createAsyncThunk

createAsyncThunk

Overview

A function that accepts a Redux action type string and a callback function that should return a promise. It generates promise lifecycle action types based on the action type prefix that you pass in, and returns a thunk action creator that will run the promise callback and dispatch the lifecycle actions based on the returned promise.

This abstracts the standard recommended approach for handling async request lifecycles.

It does not generate any reducer functions, since it does not know what data you're fetching, how you want to track loading state, or how the data you return needs to be processed. You should write your own reducer logic that handles these actions, with whatever loading state and processing logic is appropriate for your own app.

Sample usage:

import { createAsyncThunk, createSlice } from '@reduxjs/toolkit'
import { userAPI } from './userAPI'

// First, create the thunk
const fetchUserById = createAsyncThunk(
  'users/fetchByIdStatus',
  async (userId, thunkAPI) => {
    const response = await userAPI.fetchById(userId)
    return response.data
  }
)

// Then, handle actions in your reducers:
const usersSlice = createSlice({
  name: 'users',
  initialState: { entities: [], loading: 'idle' },
  reducers: {
    // standard reducer logic, with auto-generated action types per reducer
  },
  extraReducers: {
    // Add reducers for additional action types here, and handle loading state as needed
    [fetchUserById.fulfilled]: (state, action) => {
      // Add user to the state array
      state.entities.push(action.payload)
    },
  },
})

// Later, dispatch the thunk as needed in the app
dispatch(fetchUserById(123))

Parameters

createAsyncThunk accepts three parameters: a string action type value, a payloadCreator callback, and an options object.

type

A string that will be used to generate additional Redux action type constants, representing the lifecycle of an async request:

For example, a type argument of 'users/requestStatus' will generate these action types:

• pending: 'users/requestStatus/pending'

• fulfilled: 'users/requestStatus/fulfilled'

• rejected: 'users/requestStatus/rejected'

payloadCreator

A callback function that should return a promise containing the result of some asynchronous logic. It may also return a value synchronously. If there is an error, it should either return a rejected promise containing an Error instance or a plain value such as a descriptive error message or otherwise a resolved promise with a RejectWithValue argument as returned by the thunkAPI.rejectWithValue function.

The payloadCreator function can contain whatever logic you need to calculate an appropriate result. This could include a standard AJAX data fetch request, multiple AJAX calls with the results combined into a final value, interactions with React Native AsyncStorage, and so on.

The payloadCreator function will be called with two arguments:

• arg: a single value, containing the first parameter that was passed to the thunk action creator when it was dispatched. This is useful for passing in values like item IDs that may be needed as part of the request. If you need to pass in multiple values, pass them together in an object when you dispatch the thunk, like dispatch(fetchUsers({status: 'active', sortBy: 'name'})).

• thunkAPI: an object containing all of the parameters that are normally passed to a Redux thunk function, as well as additional options:

  • dispatch: the Redux store dispatch method

  • getState: the Redux store getState method

  • extra: the "extra argument" given to the thunk middleware on setup, if available

  • requestId: a unique string ID value that was automatically generated to identify this request sequence

  • signal: an AbortController.signal object that may be used to see if another part of the app logic has marked this request as needing cancelation.

  • rejectWithValue(value, [meta]): rejectWithValue is a utility function that you can return (or throw) in your action creator to return a rejected response with a defined payload and meta. It will pass whatever value you give it and return it in the payload of the rejected action. If you also pass in a meta, it will be merged with the existing rejectedAction.meta.

  • fulfillWithValue(value, meta): fulfillWithValue is a utility function that you can return in your action creator to fulfill with a value while having the ability of adding to fulfilledAction.meta.

The logic in the payloadCreator function may use any of these values as needed to calculate the result.

Options

An object with the following optional fields:

• condition(arg, { getState, extra } ): boolean: a callback that can be used to skip execution of the payload creator and all action dispatches, if desired. See Canceling Before Execution for a complete description.

• dispatchConditionRejection: if condition() returns false, the default behavior is that no actions will be dispatched at all. If you still want a "rejected" action to be dispatched when the thunk was canceled, set this flag to true.

• idGenerator(): string: a function to use when generating the requestId for the request sequence. Defaults to use nanoid.

