repo_description: React Hooks for Data Fetching
repo_topics: cache data data-fetching fetch hook hooks nextjs react react-native stale-while-revalidate suspense swr vercel
code: /// <reference types="react/experimental" />
  import React, { useCallback, useRef, useDebugValue, useMemo } from 'react'
  import { useSyncExternalStore } from 'use-sync-external-store/shim'
  
  import {
    defaultConfig,
    IS_REACT_LEGACY,
    IS_SERVER,
    rAF,
    useIsomorphicLayoutEffect,
    SWRGlobalState,
    serialize,
    isUndefined,
    UNDEFINED,
    OBJECT,
    isFunction,
    createCacheHelper,
    SWRConfig as ConfigProvider,
    withArgs,
    subscribeCallback,
    getTimestamp,
    internalMutate,
    revalidateEvents,
    mergeObjects,
    isPromiseLike,
    noop
  } from '../_internal'
  import type {
    State,
    Fetcher,
    Key,
    SWRResponse,
    RevalidatorOptions,
    FullConfiguration,
    SWRConfiguration,
    SWRHook,
    RevalidateEvent,
    StateDependencies,
    GlobalState
  } from '../_internal'
  
  const use =
    React.use ||
    // This extra generic is to avoid TypeScript mixing up the generic and JSX sytax
    // and emitting an error.
    // We assume that this is only for the `use(thenable)` case, not `use(context)`.
    // https://github.com/facebook/react/blob/aed00dacfb79d17c53218404c52b1c7aa59c4a89/packages/react-server/src/ReactFizzThenable.js#L45
    (<T, _>(
      thenable: Promise<T> & {
        status?: 'pending' | 'fulfilled' | 'rejected'
        value?: T
        reason?: unknown
      }
    ): T => {
      switch (thenable.status) {
        case 'pending':
          throw thenable
        case 'fulfilled':
          return thenable.value as T
        case 'rejected':
          throw thenable.reason
        default:
          thenable.status = 'pending'
          thenable.then(
            v => {
              thenable.status = 'fulfilled'
              thenable.value = v
            },
            e => {
              thenable.status = 'rejected'
              thenable.reason = e
            }
          )
          throw thenable
      }
    })
  
  const WITH_DEDUPE = { dedupe: true }
  
  type DefinitelyTruthy<T> = false extends T
    ? never
    : 0 extends T
    ? never
    : '' extends T
    ? never
    : null extends T
    ? never
    : undefined extends T
    ? never
    : T
  
  const resolvedUndef = Promise.resolve(UNDEFINED)
  const sub = () => noop
  /**
   * The core implementation of the useSWR hook.
   *
   * This is the main handler function that implements all SWR functionality including
   * data fetching, caching, revalidation, error handling, and state management.
   * It manages the complete lifecycle of SWR requests from initialization through
   * cleanup.
   *
   * Key responsibilities:
   * - Key serialization and normalization
   * - Cache state management and synchronization
   * - Automatic and manual revalidation
   * - Error handling and retry logic
   * - Suspense integration
   * - Loading state management
   * - Effect cleanup and memory management
   *
   * @template Data - The type of data returned by the fetcher
   * @template Error - The type of error that can be thrown
   *
   * @param _key - The SWR key (string, array, object, function, or falsy)
   * @param fetcher - The fetcher function to retrieve data, or null to disable fetching
   * @param config - Complete SWR configuration object with both public and internal options
   *
   * @returns SWRResponse object containing data, error, mutate function, and loading states
   *
   * @internal This is the internal implementation. Use `useSWR` instead.
   */
  export const useSWRHandler = <Data = any, Error = any>(
    _key: Key,
    fetcher: Fetcher<Data> | null,
    config: FullConfiguration & SWRConfiguration<Data, Error>
  ) => {
    const {
      cache,
      compare,
      suspense,
      fallbackData,
      revalidateOnMount,
      revalidateIfStale,
      refreshInterval,
      refreshWhenHidden,
      refreshWhenOffline,
      keepPreviousData,
      strictServerPrefetchWarning
    } = config
  
    const [EVENT_REVALIDATORS, MUTATION, FETCH, PRELOAD] = SWRGlobalState.get(
      cache
    ) as GlobalState
  
    // `key` is the identifier of the SWR internal state,
    // `fnArg` is the argument/arguments parsed from the key, which will be passed
    // to the fetcher.
    // All of them are derived from `_key`.
    const [key, fnArg] = serialize(_key)
  
