# Usage With TypeScript

 

## Usage With TypeScript

:::tip What You'll Learn

* Details on how to use each Redux Toolkit API with TypeScript

:::

### Introduction

Redux Toolkit is written in TypeScript, and its API is designed to enable great integration with TypeScript applications.

This page provides specific details for each of the different APIs included in Redux Toolkit and how to type them correctly with TypeScript.

**See the** [**TypeScript Quick Start tutorial page**](https://bryan-guner.gitbook.io/my-docs/redux/repos/redux-toolkit/docs/tutorials/typescript) **for a brief overview of how to set up and use Redux Toolkit and React Redux to work with TypeScript**.

:::info

If you encounter any problems with the types that are not described on this page, please [open an issue](https://github.com/reduxjs/redux-toolkit/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc) for discussion.

:::

### `configureStore`

The basics of using `configureStore` are shown in [TypeScript Quick Start tutorial page](https://bryan-guner.gitbook.io/my-docs/redux/repos/redux-toolkit/docs/tutorials/typescript). Here are some additional details that you might find useful.

#### Getting the `State` type

The easiest way of getting the `State` type is to define the root reducer in advance and extract its `ReturnType`.\
It is recommended to give the type a different name like `RootState` to prevent confusion, as the type name `State` is usually overused.

```typescript
import { combineReducers } from '@reduxjs/toolkit'
const rootReducer = combineReducers({})
// highlight-start
export type RootState = ReturnType<typeof rootReducer>
// highlight-end
```

Alternatively, if you choose to not create a `rootReducer` yourself and instead pass the slice reducers directly to `configureStore()`, you need to slightly modify the typing to correctly infer the root reducer:

```ts
import { configureStore } from '@reduxjs/toolkit'
// ...
const store = configureStore({
  reducer: {
    one: oneSlice.reducer,
    two: twoSlice.reducer,
  },
})
export type RootState = ReturnType<typeof store.getState>

export default store
```

#### Getting the `Dispatch` type

If you want to get the `Dispatch` type from your store, you can extract it after creating the store. It is recommended to give the type a different name like `AppDispatch` to prevent confusion, as the type name `Dispatch` is usually overused. You may also find it to be more convenient to export a hook like `useAppDispatch` shown below, then using it wherever you'd call `useDispatch`.

```typescript
import { configureStore } from '@reduxjs/toolkit'
import { useDispatch } from 'react-redux'
import rootReducer from './rootReducer'

const store = configureStore({
  reducer: rootReducer,
})

// highlight-start
export type AppDispatch = typeof store.dispatch
export const useAppDispatch = () => useDispatch<AppDispatch>() // Export a hook that can be reused to resolve types
// highlight-end

export default store
```

#### Correct typings for the `Dispatch` type

The type of the `dispatch` function type will be directly inferred from the `middleware` option. So if you add *correctly typed* middlewares, `dispatch` should already be correctly typed.

As TypeScript often widens array types when combining arrays using the spread operator, we suggest using the `.concat(...)` and `.prepend(...)` methods of the `MiddlewareArray` returned by `getDefaultMiddleware()`.

Also, we suggest using the callback notation for the `middleware` option to get a correctly pre-typed version of `getDefaultMiddleware` that does not require you to specify any generics by hand.

```ts
import { configureStore } from '@reduxjs/toolkit'
import additionalMiddleware from 'additional-middleware'
import logger from 'redux-logger'
// @ts-ignore
import untypedMiddleware from 'untyped-middleware'
import rootReducer from './rootReducer'

export type RootState = ReturnType<typeof rootReducer>
const store = configureStore({
  reducer: rootReducer,
  // highlight-start
  middleware: (getDefaultMiddleware) =>
    getDefaultMiddleware()
      .prepend(
        // correctly typed middlewares can just be used
        additionalMiddleware,
        // you can also type middlewares manually
        untypedMiddleware as Middleware<
          (action: Action<'specialAction'>) => number,
          RootState
        >
      )
      // prepend and concat calls can be chained
      .concat(logger),
  // highlight-end
})

export type AppDispatch = typeof store.dispatch

export default store
```

**Using `MiddlewareArray` without `getDefaultMiddleware`**

If you want to skip the usage of `getDefaultMiddleware` altogether, you can still use `MiddlewareArray` for type-safe concatenation of your `middleware` array. This class extends the default JavaScript `Array` type, only with modified typings for `.concat(...)` and the additional `.prepend(...)` method.

