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Customizing queries

RTK Query is agnostic as to how your requests resolve. You can use any library you like to handle requests, or no library at all. RTK Query provides reasonable defaults expected to cover the majority of use cases, while also allowing room for customization to alter query handling to fit specific needs.

Customizing queries with baseQuery​

The default method to handle queries is via the baseQuery option on createApi, in combination with the query option on an endpoint definition.

To process queries, endpoints are defined with a query option, which passes its return value to a common baseQuery function used for the API.

By default, RTK Query ships with fetchBaseQuery, which is a lightweight fetch wrapper that automatically handles request headers and response parsing in a manner similar to common libraries like axios. If fetchBaseQuery alone does not meet your needs, you can customize its behaviour with a wrapper function, or create your own baseQuery function from scratch for createApi to use.

See also baseQuery API Reference.

Implementing a custom baseQuery​

RTK Query expects a baseQuery function to be called with three arguments: args, api, and extraOptions. It is expected to return an object with either a data or error property, or a promise that resolves to return such an object.

tip

Base query and query functions must always catch errors themselves, and return it in an object!

function brokenCustomBaseQuery() {
  // ❌ Don't let this throw by itself
  const data = await fetchSomeData()
  return { data }
}

function correctCustomBaseQuery() {
  // ✅ Catch errors and _return_ them so the RTKQ logic can track it
  try {
    const data = await fetchSomeData()
    return { data }
  } catch (error) {
    return { error }
  }
}

baseQuery function arguments​

baseQuery example arguments
const customBaseQuery = (
  args,
  { signal, dispatch, getState },
  extraOptions
) => {
  // omitted
}

baseQuery function return value​

  1. Expected success result format
    return { data: YourData }
  2. Expected error result format
    return { error: YourError }
baseQuery example return value
const customBaseQuery = (
  args,
  { signal, dispatch, getState },
  extraOptions
) => {
  if (Math.random() > 0.5) return { error: 'Too high!' }
  return { data: 'All good!' }
}
note

This format is required so that RTK Query can infer the return types for your responses.

At its core, a baseQuery function only needs to have the minimum return value to be valid; an object with a data or error property. It is up to the user to determine how they wish to use the provided arguments, and how requests are handled within the function itself.

fetchBaseQuery defaults​

For fetchBaseQuery specifically, the return type is as follows:

Return types of fetchBaseQuery
Promise<{
    data: any;
    error?: undefined;
    meta?: { request: Request; response: Response };
} | {
    error: {
        status: number;
        data: any;
    };
    data?: undefined;
    meta?: { request: Request; response: Response };
}>
  1. Expected success result format with fetchBaseQuery
    return { data: YourData }
  2. Expected error result format with fetchBaseQuery
    return { error: { status: number, data: YourErrorData } }

Customizing query responses with transformResponse​

Individual endpoints on createApi accept a transformResponse property which allows manipulation of the data returned by a query or mutation before it hits the cache.

transformResponse is called with the data that a successful baseQuery returns for the corresponding endpoint, and the return value of transformResponse is used as the cached data associated with that endpoint call.

By default, the payload from the server is returned directly.

function defaultTransformResponse(
  baseQueryReturnValue: unknown,
  meta: unknown,
  arg: unknown
) {
  return baseQueryReturnValue
}

To change it, provide a function that looks like:

Unpack a deeply nested collection
transformResponse: (response, meta, arg) =>
  response.some.deeply.nested.collection

transformResponse is called with the meta property returned from the baseQuery as its second argument, which can be used while determining the transformed response. The value for meta is dependent on the baseQuery used.

transformResponse meta example
transformResponse: (response: { sideA: Tracks; sideB: Tracks }, meta, arg) => {
  if (meta?.coinFlip === 'heads') {
    return response.sideA
  }
  return response.sideB
}

transformResponse is called with the arg property provided to the endpoint as its third argument, which can be used while determining the transformed response. The value for arg is dependent on the endpoint used, as well as the argument used when calling the query/mutation.

transformResponse arg example
transformResponse: (response: Posts, meta, arg) => {
  return {
    originalArg: arg,
    data: response,
  }
}

While there is less need to store the response in a normalized lookup table with RTK Query managing caching data, transformResponse can be leveraged to do so if desired.