    // If it's the initial render of this hook.
    const initialMountedRef = useRef(false)
  
    // If the hook is unmounted already. This will be used to prevent some effects
    // to be called after unmounting.
    const unmountedRef = useRef(false)
  
    // Refs to keep the key and config.
    const keyRef = useRef(key)
    const fetcherRef = useRef(fetcher)
    const configRef = useRef(config)
    const getConfig = () => configRef.current
    const isActive = () => getConfig().isVisible() && getConfig().isOnline()
  
    const [getCache, setCache, subscribeCache, getInitialCache] =
      createCacheHelper<
        Data,
        State<Data, any> & {
          // The original key arguments.
          _k?: Key
        }
      >(cache, key)
  
    const stateDependencies = useRef<StateDependencies>({}).current
  
    // Resolve the fallback data from either the inline option, or the global provider.
    // If it's a promise, we simply let React suspend and resolve it for us.
    const fallback = isUndefined(fallbackData)
      ? isUndefined(config.fallback)
        ? UNDEFINED
        : config.fallback[key]
      : fallbackData
  
    const isEqual = (prev: State<Data, any>, current: State<Data, any>) => {
      for (const _ in stateDependencies) {
        const t = _ as keyof StateDependencies
        if (t === 'data') {
          if (!compare(prev[t], current[t])) {
            if (!isUndefined(prev[t])) {
              return false
            }
            if (!compare(returnedData, current[t])) {
              return false
            }
          }
        } else {
          if (current[t] !== prev[t]) {
            return false
          }
        }
      }
      return true
    }
    const isInitialMount = !initialMountedRef.current
    const getSnapshot = useMemo(() => {
      const cachedData = getCache()
      const initialData = getInitialCache()
      const getSelectedCache = (state: ReturnType<typeof getCache>) => {
        // We only select the needed fields from the state.
        const snapshot = mergeObjects(state)
        delete snapshot._k
  
        const shouldStartRequest = (() => {
          if (!key) return false
          if (!fetcher) return false
          // If it's paused, we skip revalidation.
          if (getConfig().isPaused()) return false
          // If `revalidateOnMount` is set, we take the value directly.
          if (isInitialMount && !isUndefined(revalidateOnMount))
            return revalidateOnMount
          const data = !isUndefined(fallback) ? fallback : snapshot.data
          if (suspense) return isUndefined(data) || revalidateIfStale
          return isUndefined(data) || revalidateIfStale
        })()
  
        if (!shouldStartRequest) {
          return snapshot
        }
  
        return {
          isValidating: true,
          isLoading: true,
          ...snapshot
        }
      }
  
      const clientSnapshot = getSelectedCache(cachedData)
      const serverSnapshot =
        cachedData === initialData
          ? clientSnapshot
          : getSelectedCache(initialData)
      // To make sure that we are returning the same object reference to avoid
      // unnecessary re-renders, we keep the previous snapshot and use deep
      // comparison to check if we need to return a new one.
      let memorizedSnapshot = clientSnapshot
  
      return [
        () => {
          const newSnapshot = getSelectedCache(getCache())
          const compareResult = isEqual(newSnapshot, memorizedSnapshot)
  
          if (compareResult) {
            // Mentally, we should always return the `memorizedSnapshot` here
            // as there's no change between the new and old snapshots.
            // However, since the `isEqual` function only compares selected fields,
            // the values of the unselected fields might be changed. That's
            // simply because we didn't track them.
            // To support the case in https://github.com/vercel/swr/pull/2576,
            // we need to update these fields in the `memorizedSnapshot` too
            // with direct mutations to ensure the snapshot is always up-to-date
            // even for the unselected fields, but only trigger re-renders when
            // the selected fields are changed.
            memorizedSnapshot.data = newSnapshot.data
            memorizedSnapshot.isLoading = newSnapshot.isLoading
            memorizedSnapshot.isValidating = newSnapshot.isValidating
            memorizedSnapshot.error = newSnapshot.error
            return memorizedSnapshot
          } else {
            memorizedSnapshot = newSnapshot
            return newSnapshot
          }
        },
        () => serverSnapshot
      ]
      // eslint-disable-next-line react-hooks/exhaustive-deps
    }, [cache, key])
  
