# zustand/middleware

Part of the `zustand` package: install `zustand` and import these from `zustand/middleware`.

## Functions

### `combine`

```ts
function combine<
  T extends object,
  U extends object,
  Mps extends [StoreMutatorIdentifier, unknown][] = [],
  Mcs extends [StoreMutatorIdentifier, unknown][] = [],
>(
  initialState: T,
  create: StateCreator<T, Mps, Mcs, U>,
): StateCreator<Write<T, U>, Mps, Mcs>
```

### `createJSONStorage`

```ts
function createJSONStorage<S, R = unknown>(
  getStorage: () => StateStorage<R>,
  options?: JsonStorageOptions,
): PersistStorage<S, unknown> | undefined
```

### `unstable_ssrSafe`

```ts
function ssrSafe<
  T extends object,
  U extends object,
  Mps extends [StoreMutatorIdentifier, unknown][] = [],
  Mcs extends [StoreMutatorIdentifier, unknown][] = [],
>(
  config: StateCreator<T, Mps, Mcs, U>,
  isSSR: boolean = typeof window === 'undefined',
): StateCreator<T, Mps, Mcs, U>
```

## Constants

### `devtools`

```ts
const devtools: Devtools
```

### `persist`

```ts
const persist: Persist
```

### `redux`

```ts
const redux: Redux
```

### `subscribeWithSelector`

```ts
const subscribeWithSelector: SubscribeWithSelector
```

## Interfaces

### `DevtoolsOptions`

```ts
interface DevtoolsOptions extends Config
```

**Properties**

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `name?` | `string` |  |  |
| `enabled?` | `boolean` |  |  |
| `anonymousActionType?` | `string` |  |  |
| `store?` | `string` |  |  |

### `PersistOptions`

```ts
interface PersistOptions<
  S,
  PersistedState = S,
  PersistReturn = unknown,
>
```

**Properties**

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `name` | `string` |  | Name of the storage (must be unique) |
| `storage?` | `PersistStorage<PersistedState, PersistReturn> \| undefined` | `createJSONStorage(() => window.localStorage)` | Use a custom persist storage. |
| `partialize?` | `(state: S) => PersistedState` |  | Filter the persisted value. |
| `onRehydrateStorage?` | `( state: S, ) => ((state?: S, error?: unknown) => void) \| void` |  | A function returning another (optional) function. The main function will be called before the state rehydration. The returned function will be called after the state rehydration or when an error occurred. |
| `version?` | `number` |  | If the stored state's version mismatch the one specified here, the storage will not be used. This is useful when adding a breaking change to your store. |
| `migrate?` | `( persistedState: unknown, version: number, ) => PersistedState \| Promise<PersistedState>` |  | A function to perform persisted state migration. This function will be called when persisted state versions mismatch with the one specified here. |
| `merge?` | `(persistedState: unknown, currentState: S) => S` |  | A function to perform custom hydration merges when combining the stored state with the current one. By default, this function does a shallow merge. |
| `skipHydration?` | `boolean` | `false` | An optional boolean that will prevent the persist middleware from triggering hydration on initialization, This allows you to call `rehydrate()` at a specific point in your apps rendering life-cycle. |

### `PersistStorage`

```ts
interface PersistStorage<S, R = unknown>
```

**Properties**

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `getItem` | `( name: string, ) => StorageValue<S> \| null \| Promise<StorageValue<S> \| null>` |  |  |
| `setItem` | `(name: string, value: StorageValue<S>) => R` |  |  |
| `removeItem` | `(name: string) => R` |  |  |

### `StateStorage`

```ts
interface StateStorage<R = unknown>
```

**Properties**

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `getItem` | `(name: string) => string \| null \| Promise<string \| null>` |  |  |
| `setItem` | `(name: string, value: string) => R` |  |  |
| `removeItem` | `(name: string) => R` |  |  |

## Types

### `NamedSet`

```ts
type NamedSet<T> = WithDevtools<StoreApi<T>>['setState']
```

### `StorageValue`

```ts
type StorageValue<S> = { … }
```

**Properties**

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `state` | `S` |  |  |
| `version?` | `number` |  |  |