Variable: customFetch

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• const customFetch: unique symbol

When configured on an interface that extends HttpRequestOptions, this applies to options parameter for functions that may trigger HTTP requests, this replaces the use of global fetch. As a fetch replacement the arguments and expected return are the same as fetch.

In theory any module that claims to be compatible with the Fetch API can be used but your mileage may vary. No workarounds to allow use of non-conform Responses will be considered.

If you only need to update the Request properties you do not need to use a Fetch API module, just change what you need and pass it to globalThis.fetch just like this module would normally do.

Its intended use cases are:

• Request/Response tracing and logging
• Custom caching strategies for responses of Authorization Server Metadata and JSON Web Key Set (JWKS) endpoints
• Changing the Request properties like headers, body, credentials, mode before it is passed to fetch

Known caveats:

• Expect Type-related issues when passing the inputs through to fetch-like modules, they hardly ever get their typings inline with actual fetch, you should @ts-expect-error them.

Examples

Using sindresorhus/ky for retries and its hooks feature for logging outgoing requests and their responses.

import ky from 'ky'

// example use
await oauth.discoveryRequest(new URL('https://as.example.com'), {
  [oauth.customFetch]: (...args) =>
    ky(args[0], {
      ...args[1],
      hooks: {
        beforeRequest: [
          (request) => {
            logRequest(request)
          },
        ],
        beforeRetry: [
          ({ request, error, retryCount }) => {
            logRetry(request, error, retryCount)
          },
        ],
        afterResponse: [
          (request, _, response) => {
            logResponse(request, response)
          },
        ],
      },
    }),
})

Using nodejs/undici to detect and use HTTP proxies.

import * as undici from 'undici'

// see https://undici.nodejs.org/#/docs/api/EnvHttpProxyAgent
let envHttpProxyAgent = new undici.EnvHttpProxyAgent()

// example use
await oauth.discoveryRequest(new URL('https://as.example.com'), {
  // @ts-ignore
  [oauth.customFetch](...args) {
    return undici.fetch(args[0], { ...args[1], dispatcher: envHttpProxyAgent }) // prettier-ignore
  },
})

Using nodejs/undici to automatically retry network errors.

import * as undici from 'undici'

// see https://undici.nodejs.org/#/docs/api/RetryAgent
let retryAgent = new undici.RetryAgent(new undici.Agent(), {
  statusCodes: [],
  errorCodes: [
    'ECONNRESET',
    'ECONNREFUSED',
    'ENOTFOUND',
    'ENETDOWN',
    'ENETUNREACH',
    'EHOSTDOWN',
    'UND_ERR_SOCKET',
  ],
})

// example use
await oauth.discoveryRequest(new URL('https://as.example.com'), {
  // @ts-ignore
  [oauth.customFetch](...args) {
    return undici.fetch(args[0], { ...args[1], dispatcher: retryAgent }) // prettier-ignore
  },
})

Using nodejs/undici to mock responses in tests.

import * as undici from 'undici'

// see https://undici.nodejs.org/#/docs/api/MockAgent
let mockAgent = new undici.MockAgent()
mockAgent.disableNetConnect()

// example use
await oauth.discoveryRequest(new URL('https://as.example.com'), {
  // @ts-ignore
  [oauth.customFetch](...args) {
    return undici.fetch(args[0], { ...args[1], dispatcher: mockAgent }) // prettier-ignore
  },
})