Normalize the response data
transformResponse: (response) =>
  response.reduce((acc, curr) => {
    acc[curr.id] = curr
    return acc
  }, {})

/*
  will convert:
  [
    {id: 1, name: 'Harry'},
    {id: 2, name: 'Ron'},
    {id: 3, name: 'Hermione'},
  ]

  to:
  {
    1: { id: 1, name: "Harry" },
    2: { id: 2, name: "Ron" },
    3: { id: 3, name: "Hermione" },
  }
*/

createEntityAdapter can also be used with transformResponse to normalize data, while also taking advantage of other features provided by createEntityAdapter, including providing an ids array, using sortComparer to maintain a consistently sorted list, as well as maintaining strong TypeScript support.

See also Websocket Chat API with a transformed response shape for an example of transformResponse normalizing response data in combination with createEntityAdapter, while also updating further data using streaming updates.

Customizing query responses with transformErrorResponse​

Individual endpoints on createApi accept a transformErrorResponse property which allows manipulation of the error returned by a query or mutation before it hits the cache.

transformErrorResponse is called with the error that a failed baseQuery returns for the corresponding endpoint, and the return value of transformErrorResponse is used as the cached error associated with that endpoint call.

By default, the payload from the server is returned directly.

function defaultTransformResponse(
  baseQueryReturnValue: unknown,
  meta: unknown,
  arg: unknown
) {
  return baseQueryReturnValue
}

To change it, provide a function that looks like:

Unpack a deeply nested error object
transformErrorResponse: (response, meta, arg) =>
  response.data.some.deeply.nested.errorObject

transformErrorResponse is called with the meta property returned from the baseQuery as its second argument, which can be used while determining the transformed response. The value for meta is dependent on the baseQuery used.

transformErrorResponse meta example
transformErrorResponse: (
  response: { data: { sideA: Tracks; sideB: Tracks } },
  meta,
  arg
) => {
  if (meta?.coinFlip === 'heads') {
    return response.data.sideA
  }
  return response.data.sideB
}

transformErrorResponse is called with the arg property provided to the endpoint as its third argument, which can be used while determining the transformed response. The value for arg is dependent on the endpoint used, as well as the argument used when calling the query/mutation.

transformErrorResponse arg example
transformErrorResponse: (response: Posts, meta, arg) => {
  return {
    originalArg: arg,
    error: response,
  }
}

Customizing queries with queryFn​

RTK Query comes with fetchBaseQuery out of the box, which makes it straightforward to define endpoints that talk to HTTP URLs (such as a typical REST API). We also have integrations with GraphQL as well. However, at its core, RTK Query is really about tracking loading state and cached values for any async request/response sequence, not just HTTP requests.

RTK Query supports defining endpoints that run arbitrary async logic and return a result. Individual endpoints on createApi accept a queryFn property, which let you write your own async function with whatever logic you want inside.

This can be useful for scenarios where you want to have particularly different behaviour for a single endpoint, or where the query itself is not relevant, including:

  • One-off queries that use a different base URL
  • One-off queries that use different request handling, such as automatic re-tries
  • One-off queries that use different error handling behaviour
  • Queries that make requests using a third-party library SDK, such as Firebase or Supabase
  • Queries that perform async tasks that are not a typical request/response
  • Performing multiple requests with a single query (example)
  • Leveraging invalidation behaviour with no relevant query (example)
  • Using Streaming Updates with no relevant initial request (example)

See also queryFn API Reference for the type signature and available options.

Implementing a queryFn​

A queryFn can be thought of as an inline baseQuery. It will be called with the same arguments as baseQuery, as well as the provided baseQuery function itself (arg, api, extraOptions, and baseQuery). Similarly to baseQuery, it is expected to return an object with either a data or error property, or a promise that resolves to return such an object.