    // Get the current state that SWR should return.
    const cached = useSyncExternalStore(
      useCallback(
        (callback: () => void) =>
          subscribeCache(
            key,
            (current: State<Data, any>, prev: State<Data, any>) => {
              if (!isEqual(prev, current)) callback()
            }
          ),
        // eslint-disable-next-line react-hooks/exhaustive-deps
        [cache, key]
      ),
      getSnapshot[0],
      getSnapshot[1]
    )
  
    const hasRevalidator =
      EVENT_REVALIDATORS[key] && EVENT_REVALIDATORS[key].length > 0
  
    const cachedData = cached.data
  
    const data = isUndefined(cachedData)
      ? fallback && isPromiseLike(fallback)
        ? use(fallback)
        : fallback
      : cachedData
    const error = cached.error
  
    // Use a ref to store previously returned data. Use the initial data as its initial value.
    const laggyDataRef = useRef(data)
  
    const returnedData = keepPreviousData
      ? isUndefined(cachedData)
        ? // checking undefined to avoid null being fallback as well
          isUndefined(laggyDataRef.current)
          ? data
          : laggyDataRef.current
        : cachedData
      : data
  
    const hasKeyButNoData = key && isUndefined(data)
    const hydrationRef = useRef<boolean | null>(null)
    // Note: the conditionally hook call is fine because the environment
    // `IS_SERVER` never changes.
    // @ts-expect-error -- use hydrationRef directly
    const _ =
      !IS_SERVER &&
      // getServerSnapshot is only called during hydration
      // eslint-disable-next-line react-hooks/rules-of-hooks
      useSyncExternalStore(
        sub,
        () => {
          hydrationRef.current = false
          return hydrationRef
        },
        () => {
          hydrationRef.current = true
          return hydrationRef
        }
      )
    const isHydration = hydrationRef.current
    // During the initial SSR render, warn if the key has no data pre-fetched via:
    // - fallback data
    // - preload calls
    // - initial data from the cache provider
    // We only warn once for each key during Hydration.
    if (
      strictServerPrefetchWarning &&
      isHydration &&
      !suspense &&
      hasKeyButNoData
    ) {
      console.warn(
        `Missing pre-initiated data for serialized key "${key}" during server-side rendering. Data fetching should be initiated on the server and provided to SWR via fallback data. You can set "strictServerPrefetchWarning: false" to disable this warning.`
      )
    }
  
    // Resolve the default validating state:
    // If it's able to validate, and it should revalidate when mount, this will be true.
    // - Suspense mode and there's stale data for the initial render.
    // - Not suspense mode and there is no fallback data and `revalidateIfStale` is enabled.
    // - `revalidateIfStale` is enabled but `data` is not defined.
    const shouldDoInitialRevalidation = (() => {
      if (!key || !fetcher) return false
      // If it's paused, we skip revalidation.
      if (getConfig().isPaused()) return false
      // if a key already has revalidators and also has error, we should not trigger revalidation
      if (hasRevalidator && !isUndefined(error)) return false
      // If `revalidateOnMount` is set, we take the value directly.
      if (isInitialMount && !isUndefined(revalidateOnMount))
        return revalidateOnMount
      // Under suspense mode, it will always fetch on render if there is no
      // stale data so no need to revalidate immediately mount it again.
      // If data exists, only revalidate if `revalidateIfStale` is true.
      if (suspense) return isUndefined(data) ? false : revalidateIfStale
      // If there is no stale data, we need to revalidate when mount;
      // If `revalidateIfStale` is set to true, we will always revalidate.
      return isUndefined(data) || revalidateIfStale
    })()
  
    const defaultValidatingState = isInitialMount && shouldDoInitialRevalidation
  
    const isValidating = isUndefined(cached.isValidating)
      ? defaultValidatingState
      : cached.isValidating
    const isLoading = isUndefined(cached.isLoading)
      ? defaultValidatingState
      : cached.isLoading
  
    // The revalidation function is a carefully crafted wrapper of the original
    // `fetcher`, to correctly handle the many edge cases.
    const revalidate = useCallback(
      async (revalidateOpts?: RevalidatorOptions): Promise<boolean> => {
        const currentFetcher = fetcherRef.current
  
        if (
          !key ||
          !currentFetcher ||
          unmountedRef.current ||
          getConfig().isPaused()
        ) {
          return false
        }
  
        let newData: Data
        let startAt: number
        let loading = true
        const opts = revalidateOpts || {}
  
        // If there is no ongoing concurrent request, or `dedupe` is not set, a
        // new request should be initiated.
        const shouldStartNewRequest = !FETCH[key] || !opts.dedupe
  
        /*
           For React 17
           Do unmount check for calls:
           If key has changed during the revalidation, or the component has been
           unmounted, old dispatch and old event callbacks should not take any
           effect
  