This is generally not required though, as you will probably not run into any array-type-widening issues as long as you are using `as const` and do not use the spread operator.

So the following two calls would be equivalent:

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

configureStore({
  reducer: rootReducer,
  middleware: new MiddlewareArray().concat(additionalMiddleware, logger),
})

configureStore({
  reducer: rootReducer,
  middleware: [additionalMiddleware, logger] as const,
})
```

#### Using the extracted `Dispatch` type with React Redux

By default, the React Redux `useDispatch` hook does not contain any types that take middlewares into account. If you need a more specific type for the `dispatch` function when dispatching, you may specify the type of the returned `dispatch` function, or create a custom-typed version of `useSelector`. See [the React Redux documentation](https://react-redux.js.org/using-react-redux/static-typing#typing-the-usedispatch-hook) for details.

### `createAction`

For most use cases, there is no need to have a literal definition of `action.type`, so the following can be used:

```typescript
createAction<number>('test')
```

This will result in the created action being of type `PayloadActionCreator<number, string>`.

In some setups, you will need a literal type for `action.type`, though. Unfortunately, TypeScript type definitions do not allow for a mix of manually-defined and inferred type parameters, so you'll have to specify the `type` both in the Generic definition as well as in the actual JavaScript code:

```typescript
createAction<number, 'test'>('test')
```

If you are looking for an alternate way of writing this without the duplication, you can use a prepare callback so that both type parameters can be inferred from arguments, removing the need to specify the action type.

```typescript
function withPayloadType<T>() {
  return (t: T) => ({ payload: t })
}
createAction('test', withPayloadType<string>())
```

#### Alternative to using a literally-typed `action.type`

If you are using `action.type` as a discriminator on a discriminated union, for example to correctly type your payload in `case` statements, you might be interested in this alternative:

Created action creators have a `match` method that acts as a [type predicate](https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates):

```typescript
const increment = createAction<number>('increment')
function test(action: Action) {
  if (increment.match(action)) {
    // action.payload inferred correctly here
    action.payload
  }
}
```

This `match` method is also very useful in combination with `redux-observable` and RxJS's `filter` method.

### `createReducer`

The default way of calling `createReducer` would be with a "lookup table" / "map object", like this:

```typescript
createReducer(0, {
  increment: (state, action: PayloadAction<number>) => state + action.payload,
})
```

Unfortunately, as the keys are only strings, using that API TypeScript can neither infer nor validate the action types for you:

```typescript
{
  const increment = createAction<number, 'increment'>('increment')
  const decrement = createAction<number, 'decrement'>('decrement')
  createReducer(0, {
    [increment.type]: (state, action) => {
      // action is any here
    },
    [decrement.type]: (state, action: PayloadAction<string>) => {
      // even though action should actually be PayloadAction<number>, TypeScript can't detect that and won't give a warning here.
    },
  })
}
```

As an alternative, RTK includes a type-safe reducer builder API.

#### Building Type-Safe Reducer Argument Objects

Instead of using a simple object as an argument to `createReducer`, you can also use a callback that receives a `ActionReducerMapBuilder` instance:

```typescript
const increment = createAction<number, 'increment'>('increment')
const decrement = createAction<number, 'decrement'>('decrement')
createReducer(0, (builder) =>
  builder
    .addCase(increment, (state, action) => {
      // action is inferred correctly here
    })
    .addCase(decrement, (state, action: PayloadAction<string>) => {
      // this would error out
    })
)
```

We recommend using this API if stricter type safety is necessary when defining reducer argument objects.

**Typing `builder.addMatcher`**

As the first `matcher` argument to `builder.addMatcher`, a [type predicate](https://www.typescriptlang.org/docs/handbook/advanced-types.html#using-type-predicates) function should be used. As a result, the `action` argument for the second `reducer` argument can be inferred by TypeScript:

```ts
function isNumberValueAction(action: AnyAction): action is PayloadAction<{ value: number }> {
  return typeof action.payload.value === 'number'
}

createReducer({ value: 0 }, builder =>
   builder.addMatcher(isNumberValueAction, (state, action) => {
      state.value += action.payload.value
   })
})
```

### `createSlice`

As `createSlice` creates your actions as well as your reducer for you, you don't have to worry about type safety here. Action types can just be provided inline:

```typescript
const slice = createSlice({
  name: 'test',
  initialState: 0,
  reducers: {
    increment: (state, action: PayloadAction<number>) => state + action.payload,
  },
})
// now available:
slice.actions.increment(2)
// also available:
slice.caseReducers.increment(0, { type: 'increment', payload: 5 })
```

If you have too many case reducers and defining them inline would be messy, or you want to reuse case reducers across slices, you can also define them outside the `createSlice` call and type them as `CaseReducer`:

```typescript
type State = number
const increment: CaseReducer<State, PayloadAction<number>> = (state, action) =>
  state + action.payload

createSlice({
  name: 'test',
  initialState: 0,
  reducers: {
    increment,
  },
})
```

#### Defining the Initial State Type

You might have noticed that it is not a good idea to pass your `SliceState` type as a generic to `createSlice`. This is due to the fact that in almost all cases, follow-up generic parameters to `createSlice` need to be inferred, and TypeScript cannot mix explicit declaration and inference of generic types within the same "generic block".

The standard approach is to declare an interface or type for your state, create an initial state value that uses that type, and pass the initial state value to `createSlice`. You can also use the construct `initialState: myInitialState as SliceState`.

```ts
type SliceState = { state: 'loading' } | { state: 'finished'; data: string }

// First approach: define the initial state using that type
const initialState: SliceState = { state: 'loading' }

createSlice({
  name: 'test1',
  initialState, // type SliceState is inferred for the state of the slice
  reducers: {},
})

// Or, cast the initial state as necessary
createSlice({
  name: 'test2',
  initialState: { state: 'loading' } as SliceState,
  reducers: {},
})
```

which will result in a `Slice<SliceState, ...>`.

#### Defining Action Contents with `prepare` Callbacks

If you want to add a `meta` or `error` property to your action, or customize the `payload` of your action, you have to use the `prepare` notation.

Using this notation with TypeScript looks like this:

```ts
const blogSlice = createSlice({
  name: 'blogData',
  initialState,
  reducers: {
    receivedAll: {
      reducer(
        state,
        action: PayloadAction<Page[], string, { currentPage: number }>
      ) {
        state.all = action.payload
        state.meta = action.meta
      },
      prepare(payload: Page[], currentPage: number) {
        return { payload, meta: { currentPage } }
      },
    },
  },
})
```

#### Generated Action Types for Slices

As TS cannot combine two string literals (`slice.name` and the key of `actionMap`) into a new literal, all actionCreators created by `createSlice` are of type 'string'. This is usually not a problem, as these types are only rarely used as literals.

In most cases that `type` would be required as a literal, the `slice.action.myAction.match` [type predicate](https://www.typescriptlang.org/docs/handbook/advanced-types.html#using-type-predicates) should be a viable alternative:

```ts
const slice = createSlice({
  name: 'test',
  initialState: 0,
  reducers: {
    increment: (state, action: PayloadAction<number>) => state + action.payload,
  },
})

function myCustomMiddleware(action: Action) {
  if (slice.actions.increment.match(action)) {
    // `action` is narrowed down to the type `PayloadAction<number>` here.
  }
}
```

If you actually *need* that type, unfortunately there is no other way than manual casting.

#### Type safety with `extraReducers`

Reducer lookup tables that map an action `type` string to a reducer function are not easy to fully type correctly. This affects both `createReducer` and the `extraReducers` argument for `createSlice`. So, like with `createReducer`, [you may also use the "builder callback" approach](#building-type-safe-reducer-argument-objects) for defining the reducer object argument.

This is particularly useful when a slice reducer needs to handle action types generated by other slices, or generated by specific calls to `createAction` (such as the actions generated by [`createAsyncThunk`](https://bryan-guner.gitbook.io/my-docs/redux/repos/redux-toolkit/docs/api/createasyncthunk)).

