redux-observable you provided 'undefined' where a stream was expected

Question

I'm using the fbsdk to get user details in an ajax request. So it makes sense to do this in a redux-observable epic. The way the fbsdk request goes, it doesn't have a .map() and .catch() it takes the success and failure callbacks:

code:

export const fetchUserDetailsEpic: Epic<*, *, *> = (
  action$: ActionsObservable<*>,
  store
): Observable<CategoryAction> =>
  action$.ofType(FETCH_USER_DETAILS).mergeMap(() => {
    getDetails(store)
  })

const getDetails = store => {
  console.log(store)
  let req = new GraphRequest(
    '/me',
    {
      httpMethod: 'GET',
      version: 'v2.5',
      parameters: {
        fields: {
          string: 'email,first_name,last_name'
        }
      }
    },
    (err, res) => {
      if (err) {
        store.dispatch(fetchUserDetailsRejected(err))
      } else {
        store.dispatch(fetchUserDetailsFulfilled(res))
      }
    }
  )

  return new GraphRequestManager().addRequest(req).start()
}

It gives the error:

TypeError: You provided 'undefined' where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.

How do I return an observable from the epic so this error goes away?

Attempt at bindCallback from this SO answer:

const getDetails = (callBack, details) => {
  let req = new GraphRequest(
    '/me',
    {
      httpMethod: 'GET',
      version: 'v2.5',
      parameters: {
        fields: {
          string: 'email,first_name,last_name'
        }
      }
    },
    callBack(details)
  )

  new GraphRequestManager().addRequest(req).start()
}

const someFunction = (options, cb) => {
  if (typeof options === 'function') {
    cb = options
    options = null
  }
  getDetails(cb, null)
}

const getDetailsObservable = Observable.bindCallback(someFunction)

export const fetchUserDetailsEpic: Epic<*, *, *> = (
  action$: ActionsObservable<*>
): Observable<CategoryAction> =>
  action$.ofType(FETCH_USER_DETAILS).mergeMap(() => {
    getDetailsObservable()
      .mergeMap(details => {
        return Observable.of(fetchUserDetailsFulfilled(details))
      })
      .catch(error => Observable.of(fetchUserDetailsRejected(error)))
  })

Getting the same error

Answer 1

Looking into source code of GraphRequestManager .start:

start(timeout: ?number) {
  const that = this;
  const callback = (error, result, response) => {
    if (response) {
      that.requestCallbacks.forEach((innerCallback, index, array) => {
        if (innerCallback) {
          innerCallback(response[index][0], response[index][1]);
        }
      });
    }
    if (that.batchCallback) {
      that.batchCallback(error, result);
    }
  };

  NativeGraphRequestManager.start(this.requestBatch, timeout || 0, callback);
}

As you can see it does return nothing, so effectively undefined. Rx mergeMap requires an instance of Observable or something compatible with it (more info).

Since you dispatch further actions, you can modify your original code like that:

export const fetchUserDetailsEpic: Epic<*, *, *> = (
  action$: ActionsObservable<*>,
  store
): Observable<CategoryAction> =>
  action$.ofType(FETCH_USER_DETAILS).do(() => { // .mergeMap changed to .do
    getDetails(store)
  })

const getDetails = store => {
  console.log(store)
  let req = new GraphRequest(
    '/me',
    {
      httpMethod: 'GET',
      version: 'v2.5',
      parameters: {
        fields: {
          string: 'email,first_name,last_name'
        }
      }
    },
    (err, res) => {
      if (err) {
        store.dispatch(fetchUserDetailsRejected(err))
      } else {
        store.dispatch(fetchUserDetailsFulfilled(res))
      }
    }
  )

  return new GraphRequestManager().addRequest(req).start()
}

To be honest I find your second attempt bit better / less coupled. To make it working you could do something like:

const getDetails = Observable.create((observer) => {
  let req = new GraphRequest(
    '/me',
    {
      httpMethod: 'GET',
      version: 'v2.5',
      parameters: {
        fields: {
          string: 'email,first_name,last_name'
        }
      }
    },
    (error, details) => {
      if (error) {
        observer.error(error)
      } else {
        observer.next(details)
        observer.complete()
      }
    }
  )

  new GraphRequestManager().addRequest(req).start()
})

export const fetchUserDetailsEpic: Epic<*, *, *> = (
  action$: ActionsObservable<*>
): Observable<CategoryAction> =>
  action$.ofType(FETCH_USER_DETAILS).mergeMap(() => {
    getDetails()
      .map(details => fetchUserDetailsFulfilled(details)) // regular .map should be enough here
      .catch(error => Observable.of(fetchUserDetailsRejected(error)))
  })

Answer 2

I don't remember well how was working redux-observable before using RxJS >= 6 but I'll try to help ;)

First, you don't need to dispatch yourself, redux-observable will do it for you. In this article, they show how it works under the hood, so they call dispatch, but you don't have to. In the new implementation, they removed store as a second argument in favor of a state stream:

const epic = (action$, store) => { ... //before
const epic = (action$, state$) => { ... //after

But most importantly, the problem you experience is that you don't return a stream of actions, but a single (dispatched) action. From their website:

It is a function which takes a stream of actions and returns a stream of actions.

So I think a quick solution would be to return observables from your callback:

(err, res) => {
  if (err) {
    return Observable.of(fetchUserDetailsRejected(err))
  }
  return Observable.of(fetchUserDetailsFulfilled(res))
}

I will update the answer based on your comments. Good luck!