          For React 18
          only check if key has changed
          https://github.com/reactwg/react-18/discussions/82
        */
        const callbackSafeguard = () => {
          if (IS_REACT_LEGACY) {
            return (
              !unmountedRef.current &&
              key === keyRef.current &&
              initialMountedRef.current
            )
          }
          return key === keyRef.current
        }
  
        // The final state object when the request finishes.
        const finalState: State<Data, Error> = {
          isValidating: false,
          isLoading: false
        }
        const finishRequestAndUpdateState = () => {
          setCache(finalState)
        }
        const cleanupState = () => {
          // Check if it's still the same request before deleting it.
          const requestInfo = FETCH[key]
          if (requestInfo && requestInfo[1] === startAt) {
            delete FETCH[key]
          }
        }
  
        // Start fetching. Change the `isValidating` state, update the cache.
        const initialState: State<Data, Error> = { isValidating: true }
        // It is in the `isLoading` state, if and only if there is no cached data.
        // This bypasses fallback data and laggy data.
        if (isUndefined(getCache().data)) {
          initialState.isLoading = true
        }
        try {
          if (shouldStartNewRequest) {
            setCache(initialState)
            // If no cache is being rendered currently (it shows a blank page),
            // we trigger the loading slow event.
            if (config.loadingTimeout && isUndefined(getCache().data)) {
              setTimeout(() => {
                if (loading && callbackSafeguard()) {
                  getConfig().onLoadingSlow(key, config)
                }
              }, config.loadingTimeout)
            }
  
            // Start the request and save the timestamp.
            // Key must be truthy if entering here.
            FETCH[key] = [
              currentFetcher(fnArg as DefinitelyTruthy<Key>),
              getTimestamp()
            ]
          }
  
          // Wait until the ongoing request is done. Deduplication is also
          // considered here.
          ;[newData, startAt] = FETCH[key]
          newData = await newData
  
          if (shouldStartNewRequest) {
            // If the request isn't interrupted, clean it up after the
            // deduplication interval.
            setTimeout(cleanupState, config.dedupingInterval)
          }
  
          // If there're other ongoing request(s), started after the current one,
          // we need to ignore the current one to avoid possible race conditions:
          //   req1------------------>res1        (current one)
          //        req2---------------->res2
          // the request that fired later will always be kept.
          // The timestamp maybe be `undefined` or a number
          if (!FETCH[key] || FETCH[key][1] !== startAt) {
            if (shouldStartNewRequest) {
              if (callbackSafeguard()) {
                getConfig().onDiscarded(key)
              }
            }
            return false
          }
  
          // Clear error.
          finalState.error = UNDEFINED
  
          // If there're other mutations(s), that overlapped with the current revalidation:
          // case 1:
          //   req------------------>res
          //       mutate------>end
          // case 2:
          //         req------------>res
          //   mutate------>end
          // case 3:
          //   req------------------>res
          //       mutate-------...---------->
          // we have to ignore the revalidation result (res) because it's no longer fresh.
          // meanwhile, a new revalidation should be triggered when the mutation ends.
          const mutationInfo = MUTATION[key]
          if (
            !isUndefined(mutationInfo) &&
            // case 1
            (startAt <= mutationInfo[0] ||
              // case 2
              startAt <= mutationInfo[1] ||
              // case 3
              mutationInfo[1] === 0)
          ) {
            finishRequestAndUpdateState()
            if (shouldStartNewRequest) {
              if (callbackSafeguard()) {
                getConfig().onDiscarded(key)
              }
            }
            return false
          }
          // Deep compare with the latest state to avoid extra re-renders.
          // For local state, compare and assign.
          const cacheData = getCache().data
  
          // Since the compare fn could be custom fn
          // cacheData might be different from newData even when compare fn returns True
          finalState.data = compare(cacheData, newData) ? cacheData : newData
  
          // Trigger the successful callback if it's the original request.
          if (shouldStartNewRequest) {
            if (callbackSafeguard()) {
              getConfig().onSuccess(newData, key, config)
            }
          }
        } catch (err: any) {
          cleanupState()
  
          const currentConfig = getConfig()
          const { shouldRetryOnError } = currentConfig
  
          // Not paused, we continue handling the error. Otherwise, discard it.
          if (!currentConfig.isPaused()) {
            // Get a new error, don't use deep comparison for errors.
            finalState.error = err as Error
  