Basic queryFn Example​

Basic queryFn example
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'
import { userAPI, User } from './userAPI'

const api = createApi({
  baseQuery: fetchBaseQuery({ url: '/' }),
  endpoints: (build) => ({
    // normal HTTP endpoint using fetchBaseQuery
    getPosts: build.query<PostsResponse, void>({
      query: () => ({ url: 'posts' }),
    }),
    // endpoint with a custom `queryFn` and separate async logic
    getUser: build.query<User, string>({
      queryFn: async (userId: string) => {
        try {
          const user = await userApi.getUserById(userId)
          // Return the result in an object with a `data` field
          return { data: user }
        } catch (error) {
          // Catch any errors and return them as an object with an `error` field
          return { error }
        }
      },
    }),
  }),
})

queryFn function arguments​

queryFn example arguments
const queryFn = (
  args,
  { signal, dispatch, getState },
  extraOptions,
  baseQuery
) => {
  // omitted
}

queryFn function return value​

  1. Expected success result format
    return { data: YourData }
  2. Expected error result format
    return { error: YourError }
queryFn example return value
const queryFn = (
  args,
  { signal, dispatch, getState },
  extraOptions,
  baseQuery
) => {
  if (Math.random() > 0.5) return { error: 'Too high!' }
  return { data: 'All good!' }
}

Examples - baseQuery​

Axios baseQuery​

This example implements a very basic axios-based baseQuery utility.

Basic axios baseQuery
import { createApi } from '@reduxjs/toolkit/query'
import type { BaseQueryFn } from '@reduxjs/toolkit/query'
import axios from 'axios'
import type { AxiosRequestConfig, AxiosError } from 'axios'

const axiosBaseQuery =
  (
    { baseUrl }: { baseUrl: string } = { baseUrl: '' }
  ): BaseQueryFn<
    {
      url: string
      method: AxiosRequestConfig['method']
      data?: AxiosRequestConfig['data']
      params?: AxiosRequestConfig['params']
    },
    unknown,
    unknown
  > =>
  async ({ url, method, data, params }) => {
    try {
      const result = await axios({ url: baseUrl + url, method, data, params })
      return { data: result.data }
    } catch (axiosError) {
      let err = axiosError as AxiosError
      return {
        error: {
          status: err.response?.status,
          data: err.response?.data || err.message,
        },
      }
    }
  }

const api = createApi({
  baseQuery: axiosBaseQuery({
    baseUrl: 'https://example.com',
  }),
  endpoints(build) {
    return {
      query: build.query({ query: () => ({ url: '/query', method: 'get' }) }),
      mutation: build.mutation({
        query: () => ({ url: '/mutation', method: 'post' }),
      }),
    }
  },
})

GraphQL baseQuery​

This example implements a very basic GraphQL-based baseQuery.

Basic GraphQL baseQuery
import { createApi } from '@reduxjs/toolkit/query'
import { request, gql, ClientError } from 'graphql-request'

const graphqlBaseQuery =
  ({ baseUrl }: { baseUrl: string }) =>
  async ({ body }: { body: string }) => {
    try {
      const result = await request(baseUrl, body)
      return { data: result }
    } catch (error) {
      if (error instanceof ClientError) {
        return { error: { status: error.response.status, data: error } }
      }
      return { error: { status: 500, data: error } }
    }
  }

export const api = createApi({
  baseQuery: graphqlBaseQuery({
    baseUrl: 'https://graphqlzero.almansi.me/api',
  }),
  endpoints: (builder) => ({
    getPosts: builder.query({
      query: () => ({
        body: gql`
          query {
            posts {
              data {
                id
                title
              }
            }
          }
        `,
      }),
      transformResponse: (response) => response.posts.data,
    }),
    getPost: builder.query({
      query: (id) => ({
        body: gql`
        query {
          post(id: ${id}) {
            id
            title
            body
          }
        }
        `,
      }),
      transformResponse: (response) => response.post,
    }),
  }),
})

Automatic re-authorization by extending fetchBaseQuery​

This example wraps fetchBaseQuery such that when encountering a 401 Unauthorized error, an additional request is sent to attempt to refresh an authorization token, and re-try to initial query after re-authorizing.

