persist.rehydrate();
  }, [])

  return (
    //...
  )
}
```

## API

> Version: >=3.6.3

The Persist API enables you to do a number of interactions
with the Persist middleware
from inside or outside of a React component.

### `getOptions`

> Type: `() => Partial<PersistOptions>`

> Returns: Options of the Persist middleware

For example, it can be used to obtain the storage name:

```ts
useBoundStore.persist.getOptions().name
```

### `setOptions`

> Type: `(newOptions: Partial<PersistOptions>) => void`

Changes 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:

```ts
useBoundStore.persist.setOptions({
  name: 'new-name',
})
```

Or even to change the storage engine:

```ts
useBoundStore.persist.setOptions({
  storage: createJSONStorage(() => sessionStorage),
})
```

### `clearStorage`

> Type: `() => void`

Clears everything stored under the [name](#name) key.

```ts
useBoundStore.persist.clearStorage()
```

### `rehydrate`

> Type: `() => Promise<void>`

In some cases, you might want to trigger the rehydration manually.
This can be done by calling the `rehydrate` method.

```ts
await useBoundStore.persist.rehydrate()
```

### `hasHydrated`

> Type: `() => boolean`

This is a non-reactive getter to check
if the storage has been hydrated
(note that it updates when calling [`rehydrate`](#rehydrate)).

```ts
useBoundStore.persist.hasHydrated()
```

### `onHydrate`

> Type: `(listener: (state) => void) => () => void`

> Returns: Unsubscribe function

This listener will be called when the hydration process starts.

```ts
const unsub = useBoundStore.persist.onHydrate((state) => {
  console.log('hydration starts')
})

// later on...
unsub()
```

### `onFinishHydration`

> Type: `(listener: (state) => void) => () => void`

> Returns: Unsubscribe function

This listener will be called when the hydration process ends.

```ts
const unsub = useBoundStore.persist.onFinishHydration((state) => {
  console.log('hydration finished')
})

// later on...
unsub()
```

### `createJSONStorage`

> Type: `(getStorage: () => StateStorage, options?: JsonStorageOptions) => StateStorage`

> Returns: `PersistStorage`

This helper function enables you to create a [`storage`](#storage) object which is useful when you want to use a custom storage engine.

`getStorage` is a function that returns the storage engine with the properties `getItem`, `setItem`, and `removeItem`.

`options` is an optional object that can be used to customize the serialization and deserialization of the data. `options.reviver` is a function that is passed to `JSON.parse` to deserialize the data. `options.replacer` is a function that is passed to `JSON.stringify` to serialize the data.

```ts
import { createJSONStorage } from 'zustand/middleware'

const storage = createJSONStorage(() => sessionStorage, {
  reviver: (key, value) => {
    if (value && value.type === 'date') {
      return new Date(value)
    }
    return value
  },
  replacer: (key, value) => {
    if (value instanceof Date) {
      return { type: 'date', value: value.toISOString() }
    }
    return value
  },
})
```

## Hydration and asynchronous storages

To explain what is the "cost" of asynchronous storages,
you need to understand what is hydration.

In a nutshell, hydration is a process
of retrieving 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 (shocking, I know!).

But what's the catch?
With synchronous hydration,
the Zustand store will already have been hydrated at its creation.
In contrast, 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 your app depends on the persisted value at page load,
you might want to wait until
the store has been hydrated before showing anything.
For example, your app might think the user
is not logged in because it's the default,
but 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_](#how-can-i-check-if-my-store-has-been-hydrated)
in the [FAQ](#faq) section below.

### Usage in Next.js

NextJS uses Server Side Rendering, and it will compare the rendered component on the server with the one rendered on client.
But since you are using data from browser to change your component, the two renders will differ and Next will throw a warning at you.

The errors usually are:

- Text content does not match server-rendered HTML
- Hydration failed because the initial UI does not match what was rendered on the server
- There was an error while hydrating. Because the error happened outside of a Suspense boundary, the entire root will switch to client rendering

To solve these errors, create a custom hook so that Zustand waits a little before changing your components.

Create a file with the following:

```ts
// useStore.ts
import { useState, useEffect } from 'react'

const useStore = <T, F>(
  store: (callback: (state: T) => unknown) => unknown,
  callback: (state: T) => F,
) => {
  const result = store(callback) as F
  const [data, setData] = useState<F>()

  useEffect(() => {
    setData(result)
  }, [result])

  return data
}

export default useStore
```

Now in your pages, you will use the hook a little bit differently:

