---
id: fetchBaseQuery
title: fetchBaseQuery
sidebar_label: fetchBaseQuery
hide_title: true
hide_table_of_contents: false
description: 'RTK Query > API: fetchBaseQuery reference'
---

&nbsp;

# `fetchBaseQuery`

This is a very small wrapper around [`fetch`](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) that aims to simplify HTTP requests. It is not a full-blown replacement for `axios`, `superagent`, or any other more heavyweight library, but it will cover the vast majority of your HTTP request needs.

`fetchBaseQuery` is a factory function that generates a data fetching method compatible with RTK Query's `baseQuery` configuration option. It takes all standard options from fetch's [`RequestInit`](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch) interface, as well as `baseUrl`, a `prepareHeaders` function, an optional `fetch` function, a `paramsSerializer` function, and a `timeout`.

## Basic Usage

To use it, import it when you are [creating an API service definition](../../tutorials/rtk-query#create-an-api-service), call it as `fetchBaseQuery(options)`, and pass the result as the `baseQuery` field in `createApi`:

```ts title="src/services/pokemon.ts"
// Or from '@reduxjs/toolkit/query/react'
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'

export const pokemonApi = createApi({
  // Set the baseUrl for every endpoint below
  baseQuery: fetchBaseQuery({ baseUrl: 'https://pokeapi.co/api/v2/' }),
  endpoints: (build) => ({
    getPokemonByName: build.query({
      // Will make a request like https://pokeapi.co/api/v2/pokemon/bulbasaur
      query: (name: string) => `pokemon/${name}`,
    }),
    updatePokemon: build.mutation({
      query: ({ name, patch }) => ({
        url: `pokemon/${name}`,
        // When performing a mutation, you typically use a method of
        // PATCH/PUT/POST/DELETE for REST endpoints
        method: 'PATCH',
        // fetchBaseQuery automatically adds `content-type: application/json` to
        // the Headers and calls `JSON.stringify(patch)`
        body: patch,
      }),
    }),
  }),
})
```

## Signature

```ts title="fetchBaseQuery signature" no-transpile
type FetchBaseQuery = (
  args: FetchBaseQueryArgs,
) => (
  args: string | FetchArgs,
  api: BaseQueryApi,
  extraOptions: ExtraOptions,
) => FetchBaseQueryResult

type FetchBaseQueryArgs = {
  baseUrl?: string
  prepareHeaders?: (
    headers: Headers,
    api: Pick<
      BaseQueryApi,
      'getState' | 'extra' | 'endpoint' | 'type' | 'forced'
    > & { arg: string | FetchArgs },
  ) => MaybePromise<Headers | void>
  fetchFn?: (
    input: RequestInfo,
    init?: RequestInit | undefined,
  ) => Promise<Response>
  paramsSerializer?: (params: Record<string, any>) => string
  isJsonContentType?: (headers: Headers) => boolean
  jsonContentType?: string
  timeout?: number
} & RequestInit

type FetchBaseQueryResult = Promise<
  | {
      data: any
      error?: undefined
      meta?: { request: Request; response: Response }
    }
  | {
      error: FetchBaseQueryError
      data?: undefined
      meta?: { request: Request; response: Response }
    }
>

type FetchBaseQueryError =
  | {
      /**
       * * `number`:
       *   HTTP status code
       */
      status: number
      data: unknown
    }
  | {
      /**
       * * `"FETCH_ERROR"`:
       *   An error that occurred during execution of `fetch` or the `fetchFn` callback option
       **/
      status: 'FETCH_ERROR'
      data?: undefined
      error: string
    }
  | {
      /**
       * * `"PARSING_ERROR"`:
       *   An error happened during parsing.
       *   Most likely a non-JSON-response was returned with the default `responseHandler` "JSON",
       *   or an error occurred while executing a custom `responseHandler`.
       **/
      status: 'PARSING_ERROR'
      originalStatus: number
      data: string
      error: string
    }
  | {
      /**
       * * `"TIMEOUT_ERROR"`:
       *   Request timed out
       **/
      status: 'TIMEOUT_ERROR'
      data?: undefined
      error: string
    }
  | {
      /**
       * * `"CUSTOM_ERROR"`:
       *   A custom error type that you can return from your `queryFn` where another error might not make sense.
       **/
      status: 'CUSTOM_ERROR'
      data?: unknown
      error: string
    }
```

## Parameters

### `baseUrl`

_(required)_

Typically a string like `https://api.your-really-great-app.com/v1/`. If you don't provide a `baseUrl`, it defaults to a relative path from where the request is being made. **You should most likely _always_ specify this**.