Simulating axios-like interceptors with a custom base query
import { fetchBaseQuery } from '@reduxjs/toolkit/query'
import type {
  BaseQueryFn,
  FetchArgs,
  FetchBaseQueryError,
} from '@reduxjs/toolkit/query'
import { tokenReceived, loggedOut } from './authSlice'

const baseQuery = fetchBaseQuery({ baseUrl: '/' })
const baseQueryWithReauth: BaseQueryFn<
  string | FetchArgs,
  unknown,
  FetchBaseQueryError
> = async (args, api, extraOptions) => {
  let result = await baseQuery(args, api, extraOptions)
  if (result.error && result.error.status === 401) {
    // try to get a new token
    const refreshResult = await baseQuery('/refreshToken', api, extraOptions)
    if (refreshResult.data) {
      // store the new token
      api.dispatch(tokenReceived(refreshResult.data))
      // retry the initial query
      result = await baseQuery(args, api, extraOptions)
    } else {
      api.dispatch(loggedOut())
    }
  }
  return result
}

Preventing multiple unauthorized errors​

Using async-mutex to prevent multiple calls to '/refreshToken' when multiple calls fail with 401 Unauthorized errors.

Preventing multiple calls to '/refreshToken'
import { fetchBaseQuery } from '@reduxjs/toolkit/query'
import type {
  BaseQueryFn,
  FetchArgs,
  FetchBaseQueryError,
} from '@reduxjs/toolkit/query'
import { tokenReceived, loggedOut } from './authSlice'
import { Mutex } from 'async-mutex'

// create a new mutex
const mutex = new Mutex()
const baseQuery = fetchBaseQuery({ baseUrl: '/' })
const baseQueryWithReauth: BaseQueryFn<
  string | FetchArgs,
  unknown,
  FetchBaseQueryError
> = async (args, api, extraOptions) => {
  // wait until the mutex is available without locking it
  await mutex.waitForUnlock()
  let result = await baseQuery(args, api, extraOptions)
  if (result.error && result.error.status === 401) {
    // checking whether the mutex is locked
    if (!mutex.isLocked()) {
      const release = await mutex.acquire()
      try {
        const refreshResult = await baseQuery(
          '/refreshToken',
          api,
          extraOptions
        )
        if (refreshResult.data) {
          api.dispatch(tokenReceived(refreshResult.data))
          // retry the initial query
          result = await baseQuery(args, api, extraOptions)
        } else {
          api.dispatch(loggedOut())
        }
      } finally {
        // release must be called once the mutex should be released again.
        release()
      }
    } else {
      // wait until the mutex is available without locking it
      await mutex.waitForUnlock()
      result = await baseQuery(args, api, extraOptions)
    }
  }
  return result
}

Automatic retries​

RTK Query exports a utility called retry that you can wrap the baseQuery in your API definition with. It defaults to 5 attempts with a basic exponential backoff.

The default behavior would retry at these intervals:

  1. 600ms * random(0.4, 1.4)
  2. 1200ms * random(0.4, 1.4)
  3. 2400ms * random(0.4, 1.4)
  4. 4800ms * random(0.4, 1.4)
  5. 9600ms * random(0.4, 1.4)
Retry every request 5 times by default
import { createApi, fetchBaseQuery, retry } from '@reduxjs/toolkit/query/react'
interface Post {
  id: number
  name: string
}
type PostsResponse = Post[]

// maxRetries: 5 is the default, and can be omitted. Shown for documentation purposes.
const staggeredBaseQuery = retry(fetchBaseQuery({ baseUrl: '/' }), {
  maxRetries: 5,
})
export const api = createApi({
  baseQuery: staggeredBaseQuery,
  endpoints: (build) => ({
    getPosts: build.query<PostsResponse, void>({
      query: () => ({ url: 'posts' }),
    }),
    getPost: build.query<PostsResponse, string>({
      query: (id) => ({ url: `post/${id}` }),
      extraOptions: { maxRetries: 8 }, // You can override the retry behavior on each endpoint
    }),
  }),
})

export const { useGetPostsQuery, useGetPostQuery } = api

In the event that you didn't want to retry on a specific endpoint, you can just set maxRetries: 0.

info

It is possible for a hook to return data and error at the same time. By default, RTK Query will keep whatever the last 'good' result was in data until it can be updated or garbage collected.

