CODEWITHSUNDEEP
javascriptnode.jseventsource
user3807454
user3807454

Reputation: 59

onmessage function is not fired for eventSource

I have created a server in node.js using express module. It exposes a get api which is used to send events to the client at regular intervals via the open connection which is always kept alive.

const cors = require('cors');
const app = express();
app.use(cors());
const port = 3001;

app.get('/sse-server', function (request, response) {
    response.status(200).set({
        "connection": "keep-alive",
        "cache-control": "no-cache",
        "content-Type": "text/event-stream"
    });
    let data = { name: "Mithoon Kumar"}
    setInterval(()=>{ response.write(JSON.stringify(data));}, 0)

})

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

Also I have created a client in node.js which is using eventsource to receive server sent events

console.log("EventSource", EventSource);
const eventSource = new EventSource('http://127.0.0.1:3001/sse-server');

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

eventSource.onerror = (error) => {
    console.log("error", error);
}

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

The problem is onmessage method is never fired. If I open the link (http://127.0.0.1:3001/sse-server) in chrome browser it displays the messages on the page.

Upvotes: 2

Views: 2895

Answers (2)

user3807454
user3807454

Reputation: 59

I editted the line in which the server was writing data and it started working.

setInterval(()=>{ response.write(`data: Hello world!\n\n`)}, 0)

Upvotes: 0

robinvrd
robinvrd

Reputation: 1848

Apparently, EventSource only work with specific format which is define in its documentation. As you can see in this example, the message data should be sent in a data: [YOUR_DATA]\n\n format.

You should use something like this on your server side :

app.get('/event-stream', (req, res) => {    
    res.status(200).set({
        "connection": "keep-alive",
        "cache-control": "no-cache",
        "content-Type": "text/event-stream"
    });

    let data = { name: "Mithoon Kumar" }
    const intervalId = setInterval(() => {
        res.write(`data: ${JSON.stringify(data)}\n\n`);
    }, 1000);

    req.on('close', () => {
        clearInterval(intervalId);
    });
});

Upvotes: 6

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