The official eventsource library has some reconnect bugs and I've also noticed it fails to detect dropped connections occasionally. This project approaches eventsource with a more modern implementation, and without these bugs.
const es = new EventSource('https://example.tld/path/to/sse-endpoint');
es.on('connecting', () => console.log('connecting'));
es.on('connected', () => console.log('connected'));
es.on('disconnected', () => console.log('disconnected'));
es.on('some_event_name', ({id, name, data}) => console.log(id, name, data));
This library does not take into account retry events that set reconnection interval since you can configure backoff with options.
new EventSource('https://example.tld/path/to/sse-endpoint', {
initialReconnectDelay: 2 * 1000, // 2 seconds
maximumReconnectDelay: 30 * 1000, // 30 seconds
})