fetch.ts

await getBytes(response.body!, getLines(getMessages(id => {
  if (id) {
    // store the id and send it back on the next retry:
    headers[LastEventId] = id;
  } else {
    // don't send the last-event-id header anymore:
    delete headers[LastEventId];
  }
}, retry => {
    retryInterval = retry;
}, onmessage)));