How to use server-sent-events in express.js

You can definitely achieve this without other packages.

I wrote a blog post about this, part 1 sets out the basics.

You mustn't close the SSE as that breaks the functionality. The whole point is that it is an open HTTP connection. This allows for new events to be pushed to the client at any point.

I disagree with using Socket.IO to implement basic Server-Sent Events. The browser API is dead simple and the implementation in Express requires only a couple of changes from a normal response route:

app.get('/streaming', (req, res) => {

    res.setHeader('Cache-Control', 'no-cache');
    res.setHeader('Content-Type', 'text/event-stream');
    res.setHeader('Access-Control-Allow-Origin', '*');
    res.setHeader('Connection', 'keep-alive');
    res.flushHeaders(); // flush the headers to establish SSE with client

    let counter = 0;
    let interValID = setInterval(() => {
        counter++;
        if (counter >= 10) {
            clearInterval(interValID);
            res.end(); // terminates SSE session
            return;
        }
        res.write(`data: ${JSON.stringify({num: counter})}\n\n`); // res.write() instead of res.send()
    }, 1000);

    // If client closes connection, stop sending events
    res.on('close', () => {
        console.log('client dropped me');
        clearInterval(interValID);
        res.end();
    });
});

• Set the appropriate headers as per the spec
• Use res.flushHeaders() to establish SSE connection
• Use res.write() instead of res.send() to send data
• To end stream from the server, use res.end()

The snippet above uses setInterval() to simulate sending data to the client for 10 seconds, then it ends the connection. The client will receive an error for the lost connection and automatically try to re-establish the connection. To avoid this, you can close the client on error, or have the browser send a specific event message that the client understands means to close gracefully. If the client closes the connection, we can catch the 'close' event to gracefully end the connection on the server and stop sending events.

express: 4.17.1 node: 10.16.3