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HTTP server: Server-sent events

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Server-sent events push updates from the server to the browser over a single long-lived HTTP response, with automatic reconnection built into the EventSource API. This example sends the time once per second.

An SSE response is a stream of text blocks. Each event is a line starting with data:, followed by a blank line.
function handler(_req: Request): Response {
  let timer: ReturnType<typeof setInterval>;
  const body = new ReadableStream({
    start(controller) {
      timer = setInterval(() => {
        const event = `data: ${new Date().toISOString()}\n\n`;
        controller.enqueue(new TextEncoder().encode(event));
      }, 1000);
    },
Stop producing events when the client disconnects.
    cancel() {
      clearInterval(timer);
    },
  });
The content type is what makes this server-sent events. Disabling proxy buffering and caching keeps events flowing immediately.
  return new Response(body, {
    headers: {
      "content-type": "text/event-stream",
      "cache-control": "no-store",
    },
  });
}

Deno.serve(handler);
In the browser, EventSource consumes the stream and reconnects automatically if the connection drops: const source = new EventSource("http://localhost:8000/"); source.onmessage = (event) => console.log(event.data); From the terminal, curl shows the raw protocol: curl http://localhost:8000/ data: 2026-06-11T13:37:00.000Z data: 2026-06-11T13:37:01.000Z

Run this example locally using the Deno CLI:

deno run -N https://docs.deno.com/examples/scripts/http_server_sse.ts

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