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SSE - protocol with strict format

res.write(`data: hello\n\n`);

starts with "data"
ends with "\n\n" -- event boundary. Dispatch it to the client

res.write(`lol`);

Browser sees lolololol. Browser doesn't dispatch an event. Not valid SSE → ignored by onmessage Browser keeps buffering → never emits an event

✅ Each line starts with a field:

  • data: → actual payload
  • event: → custom event name
  • id: → event ID
  • retry: → reconnect delay
res.write("data: hello");

→ Missing \n\n browser thinks, event not finishet yet, I'll wait.

res.write("data: hello\n\n");

Got full event → fire onmessage

data: { ... } 👉 this is a default event.
It does NOT have an explicit name, but internally it is treated as:
event: message