Bailing out of error re-tries​

The retry utility has a fail method property attached which can be used to bail out of retries immediately. This can be used for situations where it is known that additional re-tries would be guaranteed to all fail and would be redundant.

Bailing out of error re-tries
import { createApi, fetchBaseQuery, retry } from '@reduxjs/toolkit/query/react'
import type { FetchArgs } from '@reduxjs/toolkit/dist/query/fetchBaseQuery'
interface Post {
  id: number
  name: string
}
type PostsResponse = Post[]

const staggeredBaseQueryWithBailOut = retry(
  async (args: string | FetchArgs, api, extraOptions) => {
    const result = await fetchBaseQuery({ baseUrl: '/api/' })(
      args,
      api,
      extraOptions
    )

    // bail out of re-tries immediately if unauthorized,
    // because we know successive re-retries would be redundant
    if (result.error?.status === 401) {
      retry.fail(result.error)
    }

    return result
  },
  {
    maxRetries: 5,
  }
)

export const api = createApi({
  baseQuery: staggeredBaseQueryWithBailOut,
  endpoints: (build) => ({
    getPosts: build.query<PostsResponse, void>({
      query: () => ({ url: 'posts' }),
    }),
    getPost: build.query<Post, string>({
      query: (id) => ({ url: `post/${id}` }),
      extraOptions: { maxRetries: 8 }, // You can override the retry behavior on each endpoint
    }),
  }),
})
export const { useGetPostsQuery, useGetPostQuery } = api

Adding Meta information to queries​

A baseQuery can also include a meta property in its return value. This can be beneficial in cases where you may wish to include additional information associated with the request such as a request ID or timestamp.

In such a scenario, the return value would look like so:

  1. Expected success result format with meta
    return { data: YourData, meta: YourMeta }
  2. Expected error result format with meta
    return { error: YourError, meta: YourMeta }
baseQuery example with meta information
import { fetchBaseQuery, createApi } from '@reduxjs/toolkit/query'
import type {
  BaseQueryFn,
  FetchArgs,
  FetchBaseQueryError,
} from '@reduxjs/toolkit/query'
import type { FetchBaseQueryMeta } from '@reduxjs/toolkit/dist/query/fetchBaseQuery'
import { uuid } from './idGenerator'

type Meta = {
  requestId: string
  timestamp: number
}

const metaBaseQuery: BaseQueryFn<
  string | FetchArgs,
  unknown,
  FetchBaseQueryError,
  {},
  Meta & FetchBaseQueryMeta
> = async (args, api, extraOptions) => {
  const requestId = uuid()
  const timestamp = Date.now()

  const baseResult = await fetchBaseQuery({ baseUrl: '/' })(
    args,
    api,
    extraOptions
  )

  return {
    ...baseResult,
    meta: baseResult.meta && { ...baseResult.meta, requestId, timestamp },
  }
}

const DAY_MS = 24 * 60 * 60 * 1000

interface Post {
  id: number
  name: string
  timestamp: number
}
type PostsResponse = Post[]

const api = createApi({
  baseQuery: metaBaseQuery,
  endpoints: (build) => ({
    // a theoretical endpoint where we only want to return data
    // if request was performed past a certain date
    getRecentPosts: build.query<PostsResponse, void>({
      query: () => 'posts',
      transformResponse: (returnValue: PostsResponse, meta) => {
        // `meta` here contains our added `requestId` & `timestamp`, as well as
        // `request` & `response` from fetchBaseQuery's meta object.
        // These properties can be used to transform the response as desired.
        if (!meta) return []
        return returnValue.filter(
          (post) => post.timestamp >= meta.timestamp - DAY_MS
        )
      },
    }),
  }),
})

Constructing a Dynamic Base URL using Redux state​

In some cases, you may wish to have a dynamically altered base url determined from a property in your Redux state. A baseQuery has access to a getState method that provides the current store state at the time it is called. This can be used to construct the desired url using a partial url string, and the appropriate data from your store state.

