Challenge 3: Set Up an SSE Connection — Possible Solution
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const notificationStream = new EventSource("/api/notifications/stream");

notificationStream.onmessage = (event) => {
  const notification = JSON.parse(event.data);
  console.log("Received notification:", notification);
};

notificationStream.onerror = (error) => {
  console.log("SSE connection lost — attempting to reconnect...", error);
  // No manual reconnect() call is needed here — see the "why" below —
  // this handler exists purely to observe/log the event, not to trigger
  // recovery.
};

notificationStream.onopen = () => {
  console.log("SSE connection established.");
};

WHY THIS WORKS
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- new EventSource("/api/notifications/stream") immediately opens a
  persistent HTTP connection to that URL and starts listening for
  server-pushed events — no separate "connect" call is needed, unlike
  WebSockets' more manual connection lifecycle covered in later chapters.
- onmessage fires once per event the server sends; event.data is always
  a string, so JSON.parse() is needed to turn it back into a structured
  object if the server is sending JSON-formatted notification payloads
  (a very common convention, though SSE itself is transport-agnostic
  about the payload format).
- onerror fires when the connection is lost or fails — critically,
  the browser's EventSource implementation ALREADY handles reconnection
  automatically behind the scenes by default, retrying the connection on
  its own after an error. This handler is for observability (logging,
  updating a "reconnecting..." UI indicator) rather than for manually
  re-establishing the connection, which is exactly the "genuinely simpler
  than WebSockets" advantage the chapter highlighted — no hand-rolled
  reconnect() logic is required the way it typically is for raw
  WebSockets (covered in Chapter 5).
- onopen is optional but useful for confirming the initial connection
  succeeded, and for distinguishing "just connected" from "reconnected
  after a drop" in more detailed logging if needed.