parallel.js

export function* getMissingRates(orderNumbers, labelType) {
    const ratesDeltaByOrderNumber = {};

    const orderNumbersMissingRates = yield select(orderNumbersMissingRatesSelector, {orderNumbers, labelType});
    if (!orderNumbersMissingRates.length) {
        return;
    }

    yield put(startUpdatingRates(orderNumbersMissingRates, labelType));

    const ratesDeltaList = yield orderNumbersMissingRates.map(
        (orderNumber) => call(fetchRates, orderNumber, labelType),
    );

    ratesDeltaList.forEach((ratesDelta, i) => {
        const orderNumber = orderNumbersMissingRates[i];
        ratesDeltaByOrderNumber[orderNumber] = ratesDelta;
    });
}

sequential.js

export function* getMissingRates(orderNumbers, labelType) {
    const ratesDeltaByOrderNumber = {};

    const orderNumbersMissingRates = yield select(orderNumbersMissingRatesSelector, {orderNumbers, labelType});
    if (!orderNumbersMissingRates.length) {
        return;
    }

    yield put(startUpdatingRates(orderNumbersMissingRates, labelType));

    try {
        for (let i = 0; i < orderNumbersMissingRates.length; i++) {
            const orderNumber = orderNumbersMissingRates[i];
            const ratesDelta = yield call(fetchRates, orderNumber, labelType);
            ratesDeltaByOrderNumber[orderNumber] = ratesDelta;
        }
    }
    finally {
        // Commit the finished rates, even if the generator was cancelled.
        // This way we don't have to refresh rates we've already loaded
        yield put(updateRatesByOrderNumber(labelType, ratesDeltaByOrderNumber));
    }
}