• serializeError(error: unknown) => any to replace the internal miniSerializeError method with your own serialization logic.

• getPendingMeta({ arg, requestId }, { getState, extra }): any: a function to create an object that will be merged into the pendingAction.meta field.

Return Value

createAsyncThunk returns a standard Redux thunk action creator. The thunk action creator function will have plain action creators for the pending, fulfilled, and rejected cases attached as nested fields.

Using the fetchUserById example above, createAsyncThunk will generate four functions:

• fetchUserById, the thunk action creator that kicks off the async payload callback you wrote

  • fetchUserById.pending, an action creator that dispatches an 'users/fetchByIdStatus/pending' action

  • fetchUserById.fulfilled, an action creator that dispatches an 'users/fetchByIdStatus/fulfilled' action

  • fetchUserById.rejected, an action creator that dispatches an 'users/fetchByIdStatus/rejected' action

When dispatched, the thunk will:

• dispatch the pending action

• call the payloadCreator callback and wait for the returned promise to settle

• when the promise settles:

  • if the promise resolved successfully, dispatch the fulfilled action with the promise value as action.payload

  • if the promise resolved with a rejectWithValue(value) return value, dispatch the rejected action with the value passed into action.payload and 'Rejected' as action.error.message

  • if the promise failed and was not handled with rejectWithValue, dispatch the rejected action with a serialized version of the error value as action.error

• Return a fulfilled promise containing the final dispatched action (either the fulfilled or rejected action object)

Promise Lifecycle Actions

createAsyncThunk will generate three Redux action creators using createAction: pending, fulfilled, and rejected. Each lifecycle action creator will be attached to the returned thunk action creator so that your reducer logic can reference the action types and respond to the actions when dispatched. Each action object will contain the current unique requestId and arg values under action.meta.

The action creators will have these signatures:

interface SerializedError {
  name?: string
  message?: string
  code?: string
  stack?: string
}

interface PendingAction<ThunkArg> {
  type: string
  payload: undefined
  meta: {
    requestId: string
    arg: ThunkArg
  }
}

interface FulfilledAction<ThunkArg, PromiseResult> {
  type: string
  payload: PromiseResult
  meta: {
    requestId: string
    arg: ThunkArg
  }
}

interface RejectedAction<ThunkArg> {
  type: string
  payload: undefined
  error: SerializedError | any
  meta: {
    requestId: string
    arg: ThunkArg
    aborted: boolean
    condition: boolean
  }
}

interface RejectedWithValueAction<ThunkArg, RejectedValue> {
  type: string
  payload: RejectedValue
  error: { message: 'Rejected' }
  meta: {
    requestId: string
    arg: ThunkArg
    aborted: boolean
  }
}

type Pending = <ThunkArg>(
  requestId: string,
  arg: ThunkArg
) => PendingAction<ThunkArg>

type Fulfilled = <ThunkArg, PromiseResult>(
  payload: PromiseResult,
  requestId: string,
  arg: ThunkArg
) => FulfilledAction<ThunkArg, PromiseResult>

type Rejected = <ThunkArg>(
  requestId: string,
  arg: ThunkArg
) => RejectedAction<ThunkArg>

type RejectedWithValue = <ThunkArg, RejectedValue>(
  requestId: string,
  arg: ThunkArg
) => RejectedWithValueAction<ThunkArg, RejectedValue>

To handle these actions in your reducers, reference the action creators in createReducer or createSlice using either the object key notation or the "builder callback" notation. (Note that if you use TypeScript, you should use the "builder callback" notation to ensure the types are inferred correctly):

const reducer1 = createReducer(initialState, {
  [fetchUserById.fulfilled]: (state, action) => {},
})

const reducer2 = createReducer(initialState, (builder) => {
  builder.addCase(fetchUserById.fulfilled, (state, action) => {})
})

const reducer3 = createSlice({
  name: 'users',
  initialState,
  reducers: {},
  extraReducers: {
    [fetchUserById.fulfilled]: (state, action) => {},
  },
})

const reducer4 = createSlice({
  name: 'users',
  initialState,
  reducers: {},
  extraReducers: (builder) => {
    builder.addCase(fetchUserById.fulfilled, (state, action) => {})
  },
})

Handling Thunk Results

Unwrapping Result Actions

Thunks may return a value when dispatched. A common use case is to return a promise from the thunk, dispatch the thunk from a component, and then wait for the promise to resolve before doing additional work:

const onClick = () => {
  dispatch(fetchUserById(userId)).then(() => {
    // do additional work
  })
}

The thunks generated by createAsyncThunk will always return a resolved promise with either the fulfilled action object or rejected action object inside, as appropriate.

