Usage With TypeScript

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

This page is intended to give an overview of all common usecases and the most probable pitfalls you might encounter when using RTK with TypeScript.

If you encounter any problems with the types that are not described on this page, please open an issue for discussion.

Using `configureStore` with TypeScript

Using configureStore should not need any additional typings. You might, however, want to extract the RootState type and the Dispatch type.

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 recommend to give the type a different name like RootState to prevent confusion, as the type name State is usually overused.

import { combineReducers } from '@reduxjs/toolkit'
const rootReducer = combineReducers({})
export type RootState = ReturnType<typeof rootReducer>

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 recommend to give the type a different name like AppDispatch to prevent confusion, as the type name Dispatch is usually overused.

import { configureStore } from '@reduxjs/toolkit'
import rootReducer from './rootReducer'
const store = configureStore({
  reducer: rootReducer
})
export type AppDispatch = typeof store.dispatch

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.

There might however be cases, where TypeScript decides to simplify your provided middleware array down to just Array<Middleware>. In that case, you have to either specify the array type manually as a tuple, or in TS versions >= 3.4, just add as const to your definition.

Please note that when calling getDefaultMiddleware in TypeScript, you have to provide the state type as a generic argument.

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

type RootState = ReturnType<typeof rootReducer>
const store = configureStore({
  reducer: rootReducer,
  middleware: [
    // getDefaultMiddleware needs to be called with the state type
    ...getDefaultMiddleware<RootState>(),
    // correctly typed middlewares can just be used
    additionalMiddleware,
    // you can also manually type middlewares manually
    untypedMiddleware as Middleware<
      (action: Action<'specialAction'>) => number,
      RootState
    >
  ] as const // prevent this from becoming just `Array<Middleware>`
})

type AppDispatch = typeof store.dispatch

If you need any additional reference or examples, the type tests for configureStore contain many different scenarios on how to type this.

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 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:

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:

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.

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:

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:

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:

{
  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:

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.

`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:

{
  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 reducers and defining them inline would be messy, you can also define them outside the createSlice call and type them as CaseReducer:

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 constructinitialState: myInitialState as SliceState`.

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:

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 should be a viable alternative:

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 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).

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

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
    })
  }
})

Wrapping `createSlice`

If you need to reuse reducer logic, it is common to write "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:

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.

interface MyData {
  // ...
}

const fetchUserById = createAsyncThunk(
  'users/fetchById',
  // Declare the type your function argument here:
  async (userId: number) => {
    const response = await fetch(`https://reqres.in/api/users/${userId}`)
    // Inferred return type: Promise<MyData>
    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: {dispatch?, state?, extra?, rejectValue?}.

const fetchUserById = createAsyncThunk<
  // Return type of the payload creator
  MyData,
  // First argument to the payload creator
  number,
  {
    dispatch: AppDispatch
    state: State
    extra: {
      jwt: string
    }
  }
>('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.

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

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 `rejectType` in `updateUser`, the type information will be available here.
        state.error = action.payload.errorMessage
      } else {
        state.error = action.error
      }
    })
  }
})

In a component

const handleUpdateUser = async userData => {
  const resultAction = await dispatch(updateUser(userData))
  if (updateUser.fulfilled.match(resultAction)) {
    const user = unwrapResult(resultAction)
    showToast('success', `Updated ${user.name}`)
  } else {
    if (resultAction.payload) {
      // Since we passed in `MyKnownError` to `rejectType` 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:

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

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)
    }
  }
})

#Usage With TypeScript

#Using configureStore with TypeScript

#Getting the State type

#Getting the Dispatch type

#Correct typings for the Dispatch type

#Using the extracted Dispatch type with React-Redux

#createAction

#Alternative to using a literally-typed action.type

#createReducer

#Building Type-Safe Reducer Argument Objects

#createSlice

#Defining the Initial State Type

#Defining Action Contents with prepare Callbacks

#Generated Action Types for Slices

#Type safety with extraReducers

#Wrapping createSlice

#createAsyncThunk

#createEntityAdapter