Dynamically generated Base URL example
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query/react'
import type {
  BaseQueryFn,
  FetchArgs,
  FetchBaseQueryError,
} from '@reduxjs/toolkit/query/react'
import type { Post } from './types'
import { selectProjectId } from './projectSlice'
import type { RootState } from '../store'

const rawBaseQuery = fetchBaseQuery({
  baseUrl: 'www.my-cool-site.com/',
})

const dynamicBaseQuery: BaseQueryFn<
  string | FetchArgs,
  unknown,
  FetchBaseQueryError
> = async (args, api, extraOptions) => {
  const projectId = selectProjectId(api.getState() as RootState)
  // gracefully handle scenarios where data to generate the URL is missing
  if (!projectId) {
    return {
      error: {
        status: 400,
        statusText: 'Bad Request',
        data: 'No project ID received',
      },
    }
  }

  const urlEnd = typeof args === 'string' ? args : args.url
  // construct a dynamically generated portion of the url
  const adjustedUrl = `project/${projectId}/${urlEnd}`
  const adjustedArgs =
    typeof args === 'string' ? adjustedUrl : { ...args, url: adjustedUrl }
  // provide the amended url and other params to the raw base query
  return rawBaseQuery(adjustedArgs, api, extraOptions)
}

export const api = createApi({
  baseQuery: dynamicBaseQuery,
  endpoints: (builder) => ({
    getPosts: builder.query<Post[], void>({
      query: () => 'posts',
    }),
  }),
})

export const { useGetPostsQuery } = api

/*
  Using `useGetPostsQuery()` where a `projectId` of 500 is in the redux state will result in
  a request being sent to www.my-cool-site.com/project/500/posts
*/

Examples - transformResponse​

Unpacking deeply nested GraphQL data​

GraphQL transformation example
import { createApi } from '@reduxjs/toolkit/query'
import { graphqlBaseQuery, gql } from './graphqlBaseQuery'

interface Post {
  id: number
  title: string
}

export const api = createApi({
  baseQuery: graphqlBaseQuery({
    baseUrl: '/graphql',
  }),
  endpoints: (builder) => ({
    getPosts: builder.query<Post[], void>({
      query: () => ({
        body: gql`
          query {
            posts {
              data {
                id
                title
              }
            }
          }
        `,
      }),
      transformResponse: (response: { posts: { data: Post[] } }) =>
        response.posts.data,
    }),
  }),
})

Normalizing data with createEntityAdapter​

In the example below, transformResponse is used in conjunction with createEntityAdapter to normalize the data before storing it in the cache.

For a response such as:

[
  { id: 1, name: 'Harry' },
  { id: 2, name: 'Ron' },
  { id: 3, name: 'Hermione' },
]

The normalized cache data will be stored as:

{
  ids: [1, 3, 2],
  entities: {
    1: { id: 1, name: "Harry" },
    2: { id: 2, name: "Ron" },
    3: { id: 3, name: "Hermione" },
  }
}
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query/react'
import { createEntityAdapter } from '@reduxjs/toolkit'
import type { EntityState } from '@reduxjs/toolkit'

export interface Post {
  id: number
  name: string
}

const postsAdapter = createEntityAdapter<Post>({
  sortComparer: (a, b) => a.name.localeCompare(b.name),
})

export const api = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/' }),
  endpoints: (build) => ({
    getPosts: build.query<EntityState<Post, number>, void>({
      query: () => `posts`,
      transformResponse(response: Post[]) {
        return postsAdapter.addMany(postsAdapter.getInitialState(), response)
      },
    }),
  }),
})

export const { useGetPostsQuery } = api

Examples - queryFn​

Using a Third-Party SDK​

Many services like Firebase and Supabase provide their own SDK to make requests. You can use those SDK methods in a queryFn:

