How to correctly manage fetch errors response with observables and RxJS?

Question

I just recovered a simple auth app built with ReactJS, redux-observable and RxJS. I want to implement an error handler when the API backend send a response > 400.

Actually, there is an epic that handle login process on LOGIN_REQUEST event:

const apiLoginEpic = (action$, state$) =>
  action$.pipe(
    ofType(authConstant.LOGIN_REQUEST),
    mergeMap(action => authService.loginApi(action.username, action.password)),
    map(({ body }) => extractJwtPayload(body.token)),
    map(authAction.login.apiSuccess),
    logObservableErrorAndTriggerAction(authAction.login.apiFailure)

This epic just call a service, which is a simple fetch that refer to:

const fetchBackend = (path, options = {}, withCredentials = false) => {
  fetch(`${process.env.REACT_APP_BACKEND_URL}${path}`, options)
    .then(response =>
      response.status < 400
        ? Promise.all([response, response.json()])
        : Promise.reject(response)
    )
    .then(([response, body]) => ({
      status: response.status,
      headers: response.headers,
      body,
    }))
}

As you can see, when the response status is superior to 400, it reject the promise. Then, in the epic (first code extract), the error is catch in logObservableErrorAndTriggerAction function which is implemented as:

const logObservableErrorAndTriggerAction = action =>
  catchError((response, source) =>
    pipe(
      tap(logger.error),
      () => of(action(response)).pipe(merge(source))
    )(response)
  )

My problem is that I want to catch the response body in logObservableErrorAndTriggerAction, which can be reusable. Unfortunately, I don't have much knowledge about observalbes and RxJS.

When I try to use body of the response, it appears that it is a ReadableStream. I tried a lot of things but I can't turn it into JSON :

{
  body: ReadableStream,
  bodyUsed: false,
  headers: Headers {},
  ok: false,
  redirected: false,
  status: 400,
  statusText: "",
  type: "cors",
  url: "http://localhost/api/login"
}

Does anybody know how to manage my use case ?

Thank you very much, Sylvain