            // Error event and retry logic. Only for the actual request, not
            // deduped ones.
            if (shouldStartNewRequest && callbackSafeguard()) {
              currentConfig.onError(err, key, currentConfig)
              if (
                shouldRetryOnError === true ||
                (isFunction(shouldRetryOnError) &&
                  shouldRetryOnError(err as Error))
              ) {
                if (
                  !getConfig().revalidateOnFocus ||
                  !getConfig().revalidateOnReconnect ||
                  isActive()
                ) {
                  // If it's inactive, stop. It will auto-revalidate when
                  // refocusing or reconnecting.
                  // When retrying, deduplication is always enabled.
                  currentConfig.onErrorRetry(
                    err,
                    key,
                    currentConfig,
                    _opts => {
                      const revalidators = EVENT_REVALIDATORS[key]
                      if (revalidators && revalidators[0]) {
                        revalidators[0](
                          revalidateEvents.ERROR_REVALIDATE_EVENT,
                          _opts
                        )
                      }
                    },
                    {
                      retryCount: (opts.retryCount || 0) + 1,
                      dedupe: true
                    }
                  )
                }
              }
            }
          }
        }
  
        // Mark loading as stopped.
        loading = false
  
        // Update the current hook's state.
        finishRequestAndUpdateState()
  
        return true
      },
      // `setState` is immutable, and `eventsCallback`, `fnArg`, and
      // `keyValidating` are depending on `key`, so we can exclude them from
      // the deps array.
      //
      // FIXME:
      // `fn` and `config` might be changed during the lifecycle,
      // but they might be changed every render like this.
      // `useSWR('key', () => fetch('/api/'), { suspense: true })`
      // So we omit the values from the deps array
      // even though it might cause unexpected behaviors.
      // eslint-disable-next-line react-hooks/exhaustive-deps
      [key, cache]
    )
  
    // Similar to the global mutate but bound to the current cache and key.
    // `cache` isn't allowed to change during the lifecycle.
    const boundMutate: SWRResponse<Data, Error>['mutate'] = useCallback(
      // Use callback to make sure `keyRef.current` returns latest result every time
      (...args: any[]) => {
        return internalMutate(cache, keyRef.current, ...args)
      },
      // eslint-disable-next-line react-hooks/exhaustive-deps
      []
    )
  
    // The logic for updating refs.
    useIsomorphicLayoutEffect(() => {
      fetcherRef.current = fetcher
      configRef.current = config
      // Handle laggy data updates. If there's cached data of the current key,
      // it'll be the correct reference.
      if (!isUndefined(cachedData)) {
        laggyDataRef.current = cachedData
      }
    })
  
    // After mounted or key changed.
    useIsomorphicLayoutEffect(() => {
      if (!key) return
  
      const softRevalidate = revalidate.bind(UNDEFINED, WITH_DEDUPE)
  
      let nextFocusRevalidatedAt = 0
  
      if (getConfig().revalidateOnFocus) {
        const initNow = Date.now()
        nextFocusRevalidatedAt = initNow + getConfig().focusThrottleInterval
      }
  
      // Expose revalidators to global event listeners. So we can trigger
      // revalidation from the outside.
      const onRevalidate = (
        type: RevalidateEvent,
        opts: {
          retryCount?: number
          dedupe?: boolean
        } = {}
      ) => {
        if (type == revalidateEvents.FOCUS_EVENT) {
          const now = Date.now()
          if (
            getConfig().revalidateOnFocus &&
            now > nextFocusRevalidatedAt &&
            isActive()
          ) {
            nextFocusRevalidatedAt = now + getConfig().focusThrottleInterval
            softRevalidate()
          }
        } else if (type == revalidateEvents.RECONNECT_EVENT) {
          if (getConfig().revalidateOnReconnect && isActive()) {
            softRevalidate()
          }
        } else if (type == revalidateEvents.MUTATE_EVENT) {
          return revalidate()
        } else if (type == revalidateEvents.ERROR_REVALIDATE_EVENT) {
          return revalidate(opts)
        }
        return
      }
  
      const unsubEvents = subscribeCallback(key, EVENT_REVALIDATORS, onRevalidate)
  
      // Mark the component as mounted and update corresponding refs.
      unmountedRef.current = false
      keyRef.current = key
      initialMountedRef.current = true
  
      // Keep the original key in the cache.
      setCache({ _k: fnArg })
  