Basic Third-Party SDK
import { createApi, fakeBaseQuery } from '@reduxjs/toolkit/query/react'
import { supabase } from './supabaseApi'

export const supabaseApi = createApi({
  reducerPath: 'supabaseApi',
  baseQuery: fakeBaseQuery(),
  endpoints: (builder) => ({
    getBlogs: builder.query({
      queryFn: async () => {
        // Supabase conveniently already has `data` and `error` fields
        const { data, error } = await supabase.from('blogs').select()
        if (error) {
          return { error }
        }
        return { data }
      },
    }),
  }),
})

You could also try creating a custom base query that uses the SDK, and define endpoints that pass method names or args into that base query.

Using a no-op queryFn​

In certain scenarios, you may wish to have a query or mutation where sending a request or returning data is not relevant for the situation. Such a scenario would be to leverage the invalidatesTags property to force re-fetch specific tags that have been provided to the cache.

See also providing errors to the cache to see additional detail and an example for such a scenario to 'refetch errored queries'.

Using a no-op queryFn
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'
import type { Post, User } from './types'

const api = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/' }),
  tagTypes: ['Post', 'User'],
  endpoints: (build) => ({
    getPosts: build.query<Post[], void>({
      query: () => 'posts',
      providesTags: ['Post'],
    }),

    getUsers: build.query<User[], void>({
      query: () => 'users',
      providesTags: ['User'],
    }),

    refetchPostsAndUsers: build.mutation<null, void>({
      // The query is not relevant here, so a `null` returning `queryFn` is used
      queryFn: () => ({ data: null }),
      // This mutation takes advantage of tag invalidation behaviour to trigger
      // any queries that provide the 'Post' or 'User' tags to re-fetch if the queries
      // are currently subscribed to the cached data
      invalidatesTags: ['Post', 'User'],
    }),
  }),
})

Streaming data with no initial request​

RTK Query provides the ability for an endpoint to send an initial request for data, followed up with recurring streaming updates that perform further updates to the cached data as the updates occur. However, the initial request is optional, and you may wish to use streaming updates without any initial request fired off.

In the example below, a queryFn is used to populate the cache data with an empty array, with no initial request sent. The array is later populated using streaming updates via the onCacheEntryAdded endpoint option, updating the cached data as it is received.

Streaming data with no initial request
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'
import type { Message } from './types'

const api = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/' }),
  tagTypes: ['Message'],
  endpoints: (build) => ({
    streamMessages: build.query<Message[], void>({
      // The query is not relevant here as the data will be provided via streaming updates.
      // A queryFn returning an empty array is used, with contents being populated via
      // streaming updates below as they are received.
      queryFn: () => ({ data: [] }),
      async onCacheEntryAdded(arg, { updateCachedData, cacheEntryRemoved }) {
        const ws = new WebSocket('ws://localhost:8080')
        // populate the array with messages as they are received from the websocket
        ws.addEventListener('message', (event) => {
          updateCachedData((draft) => {
            draft.push(JSON.parse(event.data))
          })
        })
        await cacheEntryRemoved
        ws.close()
      },
    }),
  }),
})

Performing multiple requests with a single query​

In the example below, a query is written to fetch all posts for a random user. This is done using a first request for a random user, followed by getting all posts for that user. Using queryFn allows the two requests to be included within a single query, avoiding having to chain that logic within component code.

Performing multiple requests with a single query
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'
import type { FetchBaseQueryError } from '@reduxjs/toolkit/query'
import type { Post, User } from './types'

const api = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/ ' }),
  endpoints: (build) => ({
    getRandomUserPosts: build.query<Post, void>({
      async queryFn(_arg, _queryApi, _extraOptions, fetchWithBQ) {
        // get a random user
        const randomResult = await fetchWithBQ('users/random')
        if (randomResult.error)
          return { error: randomResult.error as FetchBaseQueryError }
        const user = randomResult.data as User
        const result = await fetchWithBQ(`user/${user.id}/posts`)
        return result.data
          ? { data: result.data as Post }
          : { error: result.error as FetchBaseQueryError }
      },
    }),
  }),
})
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