Note: This feature is available in Web Workers.
The readyState read-only property of the EventSource interface returns a number representing the state of the connection.
A number which is one of the three possible state constants defined on the EventSource interface:
EventSource.CONNECTING (0)The connection is not yet open.
EventSource.OPEN (1)The connection is open and ready to communicate.
EventSource.CLOSED (2)The connection is closed or couldn't be opened.
const evtSource = new EventSource("sse.php");
console.log(evtSource.readyState);Note: You can find a full example on GitHub — see Simple SSE demo using PHP.