```ts
// useBearStore.ts

import { create } from 'zustand'
import { persist } from 'zustand/middleware'

// the store itself does not need any change
export const useBearStore = create(
  persist(
    (set, get) => ({
      bears: 0,
      addABear: () => set({ bears: get().bears + 1 }),
    }),
    {
      name: 'food-storage',
    },
  ),
)
```

```ts
// yourComponent.tsx

import useStore from './useStore'
import { useBearStore } from './stores/useBearStore'

const bears = useStore(useBearStore, (state) => state.bears)
```

Credits: [This reply to an issue](https://github.com/pmndrs/zustand/issues/938#issuecomment-1481801942), which points to [this blog post](https://dev.to/abdulsamad/how-to-use-zustands-persist-middleware-in-nextjs-4lb5).

## FAQ

### How can I check if my store has been hydrated

There are a few different ways to do this.

You can use the [`onRehydrateStorage`](#onrehydratestorage)
listener function to update a field in the store:

```ts
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:

```ts
const useBoundStore = create(persist(...))

const useHydration = () => {
  const [hydrated, setHydrated] = useState(false)

  useEffect(() => {
    // Note: This is just in case you want to take into account manual rehydration.
    // You can remove the following line if you don't need it.
    const unsubHydrate = useBoundStore.persist.onHydrate(() => setHydrated(false))

    const unsubFinishHydration = useBoundStore.persist.onFinishHydration(() => setHydrated(true))

    setHydrated(useBoundStore.persist.hasHydrated())

    return () => {
      unsubHydrate()
      unsubFinishHydration()
    }
  }, [])

  return hydrated
}
```

### How can I use a custom storage engine

If the storage you want to use does not match the expected API, you can create your own storage:

```ts
import { create } from 'zustand'
import { persist, createJSONStorage, 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) => ({
      bears: 0,
      addABear: () => set({ bears: get().bears + 1 }),
    }),
    {
      name: 'food-storage', // unique name
      storage: createJSONStorage(() => storage),
    },
  ),
)
```

If you're using a type that `JSON.stringify()` doesn't support, you'll need to write your own serialization/deserialization code. However, if this is tedious, you can use third-party libraries to serialize and deserialize different types of data.

For example, [Superjson](https://github.com/blitz-js/superjson) can serialize data along with its type, allowing the data to be parsed back to its original type upon deserialization

```ts
import superjson from 'superjson' //  can use anything: serialize-javascript, devalue, etc.
import { PersistStorage } from 'zustand/middleware'

interface BearState {
  bear: Map<string, string>
  fish: Set<string>
  time: Date
  query: RegExp
}

const storage: PersistStorage<BearState> = {
  getItem: (name) => {
    const str = localStorage.getItem(name)
    if (!str) return null
    return superjson.parse(str)
  },
  setItem: (name, value) => {
    localStorage.setItem(name, superjson.stringify(value))
  },
  removeItem: (name) => localStorage.removeItem(name),
}

const initialState: BearState = {
  bear: new Map(),
  fish: new Set(),
  time: new Date(),
  query: new RegExp(''),
}

export const useBearStore = create<BearState>()(
  persist(
    (set) => ({
      ...initialState,
      // ...
    }),
    {
      name: 'food-storage',
      storage,
    },
  ),
)
```

### How can I rehydrate on storage event

You can use the Persist API to create your own implementation,
similar to the example below:

```ts
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)
```

### How do I use it with TypeScript

Basic typescript usage doesn't require anything special
except for writing `create<State>()(...)` instead of `create(...)`.

```tsx
import { create } from 'zustand'
import { persist, createJSONStorage } from 'zustand/middleware'

interface MyState {
  bears: number
  addABear: () => void
}

export const useBearStore = create<MyState>()(
  persist(
    (set, get) => ({
      bears: 0,
      addABear: () => set({ bears: get().bears + 1 }),
    }),
    {
      name: 'food-storage', // name of item in the storage (must be unique)
      storage: createJSONStorage(() => sessionStorage), // (optional) by default the 'localStorage' is used
      partialize: (state) => ({ bears: state.bears }),
    },
  ),
)
```

### How do I use it with Map and Set

In order to persist object types such as `Map` and `Set`, they will need to be converted to JSON-serializable types such as an `Array` which can be done by defining a custom `storage` engine.

