nodejs/express - stream stdout instantly to the client

I spawned the following child: var spw = spawn('ping', ['-n','10', '127.0.0.1']) and I would like to receive the ping results on the client side (browser) one by one, not as a whole.

So far I tried this:

app.get('/path', function(req, res) {
   ...
   spw.stdout.on('data', function (data) {
      var str = data.toString();
      res.write(str + "\n");
   });
   ...
}

and that:

...
spw.stdout.pipe(res);
...

In both cases browser waits 10 of the pings to complete, and then prints the result as a whole. I would like to have them one by one, how to accomplish that?

(Client is just making a call to .../path and console.logs the result)

EDIT: Although I do believe that websockets are necessary to implement this, I just want to know whether there are any other ways. I saw several confusing SO answers, and blog posts (in this post, at step one OP streams the logs to the browser) which didn't help, therefore I decided to go for a bounty for some attention.

Solution

Here's a complete example using SSE (Server sent events). This works in Firefox and probably Chrome too:

var cp = require("child_process"),
         express = require("express"),
         app = express();

app.configure(function(){
    app.use(express.static(__dirname));
});


app.get('/msg', function(req, res){
    res.writeHead(200, { "Content-Type": "text/event-stream",
                         "Cache-control": "no-cache" });

    var spw = cp.spawn('ping', ['-c', '100', '127.0.0.1']),
    str = "";

    spw.stdout.on('data', function (data) {
        str += data.toString();

        // just so we can see the server is doing something
        console.log("data");

        // Flush out line by line.
        var lines = str.split("\n");
        for(var i in lines) {
            if(i == lines.length - 1) {
                str = lines[i];
            } else{
                // Note: The double-newline is *required*
                res.write('data: ' + lines[i] + "\n\n");
            }
        }
    });

    spw.on('close', function (code) {
        res.end(str);
    });

    spw.stderr.on('data', function (data) {
        res.end('stderr: ' + data);
    });
});

app.listen(4000);

And the client HTML:

<!DOCTYPE Html>
<html> 
<body>
   <ul id="eventlist"> </ul>

   <script>              
    var eventList = document.getElementById("eventlist");
    var evtSource = new EventSource("http://localhost:4000/msg");

    var newElement = document.createElement("li");
    newElement.innerHTML = "Messages:";
    eventList.appendChild(newElement);


    evtSource.onmessage = function(e) {
        console.log("received event");
        console.log(e);
        var newElement = document.createElement("li");

        newElement.innerHTML = "message: " + e.data;
        eventList.appendChild(newElement);
    };      

    evtSource.onerror = function(e) {
        console.log("EventSource failed.");
    };

    console.log(evtSource);

    </script>

</body>
</html>

Run node index.js and point your browser at http://localhost:4000/client.html. Note that I had to use the "-c" option rather than "-n" since I'm running OS X.