SSE Middleware

sse() sets up a response for Server-Sent Events (SSE). It configures the correct headers, starts an optional heartbeat, attaches res.sendEvent() for dispatching named events, and cleans up automatically when the client disconnects.

Coming from Express?

Drop-in replacement for the sse / express-sse pattern. Mount it as route middleware, then call res.sendEvent() inside your handler.

Basic usage

import { sse } from "bunway";

app.get("/events", sse(), (req, res) => {
  res.sendEvent("update", { count: 1 });
  res.sendEvent("update", { count: 2 }, "msg-42"); // with id
  res.end();
});

The middleware sets:

• Content-Type: text/event-stream
• Cache-Control: no-cache
• Connection: keep-alive
• X-Accel-Buffering: no (disables nginx proxy buffering)

Sending events

res.sendEvent(event, data, id?) writes a formatted SSE frame:

id: msg-42
event: update
data: {"count":1}

id is optional. If omitted, the id: line is not written.

Heartbeat

A keepalive comment (: ping) is sent on an interval to prevent proxies and load balancers from closing idle connections.

app.get("/events", sse({ heartbeatInterval: 30_000 }), handler);
// Set to 0 to disable
app.get("/events", sse({ heartbeatInterval: 0 }), handler);

Default: 15000 ms (15 seconds).

Client disconnects

When the client closes the connection, the abort signal fires, the heartbeat timer is cleared, and res.end() is called automatically. No cleanup code needed in your handler.

Options reference

Option	Type	Default	Description
heartbeatInterval	number	15000	ms between keepalive pings. Set to 0 to disable.

For type details see SseOptions in the API Reference.