Let's say your state uses `Map` to handle a list of `transactions`,
then you can convert the `Map` into an `Array` in the `storage` prop which is shown below:

```ts

interface BearState {
  .
  .
  .
  transactions: Map<any>
}

  storage: {
    getItem: (name) => {
      const str = localStorage.getItem(name);
      if (!str) return null;
      const { state } = JSON.parse(str);
      return {
        state: {
          ...state,
          transactions: new Map(state.transactions),
        },
      }
    },
    setItem: (name, newValue: StorageValue<BearState>) => {
      // functions cannot be JSON encoded
      const str = JSON.stringify({
        state: {
          ...newValue.state,
          transactions: Array.from(newValue.state.transactions.entries()),
        },
      })
      localStorage.setItem(name, str)
    },
    removeItem: (name) => localStorage.removeItem(name),
  },
```


Simple example

Options

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.

name

storage

partialize

onRehydrateStorage

version

migrate

merge

skipHydration

getOptions

setOptions

clearStorage

rehydrate

hasHydrated

onHydrate

onFinishHydration

createJSONStorage

To explain what is the "cost" of asynchronous storages,
you need to understand what is hydration.

Hydration and asynchronous storages

NextJS uses Server Side Rendering, and it will compare the rendered component on the server with the one rendered on client.
But since you are using data from browser to change your component, the two renders will differ and Next will throw a warning at you.

Usage in Next.js

There are a few different ways to do this.

How can I check if my store has been hydrated

If the storage you want to use does not match the expected API, you can create your own storage:

How can I use a custom storage engine

You can use the Persist API to create your own implementation,
similar to the example below:

How can I rehydrate on storage event

Basic typescript usage doesn't require anything special
except for writing create<State>()(...) instead of create(...).

How do I use it with TypeScript

In order to persist object types such as Map and Set, they will need to be converted to JSON-serializable types such as an Array which can be done by defining a custom storage engine.

How do I use it with Map and Set

Persisting store data


A special `createContext` is provided since v3.5,
which avoids misusing the store hook.

> **Note**: This function is deprecated in v4 and will be removed in v5. See [Migration](#migration).

```jsx
import create from 'zustand'
import createContext from 'zustand/context'

const { Provider, useStore } = createContext()

const createStore = () => create(...)

const App = () => (
  <Provider createStore={createStore}>
    ...
  </Provider>
)

const Component = () => {
  const state = useStore()
  const slice = useStore(selector)
  ...
```

## createContext usage in real components

```jsx
import create from "zustand";
import createContext from "zustand/context";

// Best practice: You can move the below createContext() and createStore to a separate file(store.js) and import the Provider, useStore here/wherever you need.

const { Provider, useStore } = createContext();

const createStore = () =>
  create((set) => ({
    bears: 0,
    increasePopulation: () => set((state) => ({ bears: state.bears + 1 })),
    removeAllBears: () => set({ bears: 0 })
  }));

const Button = () => {
  return (
      {/** store() - This will create a store for each time using the Button component instead of using one store for all components **/}
    <Provider createStore={createStore}>
      <ButtonChild />
    </Provider>
  );
};

const ButtonChild = () => {
  const state = useStore();
  return (
    <div>
      {state.bears}
      <button
        onClick={() => {
          state.increasePopulation();
        }}
      >
        +
      </button>
    </div>
  );
};

export default function App() {
  return (
    <div className="App">
      <Button />
      <Button />
    </div>
  );
}
```

## createContext usage with initialization from props

```tsx
import create from 'zustand'
import createContext from 'zustand/context'

const { Provider, useStore } = createContext()

export default function App({ initialBears }) {
  return (
    <Provider
      createStore={() =>
        create((set) => ({
          bears: initialBears,
          increase: () => set((state) => ({ bears: state.bears + 1 })),
        }))
      }
    >
      <Button />
    </Provider>
  )
}
```

## Migration

Discussion: https://github.com/pmndrs/zustand/discussions/1276

Here's the new context usage with v4 API.

```jsx
import { createContext, useContext, useRef } from 'react'
import { createStore, useStore } from 'zustand'

const StoreContext = createContext(null)

const StoreProvider = ({ children }) => {
  const storeRef = useRef()
  if (!storeRef.current) {
    storeRef.current = createStore((set) => ({
      // ...
    }))
  }
  return (
    <StoreContext.Provider value={storeRef.current}>
      {children}
    </StoreContext.Provider>
  )
}

const useStoreInContext = (selector) => {
  const store = useContext(StoreContext)
  if (!store) {
    throw new Error('Missing StoreProvider')
  }
  return useStore(store, selector)
}
```

Or reach out to some third-party libraries that provide Zustand v3-like APIs:

- https://github.com/charkour/zustand-di
- https://github.com/arvinxx/zustand-utils


createContext usage in real components

createContext usage with initialization from props

Discussion: https://github.com/pmndrs/zustand/discussions/1276

Migration

createContext from zustand/context


The only breaking changes are in types.
If you are using Zustand with TypeScript
or JSDoc type annotations,
this guide applies.
Otherwise, no migration is required.