The calling logic may wish to treat these actions as if they were the original promise contents. Redux Toolkit exports an unwrapResult function that can be used to extract the payload of a fulfilled action or to throw either the error or, if available, payload created by rejectWithValue from a rejected action:

import { unwrapResult } from '@reduxjs/toolkit'

// in the component
const onClick = () => {
  dispatch(fetchUserById(userId))
    .then(unwrapResult)
    .then((originalPromiseResult) => {})
    .catch((rejectedValueOrSerializedError) => {})
}

Checking Errors After Dispatching

Note that this means a failed request or error in a thunk will never return a rejected promise. We assume that any failure is more of a handled error than an unhandled exception at this point. This is due to the fact that we want to prevent uncaught promise rejections for those who do not use the result of dispatch.

If your component needs to know if the request failed, use unwrapResult and handle the re-thrown error accordingly.

Handling Thunk Errors

When your payloadCreator returns a rejected promise (such as a thrown error in an async function), the thunk will dispatch a rejected action containing an automatically-serialized version of the error as action.error. However, to ensure serializability, everything that does not match the SerializedError interface will have been removed from it:

export interface SerializedError {
  name?: string
  message?: string
  stack?: string
  code?: string
}

If you need to customize the contents of the rejected action, you should catch any errors yourself, and then return a new value using the thunkAPI.rejectWithValue utility. Doing return rejectWithValue(errorPayload) will cause the rejected action to use that value as action.payload.

The rejectWithValue approach should also be used if your API response "succeeds", but contains some kind of additional error details that the reducer should know about. This is particularly common when expecting field-level validation errors from an API.

const updateUser = createAsyncThunk(
  'users/update',
  async (userData, { rejectWithValue }) => {
    const { id, ...fields } = userData
    try {
      const response = await userAPI.updateById(id, fields)
      return response.data.user
    } catch (err) {
      // Use `err.response.data` as `action.payload` for a `rejected` action,
      // by explicitly returning it using the `rejectWithValue()` utility
      return rejectWithValue(err.response.data)
    }
  }
)

Cancellation

Canceling Before Execution

If you need to cancel a thunk before the payload creator is called, you may provide a condition callback as an option after the payload creator. The callback will receive the thunk argument and an object with {getState, extra} as parameters, and use those to decide whether to continue or not. If the execution should be canceled, the condition callback should return a literal false value:

const fetchUserById = createAsyncThunk(
  'users/fetchByIdStatus',
  async (userId, thunkAPI) => {
    const response = await userAPI.fetchById(userId)
    return response.data
  },
  {
    condition: (userId, { getState, extra }) => {
      const { users } = getState()
      const fetchStatus = users.requests[userId]
      if (fetchStatus === 'fulfilled' || fetchStatus === 'loading') {
        // Already fetched or in progress, don't need to re-fetch
        return false
      }
    },
  }
)

If condition() returns false, the default behavior is that no actions will be dispatched at all. If you still want a "rejected" action to be dispatched when the thunk was canceled, pass in {condition, dispatchConditionRejection: true}.

Canceling While Running

If you want to cancel your running thunk before it has finished, you can use the abort method of the promise returned by dispatch(fetchUserById(userId)).

