In Spring Boot, when we try to send a Server Sent Event, it only sends an error event containing data: {"timeout":-1} when we try to connect, and the connection closes. The Spring Boot class is as follows

@RestController
@CrossOrigin(origins = "*")
public class SsePushNotificationRestController {
    private static final Logger log = LoggerFactory.getLogger(SsePushNotificationRestController.class);
    private SseEmitter emitter;

    @GetMapping("/test")
    public String getString(){
        try {
            emitter.send("User connected");
            log.info("User connected");
            emitter.complete();
        } catch (Exception e) {
            log.info("Error while sending message to client: " + e.getMessage());
        }
        return "placeholder";
    }

    @GetMapping("/emitter")
    public SseEmitter eventEmitter(@RequestParam String userId) {
        emitter = new SseEmitter(-1L);
        return emitter;
    }
}

And our client code is as follows:

const eventSource = new EventSource('http://localhost:8080/emitter?userId=testUser');

eventSource.addEventListener("message", (event) => {
    console.log(event);
});

eventSource.addEventListener("open", (event) => {
    console.log("connection opened");
});

eventSource.addEventListener("error", (e) => {
    if (e.readyState === EventSource.CLOSED) {
        console.log('closed');
    }
    else {
        console.log(e);
    }
    e.target.close();
});

document.getElementById("btn").onclick = e => {
    fetch('http://localhost:8080/test').then( data => console.log(data)).catch(data => console.log(data));
};

Immediately, an error is created before we can click the button to generate an event. What could be wrong?