message

Details

Every SSE event has an event type. When no event: field is present in the event, the type defaults to 'message'. This fires the onmessage event handler on the EventSource object.

An SSE event is terminated by a blank line (two consecutive newlines). Multiple data: lines are concatenated with a newline character between them.

For LLM streaming: each token or chunk is a separate message event. The OpenAI API sends data: {"choices":[{"delta":{"content":"token"}}]} for each chunk, then data: [DONE] as a sentinel to signal completion.

The event.data property on the MessageEvent contains the concatenated data lines as a string. Parse JSON with JSON.parse(event.data).

For a stream that sends binary data: SSE is UTF-8 only. Binary content must be base64-encoded in the data: field. For binary streaming, use WebSocket or WebTransport instead.

Examples

# Server sends:
data: Hello, world!

# Client receives MessageEvent:
# event.type = "message"
# event.data = "Hello, world!"

# Server sends:
data: {"userId": "usr_1", "score": 42}

# Client:
source.onmessage = (e) => {
  const payload = JSON.parse(e.data);
  console.log(payload.userId); // "usr_1"
};

# Server sends:
data: line one
data: line two
data: line three

# Client receives:
# event.data = "line one\nline two\nline three"

data: {"choices":[{"delta":{"content":"Hello"}}]}

data: {"choices":[{"delta":{"content":" world"}}]}

data: [DONE]

# Client:
source.onmessage = (e) => {
  if (e.data === '[DONE]') { source.close(); return; }
  const chunk = JSON.parse(e.data);
  output += chunk.choices[0].delta.content ?? '';
};

Gotchas