static overallStatus(heartbeats) {
        if (heartbeats.length === 0) {
            return -1;
        }

        let status = STATUS_PAGE_ALL_UP;
        let hasUp = false;

        for (let beat of heartbeats) {
            if (beat.status === MAINTENANCE) {
                return STATUS_PAGE_MAINTENANCE;
            } else if (beat.status === UP) {
                hasUp = true;
            } else {
                status = STATUS_PAGE_PARTIAL_DOWN;
            }
        }

        if (! hasUp) {
            status = STATUS_PAGE_ALL_DOWN;
        }

        return status;
    }
@param {heartbeats} heartbeats from getRSSPageData
@returns {number} status_page constant from util.ts