### `prepareHeaders`

_(optional)_

Allows you to inject headers on every request. You can specify headers at the endpoint level, but you'll typically want to set common headers like `authorization` here. As a convenience mechanism, the second argument allows you to use `getState` to access your redux store in the event you store information you'll need there such as an auth token. Additionally, it provides access to `arg`, `extra`, `endpoint`, `type`, and `forced` to unlock more granular conditional behaviors.

You can mutate the `headers` argument directly, and returning it is optional.

```ts title="prepareHeaders signature" no-transpile
type prepareHeaders = (
  headers: Headers,
  api: {
    getState: () => unknown
    arg: string | FetchArgs
    extra: unknown
    endpoint: string
    type: 'query' | 'mutation'
    forced: boolean | undefined
  },
) => Headers | void
```

### `paramsSerializer`

_(optional)_

A function that can be used to apply custom transformations to the data passed into [`params`](#setting-the-query-string). If you don't provide this, `params` will be stripped of keys with `undefined` values, and then passed to `new URLSearchParams()`. With some API integrations, you may need to leverage this to use something like the [`query-string`](https://github.com/sindresorhus/query-string) library to support different array types.

### `fetchFn`

_(optional)_

A fetch function that overrides the default on the window. Can be useful in SSR environments where you may need to leverage `isomorphic-fetch` or `cross-fetch`.

### `timeout`

_(optional)_

A number in milliseconds that represents the maximum time a request can take before timing out.

### `isJsonContentType`

_(optional)_

A callback that receives a `Headers` object and determines the `body` field of the `FetchArgs` argument should be stringified via `JSON.stringify()`.

The default implementation inspects the `content-type` header, and will match values like `"application/json"` and `"application/vnd.api+json"`.

### `jsonContentType`

_(optional)_

Used when automatically setting the `content-type` header for a request with a jsonifiable body that does not have an explicit `content-type` header. Defaults to `"application/json"`.

## Common Usage Patterns

### Setting default headers on requests

The most common use case for `prepareHeaders` would be to automatically include `authorization` headers for your API requests.

```ts title="Setting a token from a redux store value
// file: store.ts noEmit
export type RootState = { auth: { token: string } }

// file: baseQuery.ts
import { fetchBaseQuery } from '@reduxjs/toolkit/query'
import type { RootState } from './store'

const baseQuery = fetchBaseQuery({
  baseUrl: '/',
  prepareHeaders: (headers, { getState }) => {
    const token = (getState() as RootState).auth.token

    // If we have a token set in state, let's assume that we should be passing it.
    if (token) {
      headers.set('authorization', `Bearer ${token}`)
    }

    return headers
  },
})
```

## Individual query options

There is more behavior that you can define on a per-request basis. The `query` field may return an object containing any of the default `fetch` options available to the `RequestInit` interface, as well as these additional options:

```ts title="endpoint request options"
interface FetchArgs extends RequestInit {
  url: string
  params?: Record<string, any>
  body?: any
  responseHandler?:
    | 'json'
    | 'text'
    | `content-type`
    | ((response: Response) => Promise<any>)
  validateStatus?: (response: Response, body: any) => boolean
  timeout?: number
}

const defaultValidateStatus = (response: Response) =>
  response.status >= 200 && response.status <= 299
```

### Setting the body

By default, `fetchBaseQuery` assumes that every request you make will be `json`, so in those cases all you have to do is set the `url` and pass a `body` object when appropriate. For other implementations, you can manually set the `Headers` to specify the content type.

