rx.js

import { fromPromise } from 'rxjs/observable/fromPromise';
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mergeMap';
import 'rxjs/add/operator/switchMap';

// IGNORE (just filters)
const isValidLinkedAccount = linkedAccount => {
    const { is_linked: isLinked, link_failed: linkFailed, status, service_status: serviceStatus } = linkedAccount;

    return isLinked && !linkFailed && status === 'active' && serviceStatus === 'active';
};

const hasRequiredEnergyLinxFields = linkedAccount => {
    const { postcode, provider_code: providerCode, provider_label: providerLabel, tariff_name: tariffName } = linkedAccount;

    return postcode && providerCode && providerLabel && tariffName;
};
/--------------------------


export const getPreSwitchComparisonEpic = (action$, store) => (
    action$
        .ofType(GET_PRE_SWITCH_COMPARISON_ATTEMPT)
        .switchMap(action => {
            // Apparently it's not recommended to dispatch from within an epic, but needed here
            // to help work with existing code
            const { dispatch } = store;
            const { linkedAccounts } = action;

            const validLinkedAccounts = linkedAccounts
                .filter(isValidLinkedAccount)
                .filter(hasRequiredEnergyLinxFields);

            const requestPostcode = getRequestPostcode(validLinkedAccounts);

            if (!requestPostcode) {
                // return an action here? (Correct! see below)
                return of(noLinkedAccounts((['No suitable linked energy accounts']));
            }

            const requestObject = { postcode: requestPostcode };

            const fetchObject = getAuthenticatedFetchObject(ENERGY_INITIAL_COMPARISON_URL, 'POST', requestObject);
            const fetch$ = fromPromise(callFetch(fetchObject))
                .mergeMap(response => {
                    const { average_savings: averageSavings } = response;

                    if (averageSavings) {
                        return of(
                            getPreSwitchComparisonSuccess(),
                            showInitialEnergySavings(),
                            updateAverageSavings(averageSavings)
                        );
                    }

                    return null; // Should be mapping to some action or throwing error and going to `catch(error)` if `averageSavings` doesn't exist
                })
                .catch(error => {
                    return of(
                        getPreSwitchComparisonFailure(error),
                        hideInitialEnergySavings()
                    );
                });
                
            return of(
                updatePreSwitchFormAccount(validLinkedAccounts),
                updateAverageSavingsPostcode(requestPostcode)
               ).concat(fetch$)
                
        })
);