open

Details

The open event is fired by the browser EventSource API when the HTTP response is received successfully with the correct Content-Type. It is not a field in the event stream – the server does not send anything special to trigger it.

The open event is the right place to update UI to show 'connected' status. It fires once per connection, including after automatic reconnects.

ReadyState transitions: CONNECTING (0) → OPEN (1): open event fires OPEN (1) → CONNECTING (0): connection lost, browser queues reconnect CONNECTING (0) → CLOSED (2): permanent error (204 response, or source.close() called)

The open event is also a good place to reset any error state from a previous disconnect.

Examples

const source = new EventSource('/stream');

source.onopen = (e) => {
  console.log('Connected. readyState:', source.readyState); // 1
  updateStatus('connected');
};

// Or with addEventListener:
source.addEventListener('open', () => {
  console.log('SSE connection open');
});

// EventSource readyState values (spec §9.2.2)
console.log(EventSource.CONNECTING); // 0
console.log(EventSource.OPEN);       // 1
console.log(EventSource.CLOSED);     // 2

// Check connection state
if (source.readyState === EventSource.OPEN) {
  console.log('Connection is active');
}

Gotchas