A real-life example of that would look like this:

// file: store.ts noEmit
import { configureStore, Reducer } from '@reduxjs/toolkit'
import { useDispatch } from 'react-redux'

declare const reducer: Reducer<{}>
const store = configureStore({ reducer })
export const useAppDispatch = () => useDispatch<typeof store.dispatch>()

// file: slice.ts noEmit
import { createAsyncThunk } from '@reduxjs/toolkit'
export const fetchUserById = createAsyncThunk(
  'fetchUserById',
  (userId: string) => {
    /* ... */
  }
)

// file: MyComponent.ts
import { fetchUserById } from './slice'
import { useAppDispatch } from './store'
import React from 'react'

function MyComponent(props: { userId: string }) {
  const dispatch = useAppDispatch()
  React.useEffect(() => {
    // Dispatching the thunk returns a promise
    const promise = dispatch(fetchUserById(props.userId))
    return () => {
      // `createAsyncThunk` attaches an `abort()` method to the promise
      promise.abort()
    }
  }, [props.userId])
}

After a thunk has been cancelled this way, it will dispatch (and return) a "thunkName/rejected" action with an AbortError on the error property. The thunk will not dispatch any further actions.

Additionally, your payloadCreator can use the AbortSignal it is passed via thunkAPI.signal to actually cancel a costly asynchronous action.

The fetch api of modern browsers already comes with support for an AbortSignal:

import { createAsyncThunk } from '@reduxjs/toolkit'

const fetchUserById = createAsyncThunk(
  'users/fetchById',
  async (userId: string, thunkAPI) => {
    const response = await fetch(`https://reqres.in/api/users/${userId}`, {
      signal: thunkAPI.signal,
    })
    return await response.json()
  }
)

Checking Cancellation Status

Reading the Signal Value

You can use the signal.aborted property to regularly check if the thunk has been aborted and in that case stop costly long-running work:

import { createAsyncThunk } from '@reduxjs/toolkit'

const readStream = createAsyncThunk(
  'readStream',
  async (stream: ReadableStream, { signal }) => {
    const reader = stream.getReader()

    let done = false
    let result = ''

    while (!done) {
      if (signal.aborted) {
        throw new Error('stop the work, this has been aborted!')
      }
      const read = await reader.read()
      result += read.value
      done = read.done
    }
    return result
  }
)

Listening for Abort Events

You can also call signal.addEventListener('abort', callback) to have logic inside the thunk be notified when promise.abort() was called. This can for example be used in conjunction with an axios CancelToken:

import { createAsyncThunk } from '@reduxjs/toolkit'
import axios from 'axios'

const fetchUserById = createAsyncThunk(
  'users/fetchById',
  async (userId, { signal }) => {
    const source = axios.CancelToken.source()
    signal.addEventListener('abort', () => {
      source.cancel()
    })
    const response = await axios.get(`https://reqres.in/api/users/${userId}`, {
      cancelToken: source.token,
    })
    return response.data
  }
)

Examples

• Requesting a user by ID, with loading state, and only one request at a time:

import { createAsyncThunk, createSlice, unwrapResult } from '@reduxjs/toolkit'
import { userAPI } from './userAPI'

const fetchUserById = createAsyncThunk(
  'users/fetchByIdStatus',
  async (userId, { getState, requestId }) => {
    const { currentRequestId, loading } = getState().users
    if (loading !== 'pending' || requestId !== currentRequestId) {
      return
    }
    const response = await userAPI.fetchById(userId)
    return response.data
  }
)

const usersSlice = createSlice({
  name: 'users',
  initialState: {
    entities: [],
    loading: 'idle',
    currentRequestId: undefined,
    error: null,
  },
  reducers: {},
  extraReducers: {
    [fetchUserById.pending]: (state, action) => {
      if (state.loading === 'idle') {
        state.loading = 'pending'
        state.currentRequestId = action.meta.requestId
      }
    },
    [fetchUserById.fulfilled]: (state, action) => {
      const { requestId } = action.meta
      if (state.loading === 'pending' && state.currentRequestId === requestId) {
        state.loading = 'idle'
        state.entities.push(action.payload)
        state.currentRequestId = undefined
      }
    },
    [fetchUserById.rejected]: (state, action) => {
      const { requestId } = action.meta
      if (state.loading === 'pending' && state.currentRequestId === requestId) {
        state.loading = 'idle'
        state.error = action.error
        state.currentRequestId = undefined
      }
    },
  },
})

const UsersComponent = () => {
  const { users, loading, error } = useSelector((state) => state.users)
  const dispatch = useDispatch()

  const fetchOneUser = async (userId) => {
    try {
      const resultAction = await dispatch(fetchUserById(userId))
      const user = unwrapResult(resultAction)
      showToast('success', `Fetched ${user.name}`)
    } catch (err) {
      showToast('error', `Fetch failed: ${err.message}`)
    }
  }

