What are server-sent events?

const eventSource = new EventSource("/sse-stream");

// Fired when the connection is established.
eventSource.addEventListener("open", () => {
  console.log("Connection opened");
});

// Fired when a message is received from the server.
eventSource.addEventListener("message", (event) => {
  console.log("Received message:", event.data);
});

// Fired when an error occurs.
eventSource.addEventListener("error", (error) => {
  console.error("Error occurred:", error);
});

const express = require("express");
const app = express();

app.get("/sse-stream", (req, res) => {
  // `Content-Type` need to be set to `text/event-stream`.
  res.setHeader("Content-Type", "text/event-stream");
  res.setHeader("Cache-Control", "no-cache");
  res.setHeader("Connection", "keep-alive");

  // Each message should be prefixed with data.
  const sendEvent = (data) => res.write(`data: ${data}\n\n`);

  sendEvent("Hello from server");

  const intervalId = setInterval(() => sendEvent(new Date().toString()), 1000);

  res.on("close", () => {
    console.log("Client closed connection");
    clearInterval(intervalId);
  });
});

app.listen(3000, () => console.log("Server started on port 3000"));