      // Trigger a revalidation
      if (shouldDoInitialRevalidation) {
        // Performance optimization: if a request is already in progress for this key,
        // skip the revalidation to avoid redundant work
        if (!FETCH[key]) {
          if (isUndefined(data) || IS_SERVER) {
            // Revalidate immediately.
            softRevalidate()
          } else {
            // Delay the revalidate if we have data to return so we won't block
            // rendering.
            rAF(softRevalidate)
          }
        }
      }
  
      return () => {
        // Mark it as unmounted.
        unmountedRef.current = true
  
        unsubEvents()
      }
    }, [key])
  
    // Polling
    useIsomorphicLayoutEffect(() => {
      let timer: any
  
      function next() {
        // Use the passed interval
        // ...or invoke the function with the updated data to get the interval
        const interval = isFunction(refreshInterval)
          ? refreshInterval(getCache().data)
          : refreshInterval
  
        // We only start the next interval if `refreshInterval` is not 0, and:
        // - `force` is true, which is the start of polling
        // - or `timer` is not 0, which means the effect wasn't canceled
        if (interval && timer !== -1) {
          timer = setTimeout(execute, interval)
        }
      }
  
      function execute() {
        // Check if it's OK to execute:
        // Only revalidate when the page is visible, online, and not errored.
        if (
          !getCache().error &&
          (refreshWhenHidden || getConfig().isVisible()) &&
          (refreshWhenOffline || getConfig().isOnline())
        ) {
          revalidate(WITH_DEDUPE).then(next)
        } else {
          // Schedule the next interval to check again.
          next()
        }
      }
  
      next()
  
      return () => {
        if (timer) {
          clearTimeout(timer)
          timer = -1
        }
      }
    }, [refreshInterval, refreshWhenHidden, refreshWhenOffline, key])
  
    // Display debug info in React DevTools.
    useDebugValue(returnedData)
  
    // In Suspense mode, we can't return the empty `data` state.
    // If there is an `error`, the `error` needs to be thrown to the error boundary.
    // If there is no `error`, the `revalidation` promise needs to be thrown to
    // the suspense boundary.
    if (suspense) {
      // SWR should throw when trying to use Suspense on the server with React 18,
      // without providing any fallback data. This causes hydration errors. See:
      // https://github.com/vercel/swr/issues/1832
      if (!IS_REACT_LEGACY && IS_SERVER && hasKeyButNoData) {
        throw new Error('Fallback data is required when using Suspense in SSR.')
      }
  
      // Always update fetcher and config refs even with the Suspense mode.
      if (hasKeyButNoData) {
        fetcherRef.current = fetcher
        configRef.current = config
        unmountedRef.current = false
      }
  
      const req = PRELOAD[key]
  
      const mutateReq =
        !isUndefined(req) && hasKeyButNoData ? boundMutate(req) : resolvedUndef
      use(mutateReq)
  
      if (!isUndefined(error) && hasKeyButNoData) {
        throw error
      }
      const revalidation = hasKeyButNoData
        ? revalidate(WITH_DEDUPE)
        : resolvedUndef
      if (!isUndefined(returnedData) && hasKeyButNoData) {
        // @ts-ignore modify react promise status
        revalidation.status = 'fulfilled'
        // @ts-ignore modify react promise value
        revalidation.value = true
      }
      use(revalidation)
    }
  
    const swrResponse: SWRResponse<Data, Error> = {
      mutate: boundMutate,
      get data() {
        stateDependencies.data = true
        return returnedData
      },
      get error() {
        stateDependencies.error = true
        return error
      },
      get isValidating() {
        stateDependencies.isValidating = true
        return isValidating
      },
      get isLoading() {
        stateDependencies.isLoading = true
        return isLoading
      }
    }
    return swrResponse
  }
  
  export const SWRConfig = OBJECT.defineProperty(ConfigProvider, 'defaultValue', {
    value: defaultConfig
  }) as typeof ConfigProvider & {
    defaultValue: FullConfiguration
  }
  
  export { unstable_serialize } from './serialize'
  
  /**
   * A hook to fetch data.
   *
   * @see {@link https://swr.vercel.app}
   *
   * @example
   * ```jsx
   * import useSWR from 'swr'
   * function Profile() {
   *   const { data, error, isLoading } = useSWR('/api/user', fetcher)
   *   if (error) return <div>failed to load</div>
   *   if (isLoading) return <div>loading...</div>
   *   return <div>hello {data.name}!</div>
   * }
   * ```
   */
  const useSWR = withArgs<SWRHook>(useSWRHandler)
  
  export default useSWR
  