EventSource.onopen
 The onopen property of the EventSource interface is an event handler called when an open event is received, that is when the connection was just opened. 
Syntax
eventSource.onopen = function
Examples
evtSource.onopen = function() { console.log("Connection to server opened."); };
Note: You can find a full example on GitHub — see Simple SSE demo using PHP.
Specifications
| Specification | 
|---|
| HTML Standard (HTML) # handler-eventsource-onopen | 
Browser compatibility
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
| onopen | 6 | 79 | 6 | No | Yes | 5 | ≤37 | 18 | 45 | 12 | 5 | 1.0 | 
See also
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    https://developer.mozilla.org/en-US/docs/Web/API/EventSource/onopen