Function fetchNextData

packages/next/src/shared/lib/router/router.ts:498–643

({
  dataHref,
  inflightCache,
  isPrefetch,
  hasMiddleware,
  isServerRender,
  parseJSON,
  persistCache,
  isBackground,
  unstable_skipClientCache,
}: FetchNextDataParams)

}

function fetchNextData({
  dataHref,
  inflightCache,
  isPrefetch,
  hasMiddleware,
  isServerRender,
  parseJSON,
  persistCache,
  isBackground,
  unstable_skipClientCache,
}: FetchNextDataParams): Promise<FetchDataOutput> {
  const { href: cacheKey } = new URL(dataHref, window.location.href)
  const deploymentId = getDeploymentId()
  const getData = (params?: { method?: 'HEAD' | 'GET' }) =>
    fetchRetry(dataHref, isServerRender ? 3 : 1, {
      headers: Object.assign(
        {} as HeadersInit,
        isPrefetch ? { purpose: 'prefetch' } : {},
        isPrefetch && hasMiddleware ? { 'x-middleware-prefetch': '1' } : {},
        deploymentId ? { 'x-deployment-id': deploymentId } : {}
      ),
      method: params?.method ?? 'GET',
    })
      .then((response) => {
        if (response.ok && params?.method === 'HEAD') {
          return { dataHref, response, text: '', json: {}, cacheKey }
        }

        return response.text().then((text) => {
          if (!response.ok) {
            /**
             * When the data response is a redirect because of a middleware
             * we do not consider it an error. The headers must bring the
             * mapped location.
             * TODO: Change the status code in the handler.
             */
            if (
              hasMiddleware &&
              [301, 302, 307, 308].includes(response.status)
            ) {
              return { dataHref, response, text, json: {}, cacheKey }
            }

            if (response.status === 404) {
              if (tryToParseAsJSON(text)?.notFound) {
                return {
                  dataHref,
                  json: { notFound: SSG_DATA_NOT_FOUND },
                  response,
                  text,
                  cacheKey,
                }
              }
            }

            const error = new Error(`Failed to load static props`)

            /**