Also, it's recommended to first read
the new [TypeScript Guide](../guides/typescript.md),
so that the migration is easier to understand.

In addition to this migration guide,
you can also check the
[diff](https://github.com/pmndrs/zustand/compare/v3.7.2...v4.0.0?short_path=37e5b4c#diff-c21e24854115b390eccde717da83f91feb2d5927a76c1485e5f0fdd0135c2afa)
of the test files in the Zustand repository from v3 to v4.

## `create`

**Applicable imports**

```ts
import create from 'zustand'
import create from 'zustand/vanilla'
```

**Change**

```diff
- create:
-   < State
-   , StoreSetState = StoreApi<State>["set"]
-   , StoreGetState = StoreApi<State>["get"]
-   , Store = StoreApi<State>
-   >
-     (f: ...) => ...
+ create:
+   { <State>(): (f: ...) => ...
+   , <State, Mutators>(f: ...) => ...
+   }
```

**Migration**

If you are not passing any type parameters to `create`,
no migration is required.

If you are using a "leaf" middleware like `combine` or `redux`,
remove all type parameters from `create`.

Else, replace `create<T, ...>(...)` with `create<T>()(...)`.

## `StateCreator`

**Applicable imports**

```ts
import type { StateCreator } from 'zustand'
import type { StateCreator } from 'zustand/vanilla'
```

**Change**

```diff
- type StateCreator
-   < State
-   , StoreSetState = StoreApi<State>["set"]
-   , StoreGetState = StoreApi<State>["get"]
-   , Store = StoreApi<State>
-   > =
-     ...
+ type StateCreator
+   < State
+   , InMutators extends [StoreMutatorIdentifier, unknown][] = []
+   , OutMutators extends [StoreMutatorIdentifier, unknown][] = []
+   , Return = State
+   > =
+     ...
```

**Migration**

If you are using `StateCreator`,
you are likely authoring a middleware
or using the "slices" pattern.
For that check the
[Authoring middlewares and advanced usage](../guides/typescript.md#authoring-middlewares-and-advanced-usage)
and [Common recipes](../guides/typescript.md#common-recipes)
sections of the TypeScript Guide.

## `PartialState`

**Applicable imports**

```ts
import type { PartialState } from 'zustand'
import type { PartialState } from 'zustand/vanilla'
```

**Change**

```diff
- type PartialState
-   < T extends State
-   , K1 extends keyof T = keyof T
-   , K2 extends keyof T = K1
-   , K3 extends keyof T = K2
-   , K4 extends keyof T = K3
-   > =
-   | (Pick<T, K1> | Pick<T, K2> | Pick<T, K3> | Pick<T, K4> | T)
-   | ((state: T) => Pick<T, K1> | Pick<T, K2> | Pick<T, K3> | Pick<T, K4> | T)
+ type PartialState<T> =
+   | Partial<T>
+   | ((state: T) => Partial<T>)
```

**Migration**

Replace `PartialState<T, ...>` with `PartialState<T>`
and preferably turn on [`exactOptionalPropertyTypes`](https://www.typescriptlang.org/tsconfig#exactOptionalPropertyTypes)
in your `tsconfig.json`:

```json
{
  "compilerOptions": {
    "exactOptionalPropertyTypes": true
  }
}
```

We're no longer using the trick to disallow `{ foo: undefined }`
to be assigned to `Partial<{ foo: string }>`.
Instead, we're relying on the users to turn on `exactOptionalPropertyTypes`.