```ts
const fetchUserById = createAsyncThunk(
  'users/fetchById',
  // if you type your function argument here
  async (userId: number) => {
    const response = await fetch(`https://reqres.in/api/users/${userId}`)
    return (await response.json()) as Returned
  }
)

interface UsersState {
  entities: []
  loading: 'idle' | 'pending' | 'succeeded' | 'failed'
}

const initialState = {
  entities: [],
  loading: 'idle',
} as UsersState

const usersSlice = createSlice({
  name: 'users',
  initialState,
  reducers: {
    // fill in primary logic here
  },
  extraReducers: (builder) => {
    builder.addCase(fetchUserById.pending, (state, action) => {
      // both `state` and `action` are now correctly typed
      // based on the slice state and the `pending` action creator
    })
  },
})
```

Like the `builder` in `createReducer`, this `builder` also accepts `addMatcher` (see [typing `builder.matcher`](#typing-builderaddmatcher)) and `addDefaultCase`.

#### Wrapping `createSlice`

If you need to reuse reducer logic, it is common to write ["higher-order reducers"](https://redux.js.org/recipes/structuring-reducers/reusing-reducer-logic#customizing-behavior-with-higher-order-reducers) that wrap a reducer function with additional common behavior. This can be done with `createSlice` as well, but due to the complexity of the types for `createSlice`, you have to use the `SliceCaseReducers` and `ValidateSliceCaseReducers` types in a very specific way.

Here is an example of such a "generic" wrapped `createSlice` call:

```ts
interface GenericState<T> {
  data?: T
  status: 'loading' | 'finished' | 'error'
}

const createGenericSlice = <
  T,
  Reducers extends SliceCaseReducers<GenericState<T>>
>({
  name = '',
  initialState,
  reducers,
}: {
  name: string
  initialState: GenericState<T>
  reducers: ValidateSliceCaseReducers<GenericState<T>, Reducers>
}) => {
  return createSlice({
    name,
    initialState,
    reducers: {
      start(state) {
        state.status = 'loading'
      },
      /**
       * If you want to write to values of the state that depend on the generic
       * (in this case: `state.data`, which is T), you might need to specify the
       * State type manually here, as it defaults to `Draft<GenericState<T>>`,
       * which can sometimes be problematic with yet-unresolved generics.
       * This is a general problem when working with immer's Draft type and generics.
       */
      success(state: GenericState<T>, action: PayloadAction<T>) {
        state.data = action.payload
        state.status = 'finished'
      },
      ...reducers,
    },
  })
}

const wrappedSlice = createGenericSlice({
  name: 'test',
  initialState: { status: 'loading' } as GenericState<string>,
  reducers: {
    magic(state) {
      state.status = 'finished'
      state.data = 'hocus pocus'
    },
  },
})
```

### `createAsyncThunk`

In the most common use cases, you should not need to explicitly declare any types for the `createAsyncThunk` call itself.

Just provide a type for the first argument to the `payloadCreator` argument as you would for any function argument, and the resulting thunk will accept the same type as its input parameter. The return type of the `payloadCreator` will also be reflected in all generated action types.