  // render UI here
}

• Using rejectWithValue to access a custom rejected payload in a component

  Note: this is a contrived example assuming our userAPI only ever throws validation-specific errors

// file: store.ts noEmit
import { configureStore, Reducer } from '@reduxjs/toolkit'
import { useDispatch } from 'react-redux'
import usersReducer from './user/slice'

const store = configureStore({ reducer: { users: usersReducer } })
export const useAppDispatch = () => useDispatch<typeof store.dispatch>()
export type RootState = ReturnType<typeof store.getState>

// file: user/userAPI.ts noEmit

export declare const userAPI: {
  updateById<Response>(id: string, fields: {}): { data: Response }
}

// file: user/slice.ts
import { createAsyncThunk, createSlice, unwrapResult } from '@reduxjs/toolkit'
import { userAPI } from './userAPI'
import { AxiosError } from 'axios'

// Sample types that will be used
export interface User {
  id: string
  first_name: string
  last_name: string
  email: string
}

interface ValidationErrors {
  errorMessage: string
  field_errors: Record<string, string>
}

interface UpdateUserResponse {
  user: User
  success: boolean
}

export const updateUser = createAsyncThunk<
  User,
  { id: string } & Partial<User>,
  {
    rejectValue: ValidationErrors
  }
>('users/update', async (userData, { rejectWithValue }) => {
  try {
    const { id, ...fields } = userData
    const response = await userAPI.updateById<UpdateUserResponse>(id, fields)
    return response.data.user
  } catch (err) {
    let error: AxiosError<ValidationErrors> = err // cast the error for access
    if (!error.response) {
      throw err
    }
    // We got validation errors, let's return those so we can reference in our component and set form errors
    return rejectWithValue(error.response.data)
  }
})

interface UsersState {
  error: string | null | undefined
  entities: Record<string, User>
}

const initialState = {
  entities: {},
  error: null,
} as UsersState

const usersSlice = createSlice({
  name: 'users',
  initialState,
  reducers: {},
  extraReducers: (builder) => {
    // The `builder` callback form is used here because it provides correctly typed reducers from the action creators
    builder.addCase(updateUser.fulfilled, (state, { payload }) => {
      state.entities[payload.id] = payload
    })
    builder.addCase(updateUser.rejected, (state, action) => {
      if (action.payload) {
        // Being that we passed in ValidationErrors to rejectType in `createAsyncThunk`, the payload will be available here.
        state.error = action.payload.errorMessage
      } else {
        state.error = action.error.message
      }
    })
  },
})

export default usersSlice.reducer

// file: externalModules.d.ts noEmit

declare module 'some-toast-library' {
  export function showToast(type: string, message: string)
}

// file: user/UsersComponent.ts

import React from 'react'
import { useAppDispatch, RootState } from '../store'
import { useSelector } from 'react-redux'
import { User, updateUser } from './slice'
import { FormikHelpers } from 'formik'
import { showToast } from 'some-toast-library'

interface FormValues extends Omit<User, 'id'> {}

const UsersComponent = (props: { id: string }) => {
  const { entities, error } = useSelector((state: RootState) => state.users)
  const dispatch = useAppDispatch()

  // This is an example of an onSubmit handler using Formik meant to demonstrate accessing the payload of the rejected action
  const handleUpdateUser = async (
    values: FormValues,
    formikHelpers: FormikHelpers<FormValues>
  ) => {
    const resultAction = await dispatch(updateUser({ id: props.id, ...values }))
    if (updateUser.fulfilled.match(resultAction)) {
      // user will have a type signature of User as we passed that as the Returned parameter in createAsyncThunk
      const user = resultAction.payload
      showToast('success', `Updated ${user.first_name} ${user.last_name}`)
    } else {
      if (resultAction.payload) {
        // Being that we passed in ValidationErrors to rejectType in `createAsyncThunk`, those types will be available here.
        formikHelpers.setErrors(resultAction.payload.field_errors)
      } else {
        showToast('error', `Update failed: ${resultAction.error}`)
      }
    }
  }

  // render UI here
}