const fetchProductsEpic = (action$, $store) =>
action$.ofType(PRODUCTS_DISPLAY)
    .do($store.dispatch({ type: 'ON_PRODUCTS_FETCHNG' }))
    .mergeMap(action => ajax.getJSON(`http://localhost:9000/products`)
        .map(response => productsFetchedAction(response))
        .catch(err => {
            $store.dispatch({ type: 'ON_PRODUCTS_FETCH_ERR', payload: err })
        })
    );

.do($store.dispatch({ type: 'ON_PRODUCTS_FETCHNG' }))