sse_client.js

const evtSource = new EventSource('/sse');
evtSource.onmessage = event => {
  try {
    const jsonData = JSON.parse(event.data);
    console.log(jsonData);
  } catch (e) {
    console.log(e);
  }
}

evtSource.onerror = event => {
  if (event.readyState === EventSource.CLOSED) {
    console.log('connection is closed')
  }
}

evtSource.onopen = event => {
  console.log('connection is opened')
}

setTimeout(() => {
  console.log('client closes the connection');
  evtSource.close();
}, 30000);