sse-client.js

var source = new EventSource('stream')
source.addEventListener('message', function (event) {
  console.log('new message', event.data)
}, false)

// this is our custom event
source.addEventListener('connecttime', function (event) {
  console.log('Connection was last established at: ' + event.data)
}, false)

source.addEventListener('open', function (event) {
  document.body.addEventListener('click', function () {
    source.close()
    console.log('connection closed')
  })
}, false)

source.addEventListener('error', function (event) {
  if (event.target.readyState === EventSource.CLOSED) {
    source.close()
    console.log('Connection closed!')
  } else if (event.target.readyState === EventSource.CONNECTING) {
    console.log('Connection closed. Attempting to reconnect!')
  } else {
    console.log('Connection closed. Unknown error!')
  }
}, false)

Thanks yoshua https://github.com/yoshuawuyts/knowledge/blob/master/browser/sse.md