How to get the error details of an eventSource? – Blancer.com Tutorials and projects

I am currently using an eventsource to have a list of real time notifications. But for no reason, this eventsource crashes for no reason, fortunately I have a function to reconnect it but, I can’t understand why it crashes and I don’t know how to get details (code, message..) of the “onerror” event. Here is my code:

function isFunction(functionToCheck) {
  return functionToCheck && {}.toString.call(functionToCheck) === '[object Function]';
}

function debounce(func, wait) {
  var timeout;
  var waitFunc;

  return function() {
    if (isFunction(wait)) {
      waitFunc = wait;
    } else {
      waitFunc = function() {
        return wait
      };
    }

    var context = this,
      args = arguments;
    var later = function() {
      timeout = null;
      func.apply(context, args);
    };
    clearTimeout(timeout);
    timeout = setTimeout(later, waitFunc());
  };
}

// reconnectFrequencySeconds doubles every retry
var reconnectFrequencySeconds = 1;
var evtSource;
var is_deconnected = 0;
var is_first_connected = 0;
var reconnectFunc = debounce(function() {
  is_deconnected = 1;
  setupEventSource();
  // Double every attempt to avoid overwhelming server
  reconnectFrequencySeconds *= 2;
  // Max out at ~1 minute as a compromise between user experience and server load
  if (reconnectFrequencySeconds >= 64) {
    reconnectFrequencySeconds = 64;
  }
}, function() {
  return reconnectFrequencySeconds * 1000
});

var notif_compt = 0;
var show_notif_compt = 0;

function setupEventSource() {
 document.getElementById("notifs").innerHTML ="Loading";
  evtSource = new EventSource("http://express-eventsource.herokuapp.com/events");

  evtSource.addEventListener("data", function(event) {
  console.log("Data from server: "+JSON.parse(event.data).data);
    document.getElementById("notifs").innerHTML = JSON.parse(event.data).data;
    
    if(JSON.parse(event.data).data == "the end") {
    console.log("Reloading...");
        evtSource.close();
      reconnectFunc();
    }
  });

  evtSource.onopen = function(e) {

    console.log('Connected to server.')
    // Reset reconnect frequency upon successful connection
    reconnectFrequencySeconds = 1;
  };
  evtSource.onerror = function(e) {
    evtSource.close();
    console.error(e);
    console.error(evtSource);
    console.error(evtSource.readystate); // return undefined
    reconnectFunc();
  };
}
setupEventSource();


window.onbeforeunload = () => {
  evtSource.close();

};

<h1>Hi, User123</h1>
<span>Notification number(random): <div id="notifs">Loading</div> </span>