The onerror property of the EventSource interface is an event handler called when an error occurs and the error event is dispatched on an EventSource object. 
eventSource.onerror = function
evtSource.onerror = function() { console.log("EventSource failed."); };
Note: You can find a full example on GitHub — see Simple SSE demo using PHP.
| Specification | 
|---|
| HTML Standard (HTML) # handler-eventsource-onerror  | 
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
onerror | 
6  | 
79  | 
6  | 
No  | 
Yes  | 
5  | 
≤37  | 
18  | 
45  | 
12  | 
5  | 
1.0  | 
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    https://developer.mozilla.org/en-US/docs/Web/API/EventSource/onerror