EventSource: open event

The open event of the EventSource API is fired when a connection with an event source is opened.

This event is not cancelable and does not bubble.


Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("open", (event) => {});

onopen = (event) => {};

Event type

A generic Event.


const evtSource = new EventSource("sse.php");

// addEventListener version
evtSource.addEventListener("open", (e) => {
  console.log("The connection has been established.");

// onopen version
evtSource.onopen = (e) => {
  console.log("The connection has been established.");


HTML Standard
# event-open
HTML Standard
# handler-eventsource-onopen

Browser compatibility

BCD tables only load in the browser

See also