```ts
interface MyData {
  // ...
}

const fetchUserById = createAsyncThunk(
  'users/fetchById',
  // highlight-start
  // Declare the type your function argument here:
  async (userId: number) => {
    // highlight-end
    const response = await fetch(`https://reqres.in/api/users/${userId}`)
    // Inferred return type: Promise<MyData>
    // highlight-next-line
    return (await response.json()) as MyData
  }
)

// the parameter of `fetchUserById` is automatically inferred to `number` here
// and dispatching the resulting thunkAction will return a Promise of a correctly
// typed "fulfilled" or "rejected" action.
const lastReturnedAction = await store.dispatch(fetchUserById(3))
```

The second argument to the `payloadCreator`, known as `thunkApi`, is an object containing references to the `dispatch`, `getState`, and `extra` arguments from the thunk middleware as well as a utility function called `rejectWithValue`. If you want to use these from within the `payloadCreator`, you will need to define some generic arguments, as the types for these arguments cannot be inferred. Also, as TS cannot mix explicit and inferred generic parameters, from this point on you'll have to define the `Returned` and `ThunkArg` generic parameter as well.

To define the types for these arguments, pass an object as the third generic argument, with type declarations for some or all of these fields:

```ts
type AsyncThunkConfig = {
  /** return type for `thunkApi.getState` */
  state?: unknown
  /** type for `thunkApi.dispatch` */
  dispatch?: Dispatch
  /** type of the `extra` argument for the thunk middleware, which will be passed in as `thunkApi.extra` */
  extra?: unknown
  /** type to be passed into `rejectWithValue`'s first argument that will end up on `rejectedAction.payload` */
  rejectValue?: unknown
  /** return type of the `serializeError` option callback */
  serializedErrorType?: unknown
  /** type to be returned from the `getPendingMeta` option callback & merged into `pendingAction.meta` */
  pendingMeta?: unknown
  /** type to be passed into the second argument of `fulfillWithValue` to finally be merged into `fulfilledAction.meta` */
  fulfilledMeta?: unknown
  /** type to be passed into the second argument of `rejectWithValue` to finally be merged into `rejectedAction.meta` */
  rejectedMeta?: unknown
}
```

```ts
const fetchUserById = createAsyncThunk<
  // highlight-start
  // Return type of the payload creator
  MyData,
  // First argument to the payload creator
  number,
  {
    // Optional fields for defining thunkApi field types
    dispatch: AppDispatch
    state: State
    extra: {
      jwt: string
    }
  }
  // highlight-end
>('users/fetchById', async (userId, thunkApi) => {
  const response = await fetch(`https://reqres.in/api/users/${userId}`, {
    headers: {
      Authorization: `Bearer ${thunkApi.extra.jwt}`,
    },
  })
  return (await response.json()) as MyData
})
```

If you are performing a request that you know will typically either be a success or have an expected error format, you can pass in a type to `rejectValue` and `return rejectWithValue(knownPayload)` in the action creator. This allows you to reference the error payload in the reducer as well as in a component after dispatching the `createAsyncThunk` action.

```ts
interface MyKnownError {
  errorMessage: string
  // ...
}
interface UserAttributes {
  id: string
  first_name: string
  last_name: string
  email: string
}

const updateUser = createAsyncThunk<
  // Return type of the payload creator
  MyData,
  // First argument to the payload creator
  UserAttributes,
  // Types for ThunkAPI
  {
    extra: {
      jwt: string
    }
    rejectValue: MyKnownError
  }
>('users/update', async (user, thunkApi) => {
  const { id, ...userData } = user
  const response = await fetch(`https://reqres.in/api/users/${id}`, {
    method: 'PUT',
    headers: {
      Authorization: `Bearer ${thunkApi.extra.jwt}`,
    },
    body: JSON.stringify(userData),
  })
  if (response.status === 400) {
    // Return the known error for future handling
    return thunkApi.rejectWithValue((await response.json()) as MyKnownError)
  }
  return (await response.json()) as MyData
})
```

While this notation for `state`, `dispatch`, `extra` and `rejectValue` might seem uncommon at first, it allows you to provide only the types for these you actually need - so for example, if you are not accessing `getState` within your `payloadCreator`, there is no need to provide a type for `state`. The same can be said about `rejectValue` - if you don't need to access any potential error payload, you can ignore it.

In addition, you can leverage checks against `action.payload` and `match` as provided by `createAction` as a type-guard for when you want to access known properties on defined types. Example:

* In a reducer

