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

Question

I setup my REST server with express.js. Now I want to add sse to this server. After I implemented this sse package, I get an error. I know that I get this error, when would try to use res.send twice, but I am not.

ERROR: Error: Can't set headers after they are sent.                                            
    at ServerResponse.OutgoingMessage.setHeader (http.js:690:11)                            
    at ServerResponse.header (/home/root/node_modules/express/lib/response.js:718:10)         
    at ServerResponse.send (/home/root/node_modules/express/lib/response.js:163:12)            
    at app.get.str (/home/root/.node_app_slot/main.js:1330:25)                                 
    at Layer.handle [as handle_request] (/home/root/node_modules/express/lib/router/layer.js:95:5)    
    at next (/home/root/node_modules/express/lib/router/route.js:131:13)                       
    at sse (/home/root/node_modules/server-sent-events/index.js:35:2)                          
    at Layer.handle [as handle_request] (/home/root/node_modules/express/lib/router/layer.js:95:5)  
    at next (/home/root/node_modules/express/lib/router/route.js:131:13)                          
    at Route.dispatch (/home/root/node_modules/express/lib/router/route.js:112:3)

Is it possible that I can't use the express methods anymore within the sse function? For example:

app.get('/events', sse, function(req, res) {
    res.send('...');
});

Furthermore, I found this solution and this. Is it possible to make sse with the res.write function or in another way without using another package?

Answer 1

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