Server-Sent Events (SSE): пример кода

// server (Node/Express) — a stream that never returns
app.get('/prices', (req, res) => {
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    Connection: 'keep-alive',
    'X-Accel-Buffering': 'no', // tell nginx NOT to buffer the stream
  });
  const since = req.headers['last-event-id']; // resume support
  const timer = setInterval(() => {
    res.write(`id: ${nextId()}\n`);
    res.write(`event: price\n`);
    res.write(`data: ${JSON.stringify(getPrice())}\n\n`); // blank line!
  }, 1000);
  const beat = setInterval(() => res.write(': ping\n\n'), 15000); // comment = heartbeat
  req.on('close', () => { clearInterval(timer); clearInterval(beat); });
});

// client — the browser handles reconnect + Last-Event-ID for you
const es = new EventSource('/prices'); // add { withCredentials: true } for cookies
es.addEventListener('price', (e) => render(JSON.parse(e.data)));
es.onerror = () => {
  // fired on every drop; EventSource is ALREADY retrying. Don't recreate it.
  if (es.readyState === EventSource.CLOSED) reallyGiveUp();
};