```ts
const usersSlice = createSlice({
  name: 'users',
  initialState: {
    entities: {},
    error: null,
  },
  reducers: {},
  extraReducers: (builder) => {
    builder.addCase(updateUser.fulfilled, (state, { payload }) => {
      state.entities[payload.id] = payload
    })
    builder.addCase(updateUser.rejected, (state, action) => {
      if (action.payload) {
        // Since we passed in `MyKnownError` to `rejectValue` in `updateUser`, the type information will be available here.
        state.error = action.payload.errorMessage
      } else {
        state.error = action.error
      }
    })
  },
})
```

* In a component

```ts
const handleUpdateUser = async (userData) => {
  const resultAction = await dispatch(updateUser(userData))
  if (updateUser.fulfilled.match(resultAction)) {
    const user = resultAction.payload
    showToast('success', `Updated ${user.name}`)
  } else {
    if (resultAction.payload) {
      // Since we passed in `MyKnownError` to `rejectValue` in `updateUser`, the type information will be available here.
      // Note: this would also be a good place to do any handling that relies on the `rejectedWithValue` payload, such as setting field errors
      showToast('error', `Update failed: ${resultAction.payload.errorMessage}`)
    } else {
      showToast('error', `Update failed: ${resultAction.error.message}`)
    }
  }
}
```

### `createEntityAdapter`

Typing `createEntityAdapter` only requires you to specify the entity type as the single generic argument.

The example from the `createEntityAdapter` documentation would look like this in TypeScript:

```ts
interface Book {
  bookId: number
  title: string
  // ...
}

// highlight-next-line
const booksAdapter = createEntityAdapter<Book>({
  selectId: (book) => book.bookId,
  sortComparer: (a, b) => a.title.localeCompare(b.title),
})

const booksSlice = createSlice({
  name: 'books',
  initialState: booksAdapter.getInitialState(),
  reducers: {
    bookAdded: booksAdapter.addOne,
    booksReceived(state, action: PayloadAction<{ books: Book[] }>) {
      booksAdapter.setAll(state, action.payload.books)
    },
  },
})
```

#### Using `createEntityAdapter` with `normalizr`

When using a library like [`normalizr`](https://github.com/paularmstrong/normalizr/), your normalized data will resemble this shape:

```js
{
  result: 1,
  entities: {
    1: { id: 1, other: 'property' },
    2: { id: 2, other: 'property' }
  }
}
```

The methods `addMany`, `upsertMany`, and `setAll` all allow you to pass in the `entities` portion of this directly with no extra conversion steps. However, the `normalizr` TS typings currently do not correctly reflect that multiple data types may be included in the results, so you will need to specify that type structure yourself.

Here is an example of how that would look:

```ts
type Author = { id: number; name: string }
type Article = { id: number; title: string }
type Comment = { id: number; commenter: number }

export const fetchArticle = createAsyncThunk(
  'articles/fetchArticle',
  async (id: number) => {
    const data = await fakeAPI.articles.show(id)
    // Normalize the data so reducers can responded to a predictable payload.
    // Note: at the time of writing, normalizr does not automatically infer the result,
    // so we explicitly declare the shape of the returned normalized data as a generic arg.
    const normalized = normalize<
      any,
      {
        articles: { [key: string]: Article }
        users: { [key: string]: Author }
        comments: { [key: string]: Comment }
      }
    >(data, articleEntity)
    return normalized.entities
  }
)

export const slice = createSlice({
  name: 'articles',
  initialState: articlesAdapter.getInitialState(),
  reducers: {},
  extraReducers: (builder) => {
    builder.addCase(fetchArticle.fulfilled, (state, action) => {
      // The type signature on action.payload matches what we passed into the generic for `normalize`, allowing us to access specific properties on `payload.articles` if desired
      articlesAdapter.upsertMany(state, action.payload.articles)
    })
  },
})
```