#### json

```ts no-transpile
 // omitted
  endpoints: (build) => ({
    updateUser: build.query({
      query: (user: Record<string, string>) => ({
        url: `users`,
        method: 'PUT',
        body: user // Body is automatically converted to json with the correct headers
      }),
    }),
```

#### text

```ts no-transpile
 // omitted
  endpoints: (build) => ({
    updateUser: build.query({
      query: (user: Record<string, string>) => ({
        url: `users`,
        method: 'PUT',
        headers: {
            'content-type': 'text/plain',
        },
        body: user
      }),
    }),
```

### Setting the query string

`fetchBaseQuery` provides a simple mechanism that converts an `object` to a serialized query string by passing the object to `new URLSearchParms()`. If this doesn't suit your needs, you have two options:

1. Pass the `paramsSerializer` option to `fetchBaseQuery` to apply custom transformations
2. Build your own querystring and set it in the `url`

```ts no-transpile
 // omitted
  endpoints: (build) => ({
    updateUser: build.query({
      query: (user: Record<string, string>) => ({
        url: `users`,
        // Assuming no `paramsSerializer` is specified, the user object is automatically converted
        // and produces a url like /api/users?first_name=test&last_name=example
        params: user
      }),
    }),
```

### Parsing a Response

By default, `fetchBaseQuery` assumes that every `Response` you get will be parsed as `json`. In the event that you don't want that to happen, you can customize the behavior by specifying an alternative response handler like `text`, or take complete control and use a custom function that accepts the raw `Response` object &mdash; allowing you to use any [`Response` method](https://developer.mozilla.org/en-US/docs/Web/API/Response).

The `responseHandler` field can be either:

```ts
type ResponseHandler =
  | 'content-type'
  | 'json'
  | 'text'
  | ((response: Response) => Promise<any>)
```

The `"json"` and `"text"` values instruct `fetchBaseQuery` to the corresponding fetch response methods for reading the body. `content-type` will check the header field to first determine if this appears to be JSON, and then use one of those two methods. The callback allows you to process the body yourself.

```ts title="Parse a Response as text"
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'

export const customApi = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/api/' }),
  endpoints: (build) => ({
    getUsers: build.query({
      query: () => ({
        url: `users`,
        // This is the same as passing 'text'
        responseHandler: (response) => response.text(),
      }),
    }),
  }),
})
```

:::note Note about responses that return an undefined body
If you make a `json` request to an API that only returns a `200` with an undefined body, `fetchBaseQuery` will pass that through as `undefined` and will not try to parse it as `json`. This can be common with some APIs, especially on `delete` requests.
:::

#### Default response handler

The default response handler is `"json"`, which is equivalent to the following function:

```ts title="Default responseHandler"
const defaultResponseHandler = async (res: Response) => {
  const text = await res.text()
  return text.length ? JSON.parse(text) : null
}
```

### Handling non-standard Response status codes

By default, `fetchBaseQuery` will `reject` any `Response` that does not have a status code of `2xx` and set it to `error`. This is the same behavior you've most likely experienced with `axios` and other popular libraries. In the event that you have a non-standard API you're dealing with, you can use the `validateStatus` option to customize this behavior.

```ts title="Using a custom validateStatus"
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'

export const customApi = createApi({
  // Set the baseUrl for every endpoint below
  baseQuery: fetchBaseQuery({ baseUrl: '/api/' }),
  endpoints: (build) => ({
    getUsers: build.query({
      query: () => ({
        url: `users`,
        // Example: we have a backend API always returns a 200,
        // but sets an `isError` property when there is an error.
        validateStatus: (response, result) =>
          response.status === 200 && !result.isError,
      }),
    }),
  }),
})
```

### Adding a custom timeout to requests

By default, `fetchBaseQuery` has no default timeout value set, meaning your requests will stay pending until your api resolves the request(s) or it reaches the browser's default timeout (normally 5 minutes). Most of the time, this isn't what you'll want. When using `fetchBaseQuery`, you have the ability to set a `timeout` on the `baseQuery` or on individual endpoints. When specifying both options, the endpoint value will take priority.

```ts title="Setting a timeout value"
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'

export const api = createApi({
  // Set a default timeout of 10 seconds
  baseQuery: fetchBaseQuery({ baseUrl: '/api/', timeout: 10000 }),
  endpoints: (build) => ({
    getUsers: build.query({
      query: () => ({
        url: `users`,
        // Example: we know the users endpoint is _really fast_ because it's always cached.
        // We can assume if it's over > 1000ms, something is wrong and we should abort the request.
        timeout: 1000,
      }),
    }),
  }),
})
```