## `useStore`

**Applicable imports**

```ts
import { useStore } from 'zustand'
import { useStore } from 'zustand/react'
```

**Change**

```diff
- useStore:
-   { <State>(store: StoreApi<State>): State
-   , <State, StateSlice>
-       ( store: StoreApi<State>
-       , selector: StateSelector<State, StateSlice>,
-       , equals?: EqualityChecker<StateSlice>
-       ): StateSlice
-   }
+ useStore:
+   <Store, StateSlice = ExtractState<Store>>
+     ( store: Store
+     , selector?: StateSelector<State, StateSlice>,
+     , equals?: EqualityChecker<StateSlice>
+     )
+       => StateSlice
```

**Migration**

If you are not passing any type parameters to `useStore`,
no migration is required.

If you are,
it's recommended to remove all the type parameters,
or pass the **store** type instead of the **state** type as the first parameter.

## `UseBoundStore`

**Applicable imports**

```ts
import type { UseBoundStore } from 'zustand'
import type { UseBoundStore } from 'zustand/react'
```

**Change**

```diff
- type UseBoundStore<
-   State,
-   Store = StoreApi<State>
- > =
-   & { (): T
-     , <StateSlice>
-         ( selector: StateSelector<State, StateSlice>
-         , equals?: EqualityChecker<StateSlice>
-         ): U
-     }
-   & Store
+ type UseBoundStore<Store> =
+   & (<StateSlice = ExtractState<S>>
+       ( selector?: (state: ExtractState<S>) => StateSlice
+       , equals?: (a: StateSlice, b: StateSlice) => boolean
+       ) => StateSlice
+     )
+   & S
```

**Migration**

Replace `UseBoundStore<T>` with `UseBoundStore<StoreApi<T>>`,
and `UseBoundStore<T, S>` with `UseBoundStore<S>`

## `UseContextStore`

**Applicable imports**

```ts
import type { UseContextStore } from 'zustand/context'
```

**Change**

```diff
- type UseContextStore
```

**Migration**

Use `typeof MyContext.useStore` instead

## `createContext`

**Applicable imports**

```ts
import createContext from 'zustand/context'
```

**Change**

```diff
  createContext:
-   <State, Store = StoreApi<State>>() => ...
+   <Store>() => ...
```

**Migration**

Replace `createContext<T>()` with `createContext<StoreApi<T>>()`,
and `createContext<T, S>()` with `createContext<S>()`.

## `combine`, `devtools`, `subscribeWithSelector`

**Applicable imports**

```ts
import { combine } from 'zustand/middleware'
import { devtools } from 'zustand/middleware'
import { subscribeWithSelector } from 'zustand/middleware'
```

**Change**

```diff
- combine:
-   <T, U>(...) => ...
+ combine:
+   <T, U, Mps, Mcs>(...) => ...

- devtools:
-   <T>(...) => ...
+ devtools:
+   <T, Mps, Mcs>(...) => ...

- subscribeWithSelector:
-   <T>(...) => ...
+ subscribeWithSelector:
+   <T, Mps, Mcs>(...) => ...
```

**Migration**

If you are not passing any type parameters
to `combine`, `devtools`, or `subscribeWithSelector`,
no migration is required.

If you are,
remove all the type parameters,
as they are inferred automatically.

## `persist`

**Applicable imports**

```ts
import { persist } from 'zustand/middleware'
```

**Change**

```diff
- persist:
-   <T, U = Partial<T>>(...) => ...
+ persist:
+   <T, Mps, Mcs, U = T>(...) => ...
```

**Migration**

If you are passing any type parameters,
remove them as they are inferred automatically.

Next, if you are passing the `partialize` option,
there is no further steps required for migration.

If you are **not** passing the `partialize` option,
you might see some compilation errors.
If you do not see any,
there is no further migration required.

The type of partialized state is now `T` instead of `Partial<T>`,
which aligns with the runtime behavior of the default `partialize`,
which is an identity (`s => s`).

If you see some compilation errors,
you have to find and fix the errors yourself,
because they might be indicative of unsound code.
Alternatively, the workaround will be passing
`s => s as Partial<typeof s>` to `partialize`.
If your partialized state is truly `Partial<T>`,
you should not encounter any bugs.

The runtime behavior has not changed,
only the types are now correct.

## `redux`

**Applicable imports**

```ts
import { redux } from 'zustand/middleware'
```

**Change**

```diff
- redux:
-   <T, A>(...) => ...
+ redux:
+   <T, A, Mps, Mcs>(...) => ...
```

**Migration**

If you are not passing any type parameters to `redux`,
no migration is required.

If you are,
remove all the type parameters,
and annotate only the second (action) parameter.
That is, replace `redux<T, A>((state, action) => ..., ...)`
with `redux((state, action: A) => ..., ...)`.
