Response headers:
   Content-Type: text/event-stream
   Cache-Control: no-cache
   Connection: keep-alive

Then the server writes events as plain text, each terminated by a blank line:

   data: hello world\n\n

   event: price\n
   data: {"symbol":"ACME","price":42.10}\n\n

   id: 1001\n
   data: an event with an id (used for resuming)\n\n

   : this is a comment / keep-alive ping\n\n

app.get('/events', (req, res) => {
  // 1. Set the SSE headers and flush them to open the stream:
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    Connection: 'keep-alive',
  })

  // 2. Helper to write a well-formed event:
  const send = (data, event) => {
    if (event) res.write(`event: ${event}\n`)
    res.write(`data: ${JSON.stringify(data)}\n\n`)   // blank line terminates
  }

  send({ message: 'connected' })

  // 3. Push events as things happen (here: a subscription to some source):
  const onUpdate = (payload) => send(payload, 'update')
  emitter.on('update', onUpdate)

  // 4. Keep-alive comment so proxies don't close an idle connection:
  const keepAlive = setInterval(() => res.write(': ping\n\n'), 15000)

  // 5. CRITICAL: clean up when the client disconnects:
  req.on('close', () => {
    clearInterval(keepAlive)
    emitter.off('update', onUpdate)        // stop leaking listeners
    res.end()
  })
})

// Built into every modern browser — no library needed:
const source = new EventSource('/events')

source.onmessage = (e) => {                 // default (unnamed) events
  console.log('data:', JSON.parse(e.data))
}

source.addEventListener('update', (e) => {  // named events (event: update)
  render(JSON.parse(e.data))
})

source.onerror = () => {
  // EventSource reconnects on its own; this just signals a transient drop.
  console.log('connection lost — auto-reconnecting...')
}

// Server: tag each event with an id so the client can resume after a drop:
res.write(`id: ${event.id}\n`)
res.write(`data: ${JSON.stringify(event)}\n\n`)

// On reconnect, the browser AUTOMATICALLY sends the last id it saw:
app.get('/events', (req, res) => {
  const lastId = req.headers['last-event-id']   // browser sets this on reconnect
  if (lastId) {
    // Replay events the client missed while disconnected:
    for (const ev of getEventsSince(lastId)) send(ev)
  